bash5.0参考手册
Bash Reference Manual
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This text is a brief description of the features that are present in the Bash shell (version 5.0, 7 December 2018). The Bash home page is http://www.gnu.org/software/bash/.
This is Edition 5.0, last updated 7 December 2018, of The GNU Bash Reference Manual, for Bash
, Version 5.0.
Bash contains features that appear in other popular shells, and some features that only appear in Bash. Some of the shells that Bash has borrowed concepts from are the Bourne Shell (sh), the Korn Shell (ksh), and the C-shell (cshand its successor,tcsh). The following menu breaks the features up into categories, noting which features were inspired by other shells and which are specific to Bash.
This manual is meant as a brief introduction to features found in Bash. The Bash manual page should be used as the definitive reference on shell behavior.
• Introduction: | An introduction to the shell. | |
• Definitions: | Some definitions used in the rest of this manual. | |
• Basic Shell Features: | The shell "building blocks". | |
• Shell Builtin Commands: | Commands that are a part of the shell. | |
• Shell Variables: | Variables used or set by Bash. | |
• Bash Features: | Features found only in Bash. | |
• Job Control: | What job control is and how Bash allows you to use it. | |
• Command Line Editing: | Chapter describing the command line editing features. | |
• Using History Interactively: | Command History Expansion | |
• Installing Bash: | How to build and install Bash on your system. | |
• Reporting Bugs: | How to report bugs in Bash. | |
• Major Differences From The Bourne Shell: | A terse list of the differences between Bash and historical versions of /bin/sh. | |
• GNU Free Documentation License: | Copying and sharing this documentation. | |
• Indexes: | Various indexes for this manual. |
Next: Definitions, Up: Top [Contents][Index]
• What is Bash?: | A short description of Bash. | |
• What is a shell?: | A brief introduction to shells. |
Next: What is a shell?, Up: Introduction [Contents][Index]
Bash is the shell, or command language interpreter, for the GNU operating system. The name is an acronym for the ‘Bourne-Again SHell’, a pun on Stephen Bourne, the author of the direct ancestor of the current Unix shell sh
, which appeared in the Seventh Edition Bell Labs Research version of Unix.
Bash is largely compatible with sh
and incorporates useful features from the Korn shell ksh
and the C shell csh
. It is intended to be a conformant implementation of the IEEE POSIX Shell and Tools portion of the IEEE POSIX specification (IEEE Standard 1003.1). It offers functional improvements over sh
for both interactive and programming use.
While the GNU operating system provides other shells, including a version of csh
, Bash is the default shell. Like other GNU software, Bash is quite portable. It currently runs on nearly every version of Unix and a few other operating systems - independently-supported ports exist for MS-DOS, OS/2, and Windows platforms.
Previous: What is Bash?, Up: Introduction [Contents][Index]
At its base, a shell is simply a macro processor that executes commands. The term macro processor means functionality where text and symbols are expanded to create larger expressions.
A Unix shell is both a command interpreter and a programming language. As a command interpreter, the shell provides the user interface to the rich set of GNU utilities. The programming language features allow these utilities to be combined. Files containing commands can be created, and become commands themselves. These new commands have the same status as system commands in directories such as/bin, allowing users or groups to establish custom environments to automate their common tasks.
Shells may be used interactively or non-interactively. In interactive mode, they accept input typed from the keyboard. When executing non-interactively, shells execute commands read from a file.
A shell allows execution of GNU commands, both synchronously and asynchronously. The shell waits for synchronous commands to complete before accepting more input; asynchronous commands continue to execute in parallel with the shell while it reads and executes additional commands. The redirection constructs permit fine-grained control of the input and output of those commands. Moreover, the shell allows control over the contents of commands’ environments.
Shells also provide a small set of built-in commands (builtins) implementing functionality impossible or inconvenient to obtain via separate utilities. For example, cd
, break
, continue
, and exec
cannot be implemented outside of the shell because they directly manipulate the shell itself. The history
, getopts
, kill
, or pwd
builtins, among others, could be implemented in separate utilities, but they are more convenient to use as builtin commands. All of the shell builtins are described in subsequent sections.
While executing commands is essential, most of the power (and complexity) of shells is due to their embedded programming languages. Like any high-level language, the shell provides variables, flow control constructs, quoting, and functions.
Shells offer features geared specifically for interactive use rather than to augment the programming language. These interactive features include job control, command line editing, command history and aliases. Each of these features is described in this manual.
Next: Basic Shell Features, Previous: Introduction, Up: Top [Contents][Index]
These definitions are used throughout the remainder of this manual.
POSIX
A family of open system standards based on Unix. Bash is primarily concerned with the Shell and Utilities portion of the POSIX 1003.1 standard.
blank
A space or tab character.
builtin
A command that is implemented internally by the shell itself, rather than by an executable program somewhere in the file system.
control operator
A token
that performs a control function. It is a newline
or one of the following: ‘||’, ‘&&’, ‘&’, ‘;’, ‘;;’, ‘;&’, ‘;;&’, ‘|’, ‘|&’, ‘(’, or ‘)’.
exit status
The value returned by a command to its caller. The value is restricted to eight bits, so the maximum value is 255.
field
A unit of text that is the result of one of the shell expansions. After expansion, when executing a command, the resulting fields are used as the command name and arguments.
filename
A string of characters used to identify a file.
job
A set of processes comprising a pipeline, and any processes descended from it, that are all in the same process group.
job control
A mechanism by which users can selectively stop (suspend) and restart (resume) execution of processes.
metacharacter
A character that, when unquoted, separates words. A metacharacter is a space
, tab
, newline
, or one of the following characters: ‘|’, ‘&’, ‘;’, ‘(’, ‘)’, ‘<’, or ‘>’.
name
A word
consisting solely of letters, numbers, and underscores, and beginning with a letter or underscore. Name
s are used as shell variable and function names. Also referred to as an identifier
.
operator
A control operator
or a redirection operator
. See Redirections, for a list of redirection operators. Operators contain at least one unquoted metacharacter
.
process group
A collection of related processes each having the same process group ID.
process group ID
A unique identifier that represents a process group
during its lifetime.
reserved word
A word
that has a special meaning to the shell. Most reserved words introduce shell flow control constructs, such as for
and while
.
return status
A synonym for exit status
.
signal
A mechanism by which a process may be notified by the kernel of an event occurring in the system.
special builtin
A shell builtin command that has been classified as special by the POSIX standard.
token
A sequence of characters considered a single unit by the shell. It is either a word
or an operator
.
word
A sequence of characters treated as a unit by the shell. Words may not include unquoted metacharacters
.
Next: Shell Builtin Commands, Previous: Definitions, Up: Top [Contents][Index]
Bash is an acronym for ‘Bourne-Again SHell’. The Bourne shell is the traditional Unix shell originally written by Stephen Bourne. All of the Bourne shell builtin commands are available in Bash, The rules for evaluation and quoting are taken from the POSIX specification for the ‘standard’ Unix shell.
This chapter briefly summarizes the shell’s ‘building blocks’: commands, control structures, shell functions, shell parameters, shell expansions, redirections, which are a way to direct input and output from and to named files, and how the shell executes commands.
• Shell Syntax: | What your input means to the shell. | |
• Shell Commands: | The types of commands you can use. | |
• Shell Functions: | Grouping commands by name. | |
• Shell Parameters: | How the shell stores values. | |
• Shell Expansions: | How Bash expands parameters and the various expansions available. | |
• Redirections: | A way to control where input and output go. | |
• Executing Commands: | What happens when you run a command. | |
• Shell Scripts: | Executing files of shell commands. |
Next: Shell Commands, Up: Basic Shell Features [Contents][Index]
• Shell Operation: | The basic operation of the shell. | |
• Quoting: | How to remove the special meaning from characters. | |
• Comments: | How to specify comments. |
When the shell reads input, it proceeds through a sequence of operations. If the input indicates the beginning of a comment, the shell ignores the comment symbol (‘#’), and the rest of that line.
Otherwise, roughly speaking, the shell reads its input and divides the input into words and operators, employing the quoting rules to select which meanings to assign various words and characters.
The shell then parses these tokens into commands and other constructs, removes the special meaning of certain words or characters, expands others, redirects input and output as needed, executes the specified command, waits for the command’s exit status, and makes that exit status available for further inspection or processing.
Next: Quoting, Up: Shell Syntax [Contents][Index]
The following is a brief description of the shell’s operation when it reads and executes a command. Basically, the shell does the following:
metacharacters
. Alias expansion is performed by this step (see Aliases).Next: Comments, Previous: Shell Operation, Up: Shell Syntax [Contents][Index]
• Escape Character: | How to remove the special meaning from a single character. | |
• Single Quotes: | How to inhibit all interpretation of a sequence of characters. | |
• Double Quotes: | How to suppress most of the interpretation of a sequence of characters. | |
• ANSI-C Quoting: | How to expand ANSI-C sequences in quoted strings. | |
• Locale Translation: | How to translate strings into different languages. |
Quoting is used to remove the special meaning of certain characters or words to the shell. Quoting can be used to disable special treatment for special characters, to prevent reserved words from being recognized as such, and to prevent parameter expansion.
Each of the shell metacharacters (see Definitions) has special meaning to the shell and must be quoted if it is to represent itself. When the command history expansion facilities are being used (see History Interaction), the history expansion character, usually ‘!’, must be quoted to prevent history expansion. See Bash History Facilities, for more details concerning history expansion.
There are three quoting mechanisms: the escape character, single quotes, and double quotes.
Next: Single Quotes, Up: Quoting [Contents][Index]
A non-quoted backslash ‘\’ is the Bash escape character. It preserves the literal value of the next character that follows, with the exception of newline
. If a \newline
pair appears, and the backslash itself is not quoted, the \newline
is treated as a line continuation (that is, it is removed from the input stream and effectively ignored).
Next: Double Quotes, Previous: Escape Character, Up: Quoting [Contents][Index]
Enclosing characters in single quotes (‘'’) preserves the literal value of each character within the quotes. A single quote may not occur between single quotes, even when preceded by a backslash.
Next: ANSI-C Quoting, Previous: Single Quotes, Up: Quoting [Contents][Index]
Enclosing characters in double quotes (‘"’) preserves the literal value of all characters within the quotes, with the exception of ‘$’, ‘`’, ‘\’, and, when history expansion is enabled, ‘!’. When the shell is in POSIX mode (see Bash POSIX Mode), the ‘!’ has no special meaning within double quotes, even when history expansion is enabled. The characters ‘$’ and ‘`’ retain their special meaning within double quotes (see Shell Expansions). The backslash retains its special meaning only when followed by one of the following characters: ‘$’, ‘`’, ‘"’, ‘\’, or newline
. Within double quotes, backslashes that are followed by one of these characters are removed. Backslashes preceding characters without a special meaning are left unmodified. A double quote may be quoted within double quotes by preceding it with a backslash. If enabled, history expansion will be performed unless an ‘!’ appearing in double quotes is escaped using a backslash. The backslash preceding the ‘!’ is not removed.
The special parameters ‘*’ and ‘@’ have special meaning when in double quotes (see Shell Parameter Expansion).
Next: Locale Translation, Previous: Double Quotes, Up: Quoting [Contents][Index]
Words of the form $'string'
are treated specially. The word expands to string, with backslash-escaped characters replaced as specified by the ANSI C standard. Backslash escape sequences, if present, are decoded as follows:
\a
alert (bell)
\b
backspace
\e
\E
an escape character (not ANSI C)
\f
form feed
\n
newline
\r
carriage return
\t
horizontal tab
\v
vertical tab
\\
backslash
\'
single quote
\"
double quote
\?
question mark
\nnn
the eight-bit character whose value is the octal value nnn (one to three octal digits)
\xHH
the eight-bit character whose value is the hexadecimal value HH (one or two hex digits)
\uHHHH
the Unicode (ISO/IEC 10646) character whose value is the hexadecimal value HHHH (one to four hex digits)
\UHHHHHHHH
the Unicode (ISO/IEC 10646) character whose value is the hexadecimal value HHHHHHHH (one to eight hex digits)
\cx
a control-x character
The expanded result is single-quoted, as if the dollar sign had not been present.
Previous: ANSI-C Quoting, Up: Quoting [Contents][Index]
A double-quoted string preceded by a dollar sign (‘$’) will cause the string to be translated according to the current locale. If the current locale is C
or POSIX
, the dollar sign is ignored. If the string is translated and replaced, the replacement is double-quoted.
Some systems use the message catalog selected by the LC_MESSAGES
shell variable. Others create the name of the message catalog from the value of the TEXTDOMAIN
shell variable, possibly adding a suffix of ‘.mo’. If you use the TEXTDOMAIN
variable, you may need to set the TEXTDOMAINDIR
variable to the location of the message catalog files. Still others use both variables in this fashion: TEXTDOMAINDIR
/LC_MESSAGES
/LC_MESSAGES/TEXTDOMAIN
.mo.
Previous: Quoting, Up: Shell Syntax [Contents][Index]
In a non-interactive shell, or an interactive shell in which the interactive_comments
option to the shopt
builtin is enabled (see The Shopt Builtin), a word beginning with ‘#’ causes that word and all remaining characters on that line to be ignored. An interactive shell without the interactive_comments
option enabled does not allow comments. The interactive_comments
option is on by default in interactive shells. See Interactive Shells, for a description of what makes a shell interactive.
Next: Shell Functions, Previous: Shell Syntax, Up: Basic Shell Features [Contents][Index]
A simple shell command such as echo a b c
consists of the command itself followed by arguments, separated by spaces.
More complex shell commands are composed of simple commands arranged together in a variety of ways: in a pipeline in which the output of one command becomes the input of a second, in a loop or conditional construct, or in some other grouping.
• Simple Commands: | The most common type of command. | |
• Pipelines: | Connecting the input and output of several commands. | |
• Lists: | How to execute commands sequentially. | |
• Compound Commands: | Shell commands for control flow. | |
• Coprocesses: | Two-way communication between commands. | |
• GNU Parallel: | Running commands in parallel. |
Next: Pipelines, Up: Shell Commands [Contents][Index]
A simple command is the kind of command encountered most often. It’s just a sequence of words separated by blank
s, terminated by one of the shell’s control operators (see Definitions). The first word generally specifies a command to be executed, with the rest of the words being that command’s arguments.
The return status (see Exit Status) of a simple command is its exit status as provided by the POSIX 1003.1 waitpid
function, or 128+n if the command was terminated by signal n.
Next: Lists, Previous: Simple Commands, Up: Shell Commands [Contents][Index]
A pipeline
is a sequence of one or more commands separated by one of the control operators ‘|’ or ‘|&’.
The format for a pipeline is
[time [-p]] [!] command1 [ | or |& command2 ] …
The output of each command in the pipeline is connected via a pipe to the input of the next command. That is, each command reads the previous command’s output. This connection is performed before any redirections specified by the command.
If ‘|&’ is used, command1’s standard error, in addition to its standard output, is connected to command2’s standard input through the pipe; it is shorthand for 2>&1 |
. This implicit redirection of the standard error to the standard output is performed after any redirections specified by the command.
The reserved word time
causes timing statistics to be printed for the pipeline once it finishes. The statistics currently consist of elapsed (wall-clock) time and user and system time consumed by the command’s execution. The-poption changes the output format to that specified by POSIX. When the shell is in POSIX mode (see Bash POSIX Mode), it does not recognize time
as a reserved word if the next token begins with a ‘-’. The TIMEFORMAT
variable may be set to a format string that specifies how the timing information should be displayed. See Bash Variables, for a description of the available formats. The use of time
as a reserved word permits the timing of shell builtins, shell functions, and pipelines. An external time
command cannot time these easily.
When the shell is in POSIX mode (see Bash POSIX Mode), time
may be followed by a newline. In this case, the shell displays the total user and system time consumed by the shell and its children. The TIMEFORMAT
variable may be used to specify the format of the time information.
If the pipeline is not executed asynchronously (see Lists), the shell waits for all commands in the pipeline to complete.
Each command in a pipeline is executed in its own subshell, which is a separate process (see Command Execution Environment). If the lastpipe
option is enabled using the shopt
builtin (see The Shopt Builtin), the last element of a pipeline may be run by the shell process.
The exit status of a pipeline is the exit status of the last command in the pipeline, unless the pipefail
option is enabled (see The Set Builtin). If pipefail
is enabled, the pipeline’s return status is the value of the last (rightmost) command to exit with a non-zero status, or zero if all commands exit successfully. If the reserved word ‘!’ precedes the pipeline, the exit status is the logical negation of the exit status as described above. The shell waits for all commands in the pipeline to terminate before returning a value.
Next: Compound Commands, Previous: Pipelines, Up: Shell Commands [Contents][Index]
A list
is a sequence of one or more pipelines separated by one of the operators ‘;’, ‘&’, ‘&&’, or ‘||’, and optionally terminated by one of ‘;’, ‘&’, or a newline
.
Of these list operators, ‘&&’ and ‘||’ have equal precedence, followed by ‘;’ and ‘&’, which have equal precedence.
A sequence of one or more newlines may appear in a list
to delimit commands, equivalent to a semicolon.
If a command is terminated by the control operator ‘&’, the shell executes the command asynchronously in a subshell. This is known as executing the command in the background, and these are referred to as asynchronous commands. The shell does not wait for the command to finish, and the return status is 0 (true). When job control is not active (see Job Control), the standard input for asynchronous commands, in the absence of any explicit redirections, is redirected from /dev/null
.
Commands separated by a ‘;’ are executed sequentially; the shell waits for each command to terminate in turn. The return status is the exit status of the last command executed.
AND and OR lists are sequences of one or more pipelines separated by the control operators ‘&&’ and ‘||’, respectively. AND and OR lists are executed with left associativity.
An AND list has the form
command1 && command2
command2 is executed if, and only if, command1 returns an exit status of zero (success).
An OR list has the form
command1 || command2
command2 is executed if, and only if, command1 returns a non-zero exit status.
The return status of AND and OR lists is the exit status of the last command executed in the list.
Next: Coprocesses, Previous: Lists, Up: Shell Commands [Contents][Index]
• Looping Constructs: | Shell commands for iterative action. | |
• Conditional Constructs: | Shell commands for conditional execution. | |
• Command Grouping: | Ways to group commands. |
Compound commands are the shell programming language constructs. Each construct begins with a reserved word or control operator and is terminated by a corresponding reserved word or operator. Any redirections (see Redirections) associated with a compound command apply to all commands within that compound command unless explicitly overridden.
In most cases a list of commands in a compound command’s description may be separated from the rest of the command by one or more newlines, and may be followed by a newline in place of a semicolon.
Bash provides looping constructs, conditional commands, and mechanisms to group commands and execute them as a unit.
Next: Conditional Constructs, Up: Compound Commands [Contents][Index]
Bash supports the following looping constructs.
Note that wherever a ‘;’ appears in the description of a command’s syntax, it may be replaced with one or more newlines.
until
The syntax of the until
command is:
until test-commands; do consequent-commands; done
Execute consequent-commands as long as test-commands has an exit status which is not zero. The return status is the exit status of the last command executed in consequent-commands, or zero if none was executed.
while
The syntax of the while
command is:
while test-commands; do consequent-commands; done
Execute consequent-commands as long as test-commands has an exit status of zero. The return status is the exit status of the last command executed in consequent-commands, or zero if none was executed.
for
The syntax of the for
command is:
for name [ [in [words …] ] ; ] do commands; done
Expand words (see Shell Expansions), and execute commands once for each member in the resultant list, with name bound to the current member. If ‘in words’ is not present, the for
command executes the commands once for each positional parameter that is set, as if ‘in "$@"’ had been specified (see Special Parameters).
The return status is the exit status of the last command that executes. If there are no items in the expansion of words, no commands are executed, and the return status is zero.
An alternate form of the for
command is also supported:
for (( expr1 ; expr2 ; expr3 )) ; do commands ; done
First, the arithmetic expression expr1 is evaluated according to the rules described below (see Shell Arithmetic). The arithmetic expression expr2 is then evaluated repeatedly until it evaluates to zero. Each time expr2 evaluates to a non-zero value, commands are executed and the arithmetic expression expr3 is evaluated. If any expression is omitted, it behaves as if it evaluates to 1. The return value is the exit status of the last command in commands that is executed, or false if any of the expressions is invalid.
The break
and continue
builtins (see Bourne Shell Builtins) may be used to control loop execution.
Next: Command Grouping, Previous: Looping Constructs, Up: Compound Commands [Contents][Index]
if
The syntax of the if
command is:
if test-commands; then consequent-commands; [elif more-test-commands; then more-consequents;] [else alternate-consequents;] fi
The test-commands list is executed, and if its return status is zero, the consequent-commands list is executed. If test-commands returns a non-zero status, each elif
list is executed in turn, and if its exit status is zero, the corresponding more-consequents is executed and the command completes. If ‘else alternate-consequents’ is present, and the final command in the final if
or elif
clause has a non-zero exit status, then alternate-consequents is executed. The return status is the exit status of the last command executed, or zero if no condition tested true.
case
The syntax of the case
command is:
case word in [ [(] pattern [| pattern]…) command-list ;;]… esac
case
will selectively execute the command-list corresponding to the first pattern that matches word. The match is performed according to the rules described below in Pattern Matching. If the nocasematch
shell option (see the description of shopt
in The Shopt Builtin) is enabled, the match is performed without regard to the case of alphabetic characters. The ‘|’ is used to separate multiple patterns, and the ‘)’ operator terminates a pattern list. A list of patterns and an associated command-list is known as a clause.
Each clause must be terminated with ‘;;’, ‘;&’, or ‘;;&’. The word undergoes tilde expansion, parameter expansion, command substitution, arithmetic expansion, and quote removal (see Shell Parameter Expansion) before matching is attempted. Each pattern undergoes tilde expansion, parameter expansion, command substitution, and arithmetic expansion.
There may be an arbitrary number of case
clauses, each terminated by a ‘;;’, ‘;&’, or ‘;;&’. The first pattern that matches determines the command-list that is executed. It’s a common idiom to use ‘*’ as the final pattern to define the default case, since that pattern will always match.
Here is an example using case
in a script that could be used to describe one interesting feature of an animal:
echo -n "Enter the name of an animal: " read ANIMAL echo -n "The $ANIMAL has " case $ANIMAL in horse | dog | cat) echo -n "four";; man | kangaroo ) echo -n "two";; *) echo -n "an unknown number of";; esac echo " legs."
If the ‘;;’ operator is used, no subsequent matches are attempted after the first pattern match. Using ‘;&’ in place of ‘;;’ causes execution to continue with the command-list associated with the next clause, if any. Using ‘;;&’ in place of ‘;;’ causes the shell to test the patterns in the next clause, if any, and execute any associated command-list on a successful match.
The return status is zero if no pattern is matched. Otherwise, the return status is the exit status of the command-list executed.
select
The select
construct allows the easy generation of menus. It has almost the same syntax as the for
command:
select name [in words …]; do commands; done
The list of words following in
is expanded, generating a list of items. The set of expanded words is printed on the standard error output stream, each preceded by a number. If the ‘in words’ is omitted, the positional parameters are printed, as if ‘in "$@"’ had been specified. The PS3
prompt is then displayed and a line is read from the standard input. If the line consists of a number corresponding to one of the displayed words, then the value of name is set to that word. If the line is empty, the words and prompt are displayed again. If EOF
is read, the select
command completes. Any other value read causes name to be set to null. The line read is saved in the variable REPLY
.
The commands are executed after each selection until a break
command is executed, at which point the select
command completes.
Here is an example that allows the user to pick a filename from the current directory, and displays the name and index of the file selected.
select fname in *; do echo you picked $fname \($REPLY\) break; done
((…))
(( expression ))
The arithmetic expression is evaluated according to the rules described below (see Shell Arithmetic). If the value of the expression is non-zero, the return status is 0; otherwise the return status is 1. This is exactly equivalent to
let "expression"
See Bash Builtins, for a full description of the let
builtin.
[[…]]
[[ expression ]]
Return a status of 0 or 1 depending on the evaluation of the conditional expression expression. Expressions are composed of the primaries described below in Bash Conditional Expressions. Word splitting and filename expansion are not performed on the words between the [[
and ]]
; tilde expansion, parameter and variable expansion, arithmetic expansion, command substitution, process substitution, and quote removal are performed. Conditional operators such as ‘-f’ must be unquoted to be recognized as primaries.
When used with [[
, the ‘<’ and ‘>’ operators sort lexicographically using the current locale.
When the ‘==’ and ‘!=’ operators are used, the string to the right of the operator is considered a pattern and matched according to the rules described below in Pattern Matching, as if the extglob
shell option were enabled. The ‘=’ operator is identical to ‘==’. If the nocasematch
shell option (see the description of shopt
in The Shopt Builtin) is enabled, the match is performed without regard to the case of alphabetic characters. The return value is 0 if the string matches (‘==’) or does not match (‘!=’) the pattern, and 1 otherwise. Any part of the pattern may be quoted to force the quoted portion to be matched as a string.
An additional binary operator, ‘=~’, is available, with the same precedence as ‘==’ and ‘!=’. When it is used, the string to the right of the operator is considered a POSIX extended regular expression and matched accordingly (as in regex3)). The return value is 0 if the string matches the pattern, and 1 otherwise. If the regular expression is syntactically incorrect, the conditional expression’s return value is 2. If the nocasematch
shell option (see the description of shopt
in The Shopt Builtin) is enabled, the match is performed without regard to the case of alphabetic characters. Any part of the pattern may be quoted to force the quoted portion to be matched as a string. Bracket expressions in regular expressions must be treated carefully, since normal quoting characters lose their meanings between brackets. If the pattern is stored in a shell variable, quoting the variable expansion forces the entire pattern to be matched as a string. Substrings matched by parenthesized subexpressions within the regular expression are saved in the array variable BASH_REMATCH
. The element of BASH_REMATCH
with index 0 is the portion of the string matching the entire regular expression. The element of BASH_REMATCH
with index n is the portion of the string matching the nth parenthesized subexpression.
For example, the following will match a line (stored in the shell variable line) if there is a sequence of characters in the value consisting of any number, including zero, of space characters, zero or one instances of ‘a’, then a ‘b’:
[[ $line =~ [[:space:]]*?(a)b ]]
That means values like ‘aab’ and ‘aaaaaab’ will match, as will a line containing a ‘b’ anywhere in its value.
Storing the regular expression in a shell variable is often a useful way to avoid problems with quoting characters that are special to the shell. It is sometimes difficult to specify a regular expression literally without using quotes, or to keep track of the quoting used by regular expressions while paying attention to the shell’s quote removal. Using a shell variable to store the pattern decreases these problems. For example, the following is equivalent to the above:
pattern='[[:space:]]*?(a)b' [[ $line =~ $pattern ]]
If you want to match a character that’s special to the regular expression grammar, it has to be quoted to remove its special meaning. This means that in the pattern ‘xxx.txt’, the ‘.’ matches any character in the string (its usual regular expression meaning), but in the pattern ‘"xxx.txt"’ it can only match a literal ‘.’. Shell programmers should take special care with backslashes, since backslashes are used both by the shell and regular expressions to remove the special meaning from the following character. The following two sets of commands are not equivalent:
pattern='\.' [[ . =~ $pattern ]] [[ . =~ \. ]] [[ . =~ "$pattern" ]] [[ . =~ '\.' ]]
The first two matches will succeed, but the second two will not, because in the second two the backslash will be part of the pattern to be matched. In the first two examples, the backslash removes the special meaning from ‘.’, so the literal ‘.’ matches. If the string in the first examples were anything other than ‘.’, say ‘a’, the pattern would not match, because the quoted ‘.’ in the pattern loses its special meaning of matching any single character.
Expressions may be combined using the following operators, listed in decreasing order of precedence:
( expression )
Returns the value of expression. This may be used to override the normal precedence of operators.
! expression
True if expression is false.
expression1 && expression2
True if both expression1 and expression2 are true.
expression1 || expression2
True if either expression1 or expression2 is true.
The &&
and ||
operators do not evaluate expression2 if the value of expression1 is sufficient to determine the return value of the entire conditional expression.
Previous: Conditional Constructs, Up: Compound Commands [Contents][Index]
Bash provides two ways to group a list of commands to be executed as a unit. When commands are grouped, redirections may be applied to the entire command list. For example, the output of all the commands in the list may be redirected to a single stream.
()
( list )
Placing a list of commands between parentheses causes a subshell environment to be created (see Command Execution Environment), and each of the commands in list to be executed in that subshell. Since the list is executed in a subshell, variable assignments do not remain in effect after the subshell completes.
{}
{ list; }
Placing a list of commands between curly braces causes the list to be executed in the current shell context. No subshell is created. The semicolon (or newline) following list is required.
In addition to the creation of a subshell, there is a subtle difference between these two constructs due to historical reasons. The braces are reserved words
, so they must be separated from the list by blank
s or other shell metacharacters. The parentheses are operators
, and are recognized as separate tokens by the shell even if they are not separated from the list by whitespace.
The exit status of both of these constructs is the exit status of list.
Next: GNU Parallel, Previous: Compound Commands, Up: Shell Commands [Contents][Index]
A coprocess
is a shell command preceded by the coproc
reserved word. A coprocess is executed asynchronously in a subshell, as if the command had been terminated with the ‘&’ control operator, with a two-way pipe established between the executing shell and the coprocess.
The format for a coprocess is:
coproc [NAME] command [redirections]
This creates a coprocess named NAME. If NAME is not supplied, the default name is COPROC. NAME must not be supplied if command is a simple command (see Simple Commands); otherwise, it is interpreted as the first word of the simple command.
When the coprocess is executed, the shell creates an array variable (see Arrays) named NAME
in the context of the executing shell. The standard output of command is connected via a pipe to a file descriptor in the executing shell, and that file descriptor is assigned to NAME
[0]. The standard input of command is connected via a pipe to a file descriptor in the executing shell, and that file descriptor is assigned to NAME
[1]. This pipe is established before any redirections specified by the command (see Redirections). The file descriptors can be utilized as arguments to shell commands and redirections using standard word expansions. Other than those created to execute command and process substitutions, the file descriptors are not available in subshells.
The process ID of the shell spawned to execute the coprocess is available as the value of the variable NAME
_PID. The wait
builtin command may be used to wait for the coprocess to terminate.
Since the coprocess is created as an asynchronous command, the coproc
command always returns success. The return status of a coprocess is the exit status of command.
Previous: Coprocesses, Up: Shell Commands [Contents][Index]
There are ways to run commands in parallel that are not built into Bash. GNU Parallel is a tool to do just that.
GNU Parallel, as its name suggests, can be used to build and run commands in parallel. You may run the same command with different arguments, whether they are filenames, usernames, hostnames, or lines read from files. GNU Parallel provides shorthand references to many of the most common operations (input lines, various portions of the input line, different ways to specify the input source, and so on). Parallel can replace xargs
or feed commands from its input sources to several different instances of Bash.
For a complete description, refer to the GNU Parallel documentation. A few examples should provide a brief introduction to its use.
For example, it is easy to replace xargs
to gzip all html files in the current directory and its subdirectories:
find . -type f -name '*.html' -print | parallel gzip
If you need to protect special characters such as newlines in file names, use find’s-print0option and parallel’s-0option.
You can use Parallel to move files from the current directory when the number of files is too large to process with one mv
invocation:
ls | parallel mv {} destdir
As you can see, the {} is replaced with each line read from standard input. While using ls
will work in most instances, it is not sufficient to deal with all filenames. If you need to accommodate special characters in filenames, you can use
find . -depth 1 \! -name '.*' -print0 | parallel -0 mv {} destdir
as alluded to above.
This will run as many mv
commands as there are files in the current directory. You can emulate a parallel xargs
by adding the-Xoption:
find . -depth 1 \! -name '.*' -print0 | parallel -0 -X mv {} destdir
GNU Parallel can replace certain common idioms that operate on lines read from a file (in this case, filenames listed one per line):
while IFS= read -r x; do do-something1 "$x" "config-$x" do-something2 < "$x" done < file | process-output
with a more compact syntax reminiscent of lambdas:
cat list | parallel "do-something1 {} config-{} ; do-something2 < {}" | process-output
Parallel provides a built-in mechanism to remove filename extensions, which lends itself to batch file transformations or renaming:
ls *.gz | parallel -j+0 "zcat {} | bzip2 >{.}.bz2 && rm {}"
This will recompress all files in the current directory with names ending in .gz using bzip2, running one job per CPU (-j+0) in parallel. (We use ls
for brevity here; using find
as above is more robust in the face of filenames containing unexpected characters.) Parallel can take arguments from the command line; the above can also be written as
parallel "zcat {} | bzip2 >{.}.bz2 && rm {}" ::: *.gz
If a command generates output, you may want to preserve the input order in the output. For instance, the following command
{ echo foss.org.my ; echo debian.org ; echo freenetproject.org ; } | parallel traceroute
will display as output the traceroute invocation that finishes first. Adding the-koption
{ echo foss.org.my ; echo debian.org ; echo freenetproject.org ; } | parallel -k traceroute
will ensure that the output of traceroute foss.org.my
is displayed first.
Finally, Parallel can be used to run a sequence of shell commands in parallel, similar to ‘cat file | bash’. It is not uncommon to take a list of filenames, create a series of shell commands to operate on them, and feed that list of commands to a shell. Parallel can speed this up. Assuming thatfilecontains a list of shell commands, one per line,
parallel -j 10 < file
will evaluate the commands using the shell (since no explicit command is supplied as an argument), in blocks of ten shell jobs at a time.
Next: Shell Parameters, Previous: Shell Commands, Up: Basic Shell Features [Contents][Index]
Shell functions are a way to group commands for later execution using a single name for the group. They are executed just like a "regular" command. When the name of a shell function is used as a simple command name, the list of commands associated with that function name is executed. Shell functions are executed in the current shell context; no new process is created to interpret them.
Functions are declared using this syntax:
name () compound-command [ redirections ]
or
function name [()] compound-command [ redirections ]
This defines a shell function named name. The reserved word function
is optional. If the function
reserved word is supplied, the parentheses are optional. The body of the function is the compound command compound-command (see Compound Commands). That command is usually a list enclosed between { and }, but may be any compound command listed above, with one exception: If the function
reserved word is used, but the parentheses are not supplied, the braces are required. compound-command is executed whenever name is specified as the name of a command. When the shell is in POSIX mode (see Bash POSIX Mode), name may not be the same as one of the special builtins (see Special Builtins). Any redirections (see Redirections) associated with the shell function are performed when the function is executed.
A function definition may be deleted using the-foption to the unset
builtin (see Bourne Shell Builtins).
The exit status of a function definition is zero unless a syntax error occurs or a readonly function with the same name already exists. When executed, the exit status of a function is the exit status of the last command executed in the body.
Note that for historical reasons, in the most common usage the curly braces that surround the body of the function must be separated from the body by blank
s or newlines. This is because the braces are reserved words and are only recognized as such when they are separated from the command list by whitespace or another shell metacharacter. Also, when using the braces, the list must be terminated by a semicolon, a ‘&’, or a newline.
When a function is executed, the arguments to the function become the positional parameters during its execution (see Positional Parameters). The special parameter ‘#’ that expands to the number of positional parameters is updated to reflect the change. Special parameter 0
is unchanged. The first element of the FUNCNAME
variable is set to the name of the function while the function is executing.
All other aspects of the shell execution environment are identical between a function and its caller with these exceptions: the DEBUG
and RETURN
traps are not inherited unless the function has been given the trace
attribute using the declare
builtin or the -o functrace
option has been enabled with the set
builtin, (in which case all functions inherit the DEBUG
and RETURN
traps), and the ERR
trap is not inherited unless the -o errtrace
shell option has been enabled. See Bourne Shell Builtins, for the description of the trap
builtin.
The FUNCNEST
variable, if set to a numeric value greater than 0, defines a maximum function nesting level. Function invocations that exceed the limit cause the entire command to abort.
If the builtin command return
is executed in a function, the function completes and execution resumes with the next command after the function call. Any command associated with the RETURN
trap is executed before execution resumes. When a function completes, the values of the positional parameters and the special parameter ‘#’ are restored to the values they had prior to the function’s execution. If a numeric argument is given to return
, that is the function’s return status; otherwise the function’s return status is the exit status of the last command executed before the return
.
Variables local to the function may be declared with the local
builtin. These variables are visible only to the function and the commands it invokes. This is particularly important when a shell function calls other functions.
Local variables "shadow" variables with the same name declared at previous scopes. For instance, a local variable declared in a function hides a global variable of the same name: references and assignments refer to the local variable, leaving the global variable unmodified. When the function returns, the global variable is once again visible.
The shell uses dynamic scoping to control a variable’s visibility within functions. With dynamic scoping, visible variables and their values are a result of the sequence of function calls that caused execution to reach the current function. The value of a variable that a function sees depends on its value within its caller, if any, whether that caller is the "global" scope or another shell function. This is also the value that a local variable declaration "shadows", and the value that is restored when the function returns.
For example, if a variable var is declared as local in function func1, and func1 calls another function func2, references to var made from within func2 will resolve to the local variable var from func1, shadowing any global variable named var.
The following script demonstrates this behavior. When executed, the script displays
In func2, var = func1 local
func1() { local var='func1 local' func2 } func2() { echo "In func2, var = $var" } var=global func1
The unset
builtin also acts using the same dynamic scope: if a variable is local to the current scope, unset
will unset it; otherwise the unset will refer to the variable found in any calling scope as described above. If a variable at the current local scope is unset, it will remain so until it is reset in that scope or until the function returns. Once the function returns, any instance of the variable at a previous scope will become visible. If the unset acts on a variable at a previous scope, any instance of a variable with that name that had been shadowed will become visible.
Function names and definitions may be listed with the-foption to the declare
(typeset
) builtin command (see Bash Builtins). The-Foption to declare
or typeset
will list the function names only (and optionally the source file and line number, if the extdebug
shell option is enabled). Functions may be exported so that subshells automatically have them defined with the-foption to the export
builtin (see Bourne Shell Builtins).
Functions may be recursive. The FUNCNEST
variable may be used to limit the depth of the function call stack and restrict the number of function invocations. By default, no limit is placed on the number of recursive calls.
Next: Shell Expansions, Previous: Shell Functions, Up: Basic Shell Features [Contents][Index]
• Positional Parameters: | The shell’s command-line arguments. | |
• Special Parameters: | Parameters denoted by special characters. |
A parameter is an entity that stores values. It can be a name
, a number, or one of the special characters listed below. A variable is a parameter denoted by a name
. A variable has a value and zero or more attributes. Attributes are assigned using the declare
builtin command (see the description of the declare
builtin in Bash Builtins).
A parameter is set if it has been assigned a value. The null string is a valid value. Once a variable is set, it may be unset only by using the unset
builtin command.
A variable may be assigned to by a statement of the form
name=[value]
If value is not given, the variable is assigned the null string. All values undergo tilde expansion, parameter and variable expansion, command substitution, arithmetic expansion, and quote removal (detailed below). If the variable has its integer
attribute set, then value is evaluated as an arithmetic expression even if the $((…))
expansion is not used (see Arithmetic Expansion). Word splitting is not performed, with the exception of "$@"
as explained below. Filename expansion is not performed. Assignment statements may also appear as arguments to the alias
, declare
, typeset
, export
, readonly
, and local
builtin commands (declaration commands). When in POSIX mode (see Bash POSIX Mode), these builtins may appear in a command after one or more instances of the command
builtin and retain these assignment statement properties.
In the context where an assignment statement is assigning a value to a shell variable or array index (see Arrays), the ‘+=’ operator can be used to append to or add to the variable’s previous value. This includes arguments to builtin commands such as declare
that accept assignment statements (declaration commands). When ‘+=’ is applied to a variable for which the integer attribute has been set, value is evaluated as an arithmetic expression and added to the variable’s current value, which is also evaluated. When ‘+=’ is applied to an array variable using compound assignment (see Arrays), the variable’s value is not unset (as it is when using ‘=’), and new values are appended to the array beginning at one greater than the array’s maximum index (for indexed arrays), or added as additional key-value pairs in an associative array. When applied to a string-valued variable, value is expanded and appended to the variable’s value.
A variable can be assigned the nameref attribute using the-noption to the declare
or local
builtin commands (see Bash Builtins) to create a nameref, or a reference to another variable. This allows variables to be manipulated indirectly. Whenever the nameref variable is referenced, assigned to, unset, or has its attributes modified (other than using or changing the nameref attribute itself), the operation is actually performed on the variable specified by the nameref variable’s value. A nameref is commonly used within shell functions to refer to a variable whose name is passed as an argument to the function. For instance, if a variable name is passed to a shell function as its first argument, running
declare -n ref=$1
inside the function creates a nameref variable ref whose value is the variable name passed as the first argument. References and assignments to ref, and changes to its attributes, are treated as references, assignments, and attribute modifications to the variable whose name was passed as $1
.
If the control variable in a for
loop has the nameref attribute, the list of words can be a list of shell variables, and a name reference will be established for each word in the list, in turn, when the loop is executed. Array variables cannot be given the nameref attribute. However, nameref variables can reference array variables and subscripted array variables. Namerefs can be unset using the-noption to the unset
builtin (see Bourne Shell Builtins). Otherwise, if unset
is executed with the name of a nameref variable as an argument, the variable referenced by the nameref variable will be unset.
Next: Special Parameters, Up: Shell Parameters [Contents][Index]
A positional parameter is a parameter denoted by one or more digits, other than the single digit 0
. Positional parameters are assigned from the shell’s arguments when it is invoked, and may be reassigned using the set
builtin command. Positional parameter N
may be referenced as ${N}
, or as $N
when N
consists of a single digit. Positional parameters may not be assigned to with assignment statements. The set
and shift
builtins are used to set and unset them (see Shell Builtin Commands). The positional parameters are temporarily replaced when a shell function is executed (see Shell Functions).
When a positional parameter consisting of more than a single digit is expanded, it must be enclosed in braces.
Previous: Positional Parameters, Up: Shell Parameters [Contents][Index]
The shell treats several parameters specially. These parameters may only be referenced; assignment to them is not allowed.
*
($*) Expands to the positional parameters, starting from one. When the expansion is not within double quotes, each positional parameter expands to a separate word. In contexts where it is performed, those words are subject to further word splitting and pathname expansion. When the expansion occurs within double quotes, it expands to a single word with the value of each parameter separated by the first character of the IFS
special variable. That is, "$*"
is equivalent to "$1c$2c…"
, where c is the first character of the value of the IFS
variable. If IFS
is unset, the parameters are separated by spaces. If IFS
is null, the parameters are joined without intervening separators.
@
($@) Expands to the positional parameters, starting from one. In contexts where word splitting is performed, this expands each positional parameter to a separate word; if not within double quotes, these words are subject to word splitting. In contexts where word splitting is not performed, this expands to a single word with each positional parameter separated by a space. When the expansion occurs within double quotes, and word splitting is performed, each parameter expands to a separate word. That is, "$@"
is equivalent to "$1" "$2" …
. If the double-quoted expansion occurs within a word, the expansion of the first parameter is joined with the beginning part of the original word, and the expansion of the last parameter is joined with the last part of the original word. When there are no positional parameters, "$@"
and $@
expand to nothing (i.e., they are removed).
#
($#) Expands to the number of positional parameters in decimal.
?
($?) Expands to the exit status of the most recently executed foreground pipeline.
-
($-, a hyphen.) Expands to the current option flags as specified upon invocation, by the set
builtin command, or those set by the shell itself (such as the-ioption).
$
($$) Expands to the process ID of the shell. In a ()
subshell, it expands to the process ID of the invoking shell, not the subshell.
!
($!) Expands to the process ID of the job most recently placed into the background, whether executed as an asynchronous command or using the bg
builtin (see Job Control Builtins).
0
($0) Expands to the name of the shell or shell script. This is set at shell initialization. If Bash is invoked with a file of commands (see Shell Scripts), $0
is set to the name of that file. If Bash is started with the-coption (see Invoking Bash), then $0
is set to the first argument after the string to be executed, if one is present. Otherwise, it is set to the filename used to invoke Bash, as given by argument zero.
_
($_, an underscore.) At shell startup, set to the absolute pathname used to invoke the shell or shell script being executed as passed in the environment or argument list. Subsequently, expands to the last argument to the previous simple command executed in the foreground, after expansion. Also set to the full pathname used to invoke each command executed and placed in the environment exported to that command. When checking mail, this parameter holds the name of the mail file.
Next: Redirections, Previous: Shell Parameters, Up: Basic Shell Features [Contents][Index]
Expansion is performed on the command line after it has been split into token
s. There are seven kinds of expansion performed:
• Brace Expansion: | Expansion of expressions within braces. | |
• Tilde Expansion: | Expansion of the ~ character. | |
• Shell Parameter Expansion: | How Bash expands variables to their values. | |
• Command Substitution: | Using the output of a command as an argument. | |
• Arithmetic Expansion: | How to use arithmetic in shell expansions. | |
• Process Substitution: | A way to write and read to and from a command. | |
• Word Splitting: | How the results of expansion are split into separate arguments. | |
• Filename Expansion: | A shorthand for specifying filenames matching patterns. | |
• Quote Removal: | How and when quote characters are removed from words. |
The order of expansions is: brace expansion; tilde expansion, parameter and variable expansion, arithmetic expansion, and command substitution (done in a left-to-right fashion); word splitting; and filename expansion.
On systems that can support it, there is an additional expansion available: process substitution. This is performed at the same time as tilde, parameter, variable, and arithmetic expansion and command substitution.
After these expansions are performed, quote characters present in the original word are removed unless they have been quoted themselves (quote removal).
Only brace expansion, word splitting, and filename expansion can increase the number of words of the expansion; other expansions expand a single word to a single word. The only exceptions to this are the expansions of "$@"
and $*
(see Special Parameters), and "${name[@]}"
and ${name[*]}
(see Arrays).
After all expansions, quote removal
(see Quote Removal) is performed.
Next: Tilde Expansion, Up: Shell Expansions [Contents][Index]
Brace expansion is a mechanism by which arbitrary strings may be generated. This mechanism is similar to filename expansion (see Filename Expansion), but the filenames generated need not exist. Patterns to be brace expanded take the form of an optional preamble, followed by either a series of comma-separated strings or a sequence expression between a pair of braces, followed by an optional postscript. The preamble is prefixed to each string contained within the braces, and the postscript is then appended to each resulting string, expanding left to right.
Brace expansions may be nested. The results of each expanded string are not sorted; left to right order is preserved. For example,
bash$ echo a{d,c,b}e ade ace abe
A sequence expression takes the form {x..y[..incr]}
, where x and y are either integers or single characters, and incr, an optional increment, is an integer. When integers are supplied, the expression expands to each number between x and y, inclusive. Supplied integers may be prefixed with ‘0’ to force each term to have the same width. When either x or y begins with a zero, the shell attempts to force all generated terms to contain the same number of digits, zero-padding where necessary. When characters are supplied, the expression expands to each character lexicographically between x and y, inclusive, using the default C locale. Note that both x and y must be of the same type. When the increment is supplied, it is used as the difference between each term. The default increment is 1 or -1 as appropriate.
Brace expansion is performed before any other expansions, and any characters special to other expansions are preserved in the result. It is strictly textual. Bash does not apply any syntactic interpretation to the context of the expansion or the text between the braces.
A correctly-formed brace expansion must contain unquoted opening and closing braces, and at least one unquoted comma or a valid sequence expression. Any incorrectly formed brace expansion is left unchanged.
A { or ‘,’ may be quoted with a backslash to prevent its being considered part of a brace expression. To avoid conflicts with parameter expansion, the string ‘${’ is not considered eligible for brace expansion, and inhibits brace expansion until the closing ‘}’..
This construct is typically used as shorthand when the common prefix of the strings to be generated is longer than in the above example:
mkdir /usr/local/src/bash/{old,new,dist,bugs}
or
chown root /usr/{ucb/{ex,edit},lib/{ex?.?*,how_ex}}
Next: Shell Parameter Expansion, Previous: Brace Expansion, Up: Shell Expansions [Contents][Index]
If a word begins with an unquoted tilde character (‘~’), all of the characters up to the first unquoted slash (or all characters, if there is no unquoted slash) are considered a tilde-prefix. If none of the characters in the tilde-prefix are quoted, the characters in the tilde-prefix following the tilde are treated as a possible login name. If this login name is the null string, the tilde is replaced with the value of the HOME
shell variable. If HOME
is unset, the home directory of the user executing the shell is substituted instead. Otherwise, the tilde-prefix is replaced with the home directory associated with the specified login name.
If the tilde-prefix is ‘~+’, the value of the shell variable PWD
replaces the tilde-prefix. If the tilde-prefix is ‘~-’, the value of the shell variable OLDPWD
, if it is set, is substituted.
If the characters following the tilde in the tilde-prefix consist of a number N, optionally prefixed by a ‘+’ or a ‘-’, the tilde-prefix is replaced with the corresponding element from the directory stack, as it would be displayed by the dirs
builtin invoked with the characters following tilde in the tilde-prefix as an argument (see The Directory Stack). If the tilde-prefix, sans the tilde, consists of a number without a leading ‘+’ or ‘-’, ‘+’ is assumed.
If the login name is invalid, or the tilde expansion fails, the word is left unchanged.
Each variable assignment is checked for unquoted tilde-prefixes immediately following a ‘:’ or the first ‘=’. In these cases, tilde expansion is also performed. Consequently, one may use filenames with tildes in assignments to PATH
, MAILPATH
, and CDPATH
, and the shell assigns the expanded value.
The following table shows how Bash treats unquoted tilde-prefixes:
~
The value of $HOME
~/foo
$HOME/foo
~fred/foo
The subdirectory foo
of the home directory of the user fred
~+/foo
$PWD/foo
~-/foo
${OLDPWD-'~-'}/foo
~N
The string that would be displayed by ‘dirs +N’
~+N
The string that would be displayed by ‘dirs +N’
~-N
The string that would be displayed by ‘dirs -N’
Bash also performs tilde expansion on words satisfying the conditions of variable assignments (see Shell Parameters) when they appear as arguments to simple commands. Bash does not do this, except for the declaration commands listed above, when in POSIX mode.
Next: Command Substitution, Previous: Tilde Expansion, Up: Shell Expansions [Contents][Index]
The ‘$’ character introduces parameter expansion, command substitution, or arithmetic expansion. The parameter name or symbol to be expanded may be enclosed in braces, which are optional but serve to protect the variable to be expanded from characters immediately following it which could be interpreted as part of the name.
When braces are used, the matching ending brace is the first ‘}’ not escaped by a backslash or within a quoted string, and not within an embedded arithmetic expansion, command substitution, or parameter expansion.
The basic form of parameter expansion is ${parameter}. The value of parameter is substituted. The parameter is a shell parameter as described above (see Shell Parameters) or an array reference (see Arrays). The braces are required when parameter is a positional parameter with more than one digit, or when parameter is followed by a character that is not to be interpreted as part of its name.
If the first character of parameter is an exclamation point (!), and parameter is not a nameref, it introduces a level of indirection. Bash uses the value formed by expanding the rest of parameter as the new parameter; this is then expanded and that value is used in the rest of the expansion, rather than the expansion of the original parameter. This is known as indirect expansion
. The value is subject to tilde expansion, parameter expansion, command substitution, and arithmetic expansion. If parameter is a nameref, this expands to the name of the variable referenced by parameter instead of performing the complete indirect expansion. The exceptions to this are the expansions of ${!prefix*} and ${!name[@]} described below. The exclamation point must immediately follow the left brace in order to introduce indirection.
In each of the cases below, word is subject to tilde expansion, parameter expansion, command substitution, and arithmetic expansion.
When not performing substring expansion, using the form described below (e.g., ‘:-’), Bash tests for a parameter that is unset or null. Omitting the colon results in a test only for a parameter that is unset. Put another way, if the colon is included, the operator tests for both parameter’s existence and that its value is not null; if the colon is omitted, the operator tests only for existence.
${parameter:-word}
If parameter is unset or null, the expansion of word is substituted. Otherwise, the value of parameter is substituted.
${parameter:=word}
If parameter is unset or null, the expansion of word is assigned to parameter. The value of parameter is then substituted. Positional parameters and special parameters may not be assigned to in this way.
${parameter:?word}
If parameter is null or unset, the expansion of word (or a message to that effect if word is not present) is written to the standard error and the shell, if it is not interactive, exits. Otherwise, the value of parameter is substituted.
${parameter:+word}
If parameter is null or unset, nothing is substituted, otherwise the expansion of word is substituted.
${parameter:offset}
${parameter:offset:length}
This is referred to as Substring Expansion. It expands to up to length characters of the value of parameter starting at the character specified by offset. If parameter is ‘@’, an indexed array subscripted by ‘@’ or ‘*’, or an associative array name, the results differ as described below. If length is omitted, it expands to the substring of the value of parameter starting at the character specified by offset and extending to the end of the value. length and offset are arithmetic expressions (see Shell Arithmetic).
If offset evaluates to a number less than zero, the value is used as an offset in characters from the end of the value of parameter. If length evaluates to a number less than zero, it is interpreted as an offset in characters from the end of the value of parameter rather than a number of characters, and the expansion is the characters between offset and that result. Note that a negative offset must be separated from the colon by at least one space to avoid being confused with the ‘:-’ expansion.
Here are some examples illustrating substring expansion on parameters and subscripted arrays:
$ string=01234567890abcdefgh $ echo ${string:7} 7890abcdefgh $ echo ${string:7:0} $ echo ${string:7:2} 78 $ echo ${string:7:-2} 7890abcdef $ echo ${string: -7} bcdefgh $ echo ${string: -7:0} $ echo ${string: -7:2} bc $ echo ${string: -7:-2} bcdef $ set -- 01234567890abcdefgh $ echo ${1:7} 7890abcdefgh $ echo ${1:7:0} $ echo ${1:7:2} 78 $ echo ${1:7:-2} 7890abcdef $ echo ${1: -7} bcdefgh $ echo ${1: -7:0} $ echo ${1: -7:2} bc $ echo ${1: -7:-2} bcdef $ array[0]=01234567890abcdefgh $ echo ${array[0]:7} 7890abcdefgh $ echo ${array[0]:7:0} $ echo ${array[0]:7:2} 78 $ echo ${array[0]:7:-2} 7890abcdef $ echo ${array[0]: -7} bcdefgh $ echo ${array[0]: -7:0} $ echo ${array[0]: -7:2} bc $ echo ${array[0]: -7:-2} bcdef
If parameter is ‘@’, the result is length positional parameters beginning at offset. A negative offset is taken relative to one greater than the greatest positional parameter, so an offset of -1 evaluates to the last positional parameter. It is an expansion error if length evaluates to a number less than zero.
The following examples illustrate substring expansion using positional parameters:
$ set -- 1 2 3 4 5 6 7 8 9 0 a b c d e f g h $ echo ${@:7} 7 8 9 0 a b c d e f g h $ echo ${@:7:0} $ echo ${@:7:2} 7 8 $ echo ${@:7:-2} bash: -2: substring expression < 0 $ echo ${@: -7:2} b c $ echo ${@:0} ./bash 1 2 3 4 5 6 7 8 9 0 a b c d e f g h $ echo ${@:0:2} ./bash 1 $ echo ${@: -7:0}
If parameter is an indexed array name subscripted by ‘@’ or ‘*’, the result is the length members of the array beginning with ${parameter[offset]}
. A negative offset is taken relative to one greater than the maximum index of the specified array. It is an expansion error if length evaluates to a number less than zero.
These examples show how you can use substring expansion with indexed arrays:
$ array=(0 1 2 3 4 5 6 7 8 9 0 a b c d e f g h) $ echo ${array[@]:7} 7 8 9 0 a b c d e f g h $ echo ${array[@]:7:2} 7 8 $ echo ${array[@]: -7:2} b c $ echo ${array[@]: -7:-2} bash: -2: substring expression < 0 $ echo ${array[@]:0} 0 1 2 3 4 5 6 7 8 9 0 a b c d e f g h $ echo ${array[@]:0:2} 0 1 $ echo ${array[@]: -7:0}
Substring expansion applied to an associative array produces undefined results.
Substring indexing is zero-based unless the positional parameters are used, in which case the indexing starts at 1 by default. If offset is 0, and the positional parameters are used, $@
is prefixed to the list.
${!prefix*}
${!prefix@}
Expands to the names of variables whose names begin with prefix, separated by the first character of the IFS
special variable. When ‘@’ is used and the expansion appears within double quotes, each variable name expands to a separate word.
${!name[@]}
${!name[*]}
If name is an array variable, expands to the list of array indices (keys) assigned in name. If name is not an array, expands to 0 if name is set and null otherwise. When ‘@’ is used and the expansion appears within double quotes, each key expands to a separate word.
${#parameter}
The length in characters of the expanded value of parameter is substituted. If parameter is ‘*’ or ‘@’, the value substituted is the number of positional parameters. If parameter is an array name subscripted by ‘*’ or ‘@’, the value substituted is the number of elements in the array. If parameter is an indexed array name subscripted by a negative number, that number is interpreted as relative to one greater than the maximum index of parameter, so negative indices count back from the end of the array, and an index of -1 references the last element.
${parameter#word}
${parameter##word}
The word is expanded to produce a pattern and matched according to the rules described below (see Pattern Matching). If the pattern matches the beginning of the expanded value of parameter, then the result of the expansion is the expanded value of parameter with the shortest matching pattern (the ‘#’ case) or the longest matching pattern (the ‘##’ case) deleted. If parameter is ‘@’ or ‘*’, the pattern removal operation is applied to each positional parameter in turn, and the expansion is the resultant list. If parameter is an array variable subscripted with ‘@’ or ‘*’, the pattern removal operation is applied to each member of the array in turn, and the expansion is the resultant list.
${parameter%word}
${parameter%%word}
The word is expanded to produce a pattern and matched according to the rules described below (see Pattern Matching). If the pattern matches If the pattern matches a trailing portion of the expanded value of parameter, then the result of the expansion is the value of parameter with the shortest matching pattern (the ‘%’ case) or the longest matching pattern (the ‘%%’ case) deleted. If parameter is ‘@’ or ‘*’, the pattern removal operation is applied to each positional parameter in turn, and the expansion is the resultant list. If parameter is an array variable subscripted with ‘@’ or ‘*’, the pattern removal operation is applied to each member of the array in turn, and the expansion is the resultant list.
${parameter/pattern/string}
The pattern is expanded to produce a pattern just as in filename expansion. Parameter is expanded and the longest match of pattern against its value is replaced with string. The match is performed according to the rules described below (see Pattern Matching). If pattern begins with ‘/’, all matches of pattern are replaced with string. Normally only the first match is replaced. If pattern begins with ‘#’, it must match at the beginning of the expanded value of parameter. If pattern begins with ‘%’, it must match at the end of the expanded value of parameter. If string is null, matches of pattern are deleted and the /
following pattern may be omitted. If the nocasematch
shell option (see the description of shopt
in The Shopt Builtin) is enabled, the match is performed without regard to the case of alphabetic characters. If parameter is ‘@’ or ‘*’, the substitution operation is applied to each positional parameter in turn, and the expansion is the resultant list. If parameter is an array variable subscripted with ‘@’ or ‘*’, the substitution operation is applied to each member of the array in turn, and the expansion is the resultant list.
${parameter^pattern}
${parameter^^pattern}
${parameter,pattern}
${parameter,,pattern}
This expansion modifies the case of alphabetic characters in parameter. The pattern is expanded to produce a pattern just as in filename expansion. Each character in the expanded value of parameter is tested against pattern, and, if it matches the pattern, its case is converted. The pattern should not attempt to match more than one character. The ‘^’ operator converts lowercase letters matching pattern to uppercase; the ‘,’ operator converts matching uppercase letters to lowercase. The ‘^^’ and ‘,,’ expansions convert each matched character in the expanded value; the ‘^’ and ‘,’ expansions match and convert only the first character in the expanded value. If pattern is omitted, it is treated like a ‘?’, which matches every character. If parameter is ‘@’ or ‘*’, the case modification operation is applied to each positional parameter in turn, and the expansion is the resultant list. If parameter is an array variable subscripted with ‘@’ or ‘*’, the case modification operation is applied to each member of the array in turn, and the expansion is the resultant list.
${parameter@operator}
The expansion is either a transformation of the value of parameter or information about parameter itself, depending on the value of operator. Each operator is a single letter:
Q
The expansion is a string that is the value of parameter quoted in a format that can be reused as input.
E
The expansion is a string that is the value of parameter with backslash escape sequences expanded as with the $'…'
quoting mechanism.
P
The expansion is a string that is the result of expanding the value of parameter as if it were a prompt string (see Controlling the Prompt).
A
The expansion is a string in the form of an assignment statement or declare
command that, if evaluated, will recreate parameter with its attributes and value.
a
The expansion is a string consisting of flag values representing parameter’s attributes.
If parameter is ‘@’ or ‘*’, the operation is applied to each positional parameter in turn, and the expansion is the resultant list. If parameter is an array variable subscripted with ‘@’ or ‘*’, the operation is applied to each member of the array in turn, and the expansion is the resultant list.
The result of the expansion is subject to word splitting and pathname expansion as described below.
Next: Arithmetic Expansion, Previous: Shell Parameter Expansion, Up: Shell Expansions [Contents][Index]
Command substitution allows the output of a command to replace the command itself. Command substitution occurs when a command is enclosed as follows:
$(command)
or
`command`
Bash performs the expansion by executing command in a subshell environment and replacing the command substitution with the standard output of the command, with any trailing newlines deleted. Embedded newlines are not deleted, but they may be removed during word splitting. The command substitution $(cat file)
can be replaced by the equivalent but faster $(< file)
.
When the old-style backquote form of substitution is used, backslash retains its literal meaning except when followed by ‘$’, ‘`’, or ‘\’. The first backquote not preceded by a backslash terminates the command substitution. When using the $(command)
form, all characters between the parentheses make up the command; none are treated specially.
Command substitutions may be nested. To nest when using the backquoted form, escape the inner backquotes with backslashes.
If the substitution appears within double quotes, word splitting and filename expansion are not performed on the results.
Next: Process Substitution, Previous: Command Substitution, Up: Shell Expansions [Contents][Index]
Arithmetic expansion allows the evaluation of an arithmetic expression and the substitution of the result. The format for arithmetic expansion is:
$(( expression ))
The expression is treated as if it were within double quotes, but a double quote inside the parentheses is not treated specially. All tokens in the expression undergo parameter and variable expansion, command substitution, and quote removal. The result is treated as the arithmetic expression to be evaluated. Arithmetic expansions may be nested.
The evaluation is performed according to the rules listed below (see Shell Arithmetic). If the expression is invalid, Bash prints a message indicating failure to the standard error and no substitution occurs.
Next: Word Splitting, Previous: Arithmetic Expansion, Up: Shell Expansions [Contents][Index]
Process substitution allows a process’s input or output to be referred to using a filename. It takes the form of
<(list)
or
>(list)
The process list is run asynchronously, and its input or output appears as a filename. This filename is passed as an argument to the current command as the result of the expansion. If the >(list)
form is used, writing to the file will provide input for list. If the <(list)
form is used, the file passed as an argument should be read to obtain the output of list. Note that no space may appear between the <
or >
and the left parenthesis, otherwise the construct would be interpreted as a redirection. Process substitution is supported on systems that support named pipes (FIFOs) or the/dev/fdmethod of naming open files.
When available, process substitution is performed simultaneously with parameter and variable expansion, command substitution, and arithmetic expansion.
Next: Filename Expansion, Previous: Process Substitution, Up: Shell Expansions [Contents][Index]
The shell scans the results of parameter expansion, command substitution, and arithmetic expansion that did not occur within double quotes for word splitting.
The shell treats each character of $IFS
as a delimiter, and splits the results of the other expansions into words using these characters as field terminators. If IFS
is unset, or its value is exactly <space><tab><newline>
, the default, then sequences of <space>
, <tab>
, and <newline>
at the beginning and end of the results of the previous expansions are ignored, and any sequence of IFS
characters not at the beginning or end serves to delimit words. If IFS
has a value other than the default, then sequences of the whitespace characters space
, tab
, and newline
are ignored at the beginning and end of the word, as long as the whitespace character is in the value of IFS
(an IFS
whitespace character). Any character in IFS
that is not IFS
whitespace, along with any adjacent IFS
whitespace characters, delimits a field. A sequence of IFS
whitespace characters is also treated as a delimiter. If the value of IFS
is null, no word splitting occurs.
Explicit null arguments (""
or ''
) are retained and passed to commands as empty strings. Unquoted implicit null arguments, resulting from the expansion of parameters that have no values, are removed. If a parameter with no value is expanded within double quotes, a null argument results and is retained and passed to a command as an empty string. When a quoted null argument appears as part of a word whose expansion is non-null, the null argument is removed. That is, the word -d''
becomes -d
after word splitting and null argument removal.
Note that if no expansion occurs, no splitting is performed.
Next: Quote Removal, Previous: Word Splitting, Up: Shell Expansions [Contents][Index]
• Pattern Matching: | How the shell matches patterns. |
After word splitting, unless the-foption has been set (see The Set Builtin), Bash scans each word for the characters ‘*’, ‘?’, and ‘[’. If one of these characters appears, then the word is regarded as a pattern, and replaced with an alphabetically sorted list of filenames matching the pattern (see Pattern Matching). If no matching filenames are found, and the shell option nullglob
is disabled, the word is left unchanged. If the nullglob
option is set, and no matches are found, the word is removed. If the failglob
shell option is set, and no matches are found, an error message is printed and the command is not executed. If the shell option nocaseglob
is enabled, the match is performed without regard to the case of alphabetic characters.
When a pattern is used for filename expansion, the character ‘.’ at the start of a filename or immediately following a slash must be matched explicitly, unless the shell option dotglob
is set. The filenames ‘.’ and ‘..’ must always be matched explicitly, even if dotglob
is set. In other cases, the ‘.’ character is not treated specially.
When matching a filename, the slash character must always be matched explicitly by a slash in the pattern, but in other matching contexts it can be matched by a special pattern character as described below (see Pattern Matching).
See the description of shopt
in The Shopt Builtin, for a description of the nocaseglob
, nullglob
, failglob
, and dotglob
options.
The GLOBIGNORE
shell variable may be used to restrict the set of file names matching a pattern. If GLOBIGNORE
is set, each matching file name that also matches one of the patterns in GLOBIGNORE
is removed from the list of matches. If the nocaseglob
option is set, the matching against the patterns in GLOBIGNORE
is performed without regard to case. The filenames.and..are always ignored when GLOBIGNORE
is set and not null. However, setting GLOBIGNORE
to a non-null value has the effect of enabling the dotglob
shell option, so all other filenames beginning with a ‘.’ will match. To get the old behavior of ignoring filenames beginning with a ‘.’, make ‘.*’ one of the patterns in GLOBIGNORE
. The dotglob
option is disabled when GLOBIGNORE
is unset.
Up: Filename Expansion [Contents][Index]
Any character that appears in a pattern, other than the special pattern characters described below, matches itself. The NUL character may not occur in a pattern. A backslash escapes the following character; the escaping backslash is discarded when matching. The special pattern characters must be quoted if they are to be matched literally.
The special pattern characters have the following meanings:
*
Matches any string, including the null string. When the globstar
shell option is enabled, and ‘*’ is used in a filename expansion context, two adjacent ‘*’s used as a single pattern will match all files and zero or more directories and subdirectories. If followed by a ‘/’, two adjacent ‘*’s will match only directories and subdirectories.
?
Matches any single character.
[…]
Matches any one of the enclosed characters. A pair of characters separated by a hyphen denotes a range expression; any character that falls between those two characters, inclusive, using the current locale’s collating sequence and character set, is matched. If the first character following the ‘[’ is a ‘!’ or a ‘^’ then any character not enclosed is matched. A ‘-’ may be matched by including it as the first or last character in the set. A ‘]’ may be matched by including it as the first character in the set. The sorting order of characters in range expressions is determined by the current locale and the values of the LC_COLLATE
and LC_ALL
shell variables, if set.
For example, in the default C locale, ‘[a-dx-z]’ is equivalent to ‘[abcdxyz]’. Many locales sort characters in dictionary order, and in these locales ‘[a-dx-z]’ is typically not equivalent to ‘[abcdxyz]’; it might be equivalent to ‘[aBbCcDdxXyYz]’, for example. To obtain the traditional interpretation of ranges in bracket expressions, you can force the use of the C locale by setting the LC_COLLATE
or LC_ALL
environment variable to the value ‘C’, or enable the globasciiranges
shell option.
Within ‘[’ and ‘]’, character classes can be specified using the syntax [:
class:]
, where class is one of the following classes defined in the POSIX standard:
alnum alpha ascii blank cntrl digit graph lower print punct space upper word xdigit
A character class matches any character belonging to that class. The word
character class matches letters, digits, and the character ‘_’.
Within ‘[’ and ‘]’, an equivalence class can be specified using the syntax [=
c=]
, which matches all characters with the same collation weight (as defined by the current locale) as the character c.
Within ‘[’ and ‘]’, the syntax [.
symbol.]
matches the collating symbol symbol.
If the extglob
shell option is enabled using the shopt
builtin, several extended pattern matching operators are recognized. In the following description, a pattern-list is a list of one or more patterns separated by a ‘|’. Composite patterns may be formed using one or more of the following sub-patterns:
?(pattern-list)
Matches zero or one occurrence of the given patterns.
*(pattern-list)
Matches zero or more occurrences of the given patterns.
+(pattern-list)
Matches one or more occurrences of the given patterns.
@(pattern-list)
Matches one of the given patterns.
!(pattern-list)
Matches anything except one of the given patterns.
Complicated extended pattern matching against long strings is slow, especially when the patterns contain alternations and the strings contain multiple matches. Using separate matches against shorter strings, or using arrays of strings instead of a single long string, may be faster.
Previous: Filename Expansion, Up: Shell Expansions [Contents][Index]
After the preceding expansions, all unquoted occurrences of the characters ‘\’, ‘'’, and ‘"’ that did not result from one of the above expansions are removed.
Next: Executing Commands, Previous: Shell Expansions, Up: Basic Shell Features [Contents][Index]
Before a command is executed, its input and output may be redirected using a special notation interpreted by the shell. Redirection allows commands’ file handles to be duplicated, opened, closed, made to refer to different files, and can change the files the command reads from and writes to. Redirection may also be used to modify file handles in the current shell execution environment. The following redirection operators may precede or appear anywhere within a simple command or may follow a command. Redirections are processed in the order they appear, from left to right.
Each redirection that may be preceded by a file descriptor number may instead be preceded by a word of the form {varname}. In this case, for each redirection operator except >&- and <&-, the shell will allocate a file descriptor greater than 10 and assign it to {varname}. If >&- or <&- is preceded by {varname}, the value of varname defines the file descriptor to close. If {varname} is supplied, the redirection persists beyond the scope of the command, allowing the shell programmer to manage the file descriptor himself.
In the following descriptions, if the file descriptor number is omitted, and the first character of the redirection operator is ‘<’, the redirection refers to the standard input (file descriptor 0). If the first character of the redirection operator is ‘>’, the redirection refers to the standard output (file descriptor 1).
The word following the redirection operator in the following descriptions, unless otherwise noted, is subjected to brace expansion, tilde expansion, parameter expansion, command substitution, arithmetic expansion, quote removal, filename expansion, and word splitting. If it expands to more than one word, Bash reports an error.
Note that the order of redirections is significant. For example, the command
ls > dirlist 2>&1
directs both standard output (file descriptor 1) and standard error (file descriptor 2) to the file dirlist, while the command
ls 2>&1 > dirlist
directs only the standard output to file dirlist, because the standard error was made a copy of the standard output before the standard output was redirected to dirlist.
Bash handles several filenames specially when they are used in redirections, as described in the following table. If the operating system on which Bash is running provides these special files, bash will use them; otherwise it will emulate them internally with the behavior described below.
/dev/fd/fd
If fd is a valid integer, file descriptor fd is duplicated.
/dev/stdin
File descriptor 0 is duplicated.
/dev/stdout
File descriptor 1 is duplicated.
/dev/stderr
File descriptor 2 is duplicated.
/dev/tcp/host/port
If host is a valid hostname or Internet address, and port is an integer port number or service name, Bash attempts to open the corresponding TCP socket.
/dev/udp/host/port
If host is a valid hostname or Internet address, and port is an integer port number or service name, Bash attempts to open the corresponding UDP socket.
A failure to open or create a file causes the redirection to fail.
Redirections using file descriptors greater than 9 should be used with care, as they may conflict with file descriptors the shell uses internally.
Redirection of input causes the file whose name results from the expansion of word to be opened for reading on file descriptor n
, or the standard input (file descriptor 0) if n
is not specified.
The general format for redirecting input is:
[n]<word
Redirection of output causes the file whose name results from the expansion of word to be opened for writing on file descriptor n, or the standard output (file descriptor 1) if n is not specified. If the file does not exist it is created; if it does exist it is truncated to zero size.
The general format for redirecting output is:
[n]>[|]word
If the redirection operator is ‘>’, and the noclobber
option to the set
builtin has been enabled, the redirection will fail if the file whose name results from the expansion of word exists and is a regular file. If the redirection operator is ‘>|’, or the redirection operator is ‘>’ and the noclobber
option is not enabled, the redirection is attempted even if the file named by word exists.
Redirection of output in this fashion causes the file whose name results from the expansion of word to be opened for appending on file descriptor n, or the standard output (file descriptor 1) if n is not specified. If the file does not exist it is created.
The general format for appending output is:
[n]>>word
This construct allows both the standard output (file descriptor 1) and the standard error output (file descriptor 2) to be redirected to the file whose name is the expansion of word.
There are two formats for redirecting standard output and standard error:
&>word
and
>&word
Of the two forms, the first is preferred. This is semantically equivalent to
>word 2>&1
When using the second form, word may not expand to a number or ‘-’. If it does, other redirection operators apply (see Duplicating File Descriptors below) for compatibility reasons.
This construct allows both the standard output (file descriptor 1) and the standard error output (file descriptor 2) to be appended to the file whose name is the expansion of word.
The format for appending standard output and standard error is:
&>>word
This is semantically equivalent to
>>word 2>&1
(see Duplicating File Descriptors below).
This type of redirection instructs the shell to read input from the current source until a line containing only word (with no trailing blanks) is seen. All of the lines read up to that point are then used as the standard input (or file descriptor n if n is specified) for a command.
The format of here-documents is:
[n]<<[-]word here-document delimiter
No parameter and variable expansion, command substitution, arithmetic expansion, or filename expansion is performed on word. If any part of word is quoted, the delimiter is the result of quote removal on word, and the lines in the here-document are not expanded. If word is unquoted, all lines of the here-document are subjected to parameter expansion, command substitution, and arithmetic expansion, the character sequence \newline
is ignored, and ‘\’ must be used to quote the characters ‘\’, ‘$’, and ‘`’.
If the redirection operator is ‘<<-’, then all leading tab characters are stripped from input lines and the line containing delimiter. This allows here-documents within shell scripts to be indented in a natural fashion.
A variant of here documents, the format is:
[n]<<< word
The word undergoes tilde expansion, parameter and variable expansion, command substitution, arithmetic expansion, and quote removal. Pathname expansion and word splitting are not performed. The result is supplied as a single string, with a newline appended, to the command on its standard input (or file descriptor n if n is specified).
The redirection operator
[n]<&word
is used to duplicate input file descriptors. If word expands to one or more digits, the file descriptor denoted by n is made to be a copy of that file descriptor. If the digits in word do not specify a file descriptor open for input, a redirection error occurs. If word evaluates to ‘-’, file descriptor n is closed. If n is not specified, the standard input (file descriptor 0) is used.
The operator
[n]>&word
is used similarly to duplicate output file descriptors. If n is not specified, the standard output (file descriptor 1) is used. If the digits in word do not specify a file descriptor open for output, a redirection error occurs. If word evaluates to ‘-’, file descriptor n is closed. As a special case, if n is omitted, and word does not expand to one or more digits or ‘-’, the standard output and standard error are redirected as described previously.
The redirection operator
[n]<&digit-
moves the file descriptor digit to file descriptor n, or the standard input (file descriptor 0) if n is not specified. digit is closed after being duplicated to n.
Similarly, the redirection operator
[n]>&digit-
moves the file descriptor digit to file descriptor n, or the standard output (file descriptor 1) if n is not specified.
The redirection operator
[n]<>word
causes the file whose name is the expansion of word to be opened for both reading and writing on file descriptor n, or on file descriptor 0 if n is not specified. If the file does not exist, it is created.
Next: Shell Scripts, Previous: Redirections, Up: Basic Shell Features [Contents][Index]
• Simple Command Expansion: | How Bash expands simple commands before executing them. | |
• Command Search and Execution: | How Bash finds commands and runs them. | |
• Command Execution Environment: | The environment in which Bash executes commands that are not shell builtins. | |
• Environment: | The environment given to a command. | |
• Exit Status: | The status returned by commands and how Bash interprets it. | |
• Signals: | What happens when Bash or a command it runs receives a signal. |
Next: Command Search and Execution, Up: Executing Commands [Contents][Index]
When a simple command is executed, the shell performs the following expansions, assignments, and redirections, from left to right.
If no command name results, the variable assignments affect the current shell environment. Otherwise, the variables are added to the environment of the executed command and do not affect the current shell environment. If any of the assignments attempts to assign a value to a readonly variable, an error occurs, and the command exits with a non-zero status.
If no command name results, redirections are performed, but do not affect the current shell environment. A redirection error causes the command to exit with a non-zero status.
If there is a command name left after expansion, execution proceeds as described below. Otherwise, the command exits. If one of the expansions contained a command substitution, the exit status of the command is the exit status of the last command substitution performed. If there were no command substitutions, the command exits with a status of zero.
Next: Command Execution Environment, Previous: Simple Command Expansion, Up: Executing Commands [Contents][Index]
After a command has been split into words, if it results in a simple command and an optional list of arguments, the following actions are taken.
$PATH
for a directory containing an executable file by that name. Bash uses a hash table to remember the full pathnames of executable files to avoid multiple PATH
searches (see the description of hash
in Bourne Shell Builtins). A full search of the directories in $PATH
is performed only if the command is not found in the hash table. If the search is unsuccessful, the shell searches for a defined shell function named command_not_found_handle
. If that function exists, it is invoked in a separate execution environment with the original command and the original command’s arguments as its arguments, and the function’s exit status becomes the exit status of that subshell. If that function is not defined, the shell prints an error message and returns an exit status of 127.Next: Environment, Previous: Command Search and Execution, Up: Executing Commands [Contents][Index]
The shell has an execution environment, which consists of the following:
exec
builtincd
, pushd
, or popd
, or inherited by the shell at invocationumask
or inherited from the shell’s parenttrap
set
or inherited from the shell’s parent in the environmentset
shopt
(see The Shopt Builtin)alias
(see Aliases)$$
, and the value of $PPID
When a simple command other than a builtin or shell function is to be executed, it is invoked in a separate execution environment that consists of the following. Unless otherwise noted, the values are inherited from the shell.
A command invoked in this separate environment cannot affect the shell’s execution environment.
Command substitution, commands grouped with parentheses, and asynchronous commands are invoked in a subshell environment that is a duplicate of the shell environment, except that traps caught by the shell are reset to the values that the shell inherited from its parent at invocation. Builtin commands that are invoked as part of a pipeline are also executed in a subshell environment. Changes made to the subshell environment cannot affect the shell’s execution environment.
Subshells spawned to execute command substitutions inherit the value of the-eoption from the parent shell. When not in POSIX mode, Bash clears the-eoption in such subshells.
If a command is followed by a ‘&’ and job control is not active, the default standard input for the command is the empty file/dev/null. Otherwise, the invoked command inherits the file descriptors of the calling shell as modified by redirections.
Next: Exit Status, Previous: Command Execution Environment, Up: Executing Commands [Contents][Index]
When a program is invoked it is given an array of strings called the environment. This is a list of name-value pairs, of the form name=value
.
Bash provides several ways to manipulate the environment. On invocation, the shell scans its own environment and creates a parameter for each name found, automatically marking it for export to child processes. Executed commands inherit the environment. The export
and ‘declare -x’ commands allow parameters and functions to be added to and deleted from the environment. If the value of a parameter in the environment is modified, the new value becomes part of the environment, replacing the old. The environment inherited by any executed command consists of the shell’s initial environment, whose values may be modified in the shell, less any pairs removed by the unset
and ‘export -n’ commands, plus any additions via the export
and ‘declare -x’ commands.
The environment for any simple command or function may be augmented temporarily by prefixing it with parameter assignments, as described in Shell Parameters. These assignment statements affect only the environment seen by that command.
If the-koption is set (see The Set Builtin), then all parameter assignments are placed in the environment for a command, not just those that precede the command name.
When Bash invokes an external command, the variable ‘$_’ is set to the full pathname of the command and passed to that command in its environment.
Next: Signals, Previous: Environment, Up: Executing Commands [Contents][Index]
The exit status of an executed command is the value returned by the waitpid system call or equivalent function. Exit statuses fall between 0 and 255, though, as explained below, the shell may use values above 125 specially. Exit statuses from shell builtins and compound commands are also limited to this range. Under certain circumstances, the shell will use special values to indicate specific failure modes.
For the shell’s purposes, a command which exits with a zero exit status has succeeded. A non-zero exit status indicates failure. This seemingly counter-intuitive scheme is used so there is one well-defined way to indicate success and a variety of ways to indicate various failure modes. When a command terminates on a fatal signal whose number is N, Bash uses the value 128+N as the exit status.
If a command is not found, the child process created to execute it returns a status of 127. If a command is found but is not executable, the return status is 126.
If a command fails because of an error during expansion or redirection, the exit status is greater than zero.
The exit status is used by the Bash conditional commands (see Conditional Constructs) and some of the list constructs (see Lists).
All of the Bash builtins return an exit status of zero if they succeed and a non-zero status on failure, so they may be used by the conditional and list constructs. All builtins return an exit status of 2 to indicate incorrect usage, generally invalid options or missing arguments.
Previous: Exit Status, Up: Executing Commands [Contents][Index]
When Bash is interactive, in the absence of any traps, it ignores SIGTERM
(so that ‘kill 0’ does not kill an interactive shell), and SIGINT
is caught and handled (so that the wait
builtin is interruptible). When Bash receives a SIGINT
, it breaks out of any executing loops. In all cases, Bash ignores SIGQUIT
. If job control is in effect (see Job Control), Bash ignores SIGTTIN
, SIGTTOU
, and SIGTSTP
.
Non-builtin commands started by Bash have signal handlers set to the values inherited by the shell from its parent. When job control is not in effect, asynchronous commands ignore SIGINT
and SIGQUIT
in addition to these inherited handlers. Commands run as a result of command substitution ignore the keyboard-generated job control signals SIGTTIN
, SIGTTOU
, and SIGTSTP
.
The shell exits by default upon receipt of a SIGHUP
. Before exiting, an interactive shell resends the SIGHUP
to all jobs, running or stopped. Stopped jobs are sent SIGCONT
to ensure that they receive the SIGHUP
. To prevent the shell from sending the SIGHUP
signal to a particular job, it should be removed from the jobs table with the disown
builtin (see Job Control Builtins) or marked to not receive SIGHUP
using disown -h
.
If the huponexit
shell option has been set with shopt
(see The Shopt Builtin), Bash sends a SIGHUP
to all jobs when an interactive login shell exits.
If Bash is waiting for a command to complete and receives a signal for which a trap has been set, the trap will not be executed until the command completes. When Bash is waiting for an asynchronous command via the wait
builtin, the reception of a signal for which a trap has been set will cause the wait
builtin to return immediately with an exit status greater than 128, immediately after which the trap is executed.
Previous: Executing Commands, Up: Basic Shell Features [Contents][Index]
A shell script is a text file containing shell commands. When such a file is used as the first non-option argument when invoking Bash, and neither the-cnor-soption is supplied (see Invoking Bash), Bash reads and executes commands from the file, then exits. This mode of operation creates a non-interactive shell. The shell first searches for the file in the current directory, and looks in the directories in $PATH
if not found there.
When Bash runs a shell script, it sets the special parameter 0
to the name of the file, rather than the name of the shell, and the positional parameters are set to the remaining arguments, if any are given. If no additional arguments are supplied, the positional parameters are unset.
A shell script may be made executable by using the chmod
command to turn on the execute bit. When Bash finds such a file while searching the $PATH
for a command, it spawns a subshell to execute it. In other words, executing
filename arguments
is equivalent to executing
bash filename arguments
if filename
is an executable shell script. This subshell reinitializes itself, so that the effect is as if a new shell had been invoked to interpret the script, with the exception that the locations of commands remembered by the parent (see the description of hash
in Bourne Shell Builtins) are retained by the child.
Most versions of Unix make this a part of the operating system’s command execution mechanism. If the first line of a script begins with the two characters ‘#!’, the remainder of the line specifies an interpreter for the program. Thus, you can specify Bash, awk
, Perl, or some other interpreter and write the rest of the script file in that language.
The arguments to the interpreter consist of a single optional argument following the interpreter name on the first line of the script file, followed by the name of the script file, followed by the rest of the arguments. Bash will perform this action on operating systems that do not handle it themselves. Note that some older versions of Unix limit the interpreter name and argument to a maximum of 32 characters.
Bash scripts often begin with #! /bin/bash
(assuming that Bash has been installed in/bin), since this ensures that Bash will be used to interpret the script, even if it is executed under another shell.
Next: Shell Variables, Previous: Basic Shell Features, Up: Top [Contents][Index]
• Bourne Shell Builtins: | Builtin commands inherited from the Bourne Shell. | |
• Bash Builtins: | Table of builtins specific to Bash. | |
• Modifying Shell Behavior: | Builtins to modify shell attributes and optional behavior. | |
• Special Builtins: | Builtin commands classified specially by POSIX. |
Builtin commands are contained within the shell itself. When the name of a builtin command is used as the first word of a simple command (see Simple Commands), the shell executes the command directly, without invoking another program. Builtin commands are necessary to implement functionality impossible or inconvenient to obtain with separate utilities.
This section briefly describes the builtins which Bash inherits from the Bourne Shell, as well as the builtin commands which are unique to or have been extended in Bash.
Several builtin commands are described in other chapters: builtin commands which provide the Bash interface to the job control facilities (see Job Control Builtins), the directory stack (see Directory Stack Builtins), the command history (see Bash History Builtins), and the programmable completion facilities (see Programmable Completion Builtins).
Many of the builtins have been extended by POSIX or Bash.
Unless otherwise noted, each builtin command documented as accepting options preceded by ‘-’ accepts ‘--’ to signify the end of the options. The :
, true
, false
, and test
/[
builtins do not accept options and do not treat ‘--’ specially. The exit
, logout
, return
, break
, continue
, let
, and shift
builtins accept and process arguments beginning with ‘-’ without requiring ‘--’. Other builtins that accept arguments but are not specified as accepting options interpret arguments beginning with ‘-’ as invalid options and require ‘--’ to prevent this interpretation.
Next: Bash Builtins, Up: Shell Builtin Commands [Contents][Index]
The following shell builtin commands are inherited from the Bourne Shell. These commands are implemented as specified by the POSIX standard.
: (a colon)
: [arguments]
Do nothing beyond expanding arguments and performing redirections. The return status is zero.
. (a period)
. filename [arguments]
Read and execute commands from the filename argument in the current shell context. If filename does not contain a slash, the PATH
variable is used to find filename. When Bash is not in POSIX mode, the current directory is searched if filename is not found in $PATH
. If any arguments are supplied, they become the positional parameters when filename is executed. Otherwise the positional parameters are unchanged. If the-Toption is enabled, source
inherits any trap on DEBUG
; if it is not, any DEBUG
trap string is saved and restored around the call to source
, and source
unsets the DEBUG
trap while it executes. If-Tis not set, and the sourced file changes the DEBUG
trap, the new value is retained when source
completes. The return status is the exit status of the last command executed, or zero if no commands are executed. If filename is not found, or cannot be read, the return status is non-zero. This builtin is equivalent to source
.
break
break [n]
Exit from a for
, while
, until
, or select
loop. If n is supplied, the nth enclosing loop is exited. n must be greater than or equal to 1. The return status is zero unless n is not greater than or equal to 1.
cd
cd [-L|[-P [-e]] [-@] [directory]
Change the current working directory to directory. If directory is not supplied, the value of the HOME
shell variable is used. Any additional arguments following directory are ignored. If the shell variable CDPATH
exists, it is used as a search path: each directory name in CDPATH
is searched for directory, with alternative directory names in CDPATH
separated by a colon (‘:’). If directory begins with a slash, CDPATH
is not used.
The-Poption means to not follow symbolic links: symbolic links are resolved while cd
is traversing directory and before processing an instance of ‘..’ in directory.
By default, or when the-Loption is supplied, symbolic links in directory are resolved after cd
processes an instance of ‘..’ in directory.
If ‘..’ appears in directory, it is processed by removing the immediately preceding pathname component, back to a slash or the beginning of directory.
If the-eoption is supplied with-Pand the current working directory cannot be successfully determined after a successful directory change, cd
will return an unsuccessful status.
On systems that support it, the-@option presents the extended attributes associated with a file as a directory.
If directory is ‘-’, it is converted to $OLDPWD
before the directory change is attempted.
If a non-empty directory name from CDPATH
is used, or if ‘-’ is the first argument, and the directory change is successful, the absolute pathname of the new working directory is written to the standard output.
The return status is zero if the directory is successfully changed, non-zero otherwise.
continue
continue [n]
Resume the next iteration of an enclosing for
, while
, until
, or select
loop. If n is supplied, the execution of the nth enclosing loop is resumed. n must be greater than or equal to 1. The return status is zero unless n is not greater than or equal to 1.
eval
eval [arguments]
The arguments are concatenated together into a single command, which is then read and executed, and its exit status returned as the exit status of eval
. If there are no arguments or only empty arguments, the return status is zero.
exec
exec [-cl] [-a name] [command [arguments]]
If command is supplied, it replaces the shell without creating a new process. If the-loption is supplied, the shell places a dash at the beginning of the zeroth argument passed to command. This is what the login
program does. The-coption causes command to be executed with an empty environment. If-ais supplied, the shell passes name as the zeroth argument to command. If command cannot be executed for some reason, a non-interactive shell exits, unless the execfail
shell option is enabled. In that case, it returns failure. An interactive shell returns failure if the file cannot be executed. A subshell exits unconditionally if exec
fails. If no command is specified, redirections may be used to affect the current shell environment. If there are no redirection errors, the return status is zero; otherwise the return status is non-zero.
exit
exit [n]
Exit the shell, returning a status of n to the shell’s parent. If n is omitted, the exit status is that of the last command executed. Any trap on EXIT
is executed before the shell terminates.
export
export [-fn] [-p] [name[=value]]
Mark each name to be passed to child processes in the environment. If the-foption is supplied, the names refer to shell functions; otherwise the names refer to shell variables. The-noption means to no longer mark each name for export. If no names are supplied, or if the-poption is given, a list of names of all exported variables is displayed. The-poption displays output in a form that may be reused as input. If a variable name is followed by =value, the value of the variable is set to value.
The return status is zero unless an invalid option is supplied, one of the names is not a valid shell variable name, or-fis supplied with a name that is not a shell function.
getopts
getopts optstring name [args]
getopts
is used by shell scripts to parse positional parameters. optstring contains the option characters to be recognized; if a character is followed by a colon, the option is expected to have an argument, which should be separated from it by whitespace. The colon (‘:’) and question mark (‘?’) may not be used as option characters. Each time it is invoked, getopts
places the next option in the shell variable name, initializing name if it does not exist, and the index of the next argument to be processed into the variable OPTIND
. OPTIND
is initialized to 1 each time the shell or a shell script is invoked. When an option requires an argument, getopts
places that argument into the variable OPTARG
. The shell does not reset OPTIND
automatically; it must be manually reset between multiple calls to getopts
within the same shell invocation if a new set of parameters is to be used.
When the end of options is encountered, getopts
exits with a return value greater than zero. OPTIND
is set to the index of the first non-option argument, and name is set to ‘?’.
getopts
normally parses the positional parameters, but if more arguments are given in args, getopts
parses those instead.
getopts
can report errors in two ways. If the first character of optstring is a colon, silent error reporting is used. In normal operation, diagnostic messages are printed when invalid options or missing option arguments are encountered. If the variable OPTERR
is set to 0, no error messages will be displayed, even if the first character of optstring
is not a colon.
If an invalid option is seen, getopts
places ‘?’ into name and, if not silent, prints an error message and unsets OPTARG
. If getopts
is silent, the option character found is placed in OPTARG
and no diagnostic message is printed.
If a required argument is not found, and getopts
is not silent, a question mark (‘?’) is placed in name, OPTARG
is unset, and a diagnostic message is printed. If getopts
is silent, then a colon (‘:’) is placed in name and OPTARG
is set to the option character found.
hash
hash [-r] [-p filename] [-dt] [name]
Each time hash
is invoked, it remembers the full pathnames of the commands specified as name arguments, so they need not be searched for on subsequent invocations. The commands are found by searching through the directories listed in $PATH
. Any previously-remembered pathname is discarded. The-poption inhibits the path search, and filename is used as the location of name. The-roption causes the shell to forget all remembered locations. The-doption causes the shell to forget the remembered location of each name. If the-toption is supplied, the full pathname to which each name corresponds is printed. If multiple name arguments are supplied with-t, the name is printed before the hashed full pathname. The-loption causes output to be displayed in a format that may be reused as input. If no arguments are given, or if only-lis supplied, information about remembered commands is printed. The return status is zero unless a name is not found or an invalid option is supplied.
pwd
pwd [-LP]
Print the absolute pathname of the current working directory. If the-Poption is supplied, the pathname printed will not contain symbolic links. If the-Loption is supplied, the pathname printed may contain symbolic links. The return status is zero unless an error is encountered while determining the name of the current directory or an invalid option is supplied.
readonly
readonly [-aAf] [-p] [name[=value]] …
Mark each name as readonly. The values of these names may not be changed by subsequent assignment. If the-foption is supplied, each name refers to a shell function. The-aoption means each name refers to an indexed array variable; the-Aoption means each name refers to an associative array variable. If both options are supplied,-Atakes precedence. If no name arguments are given, or if the-poption is supplied, a list of all readonly names is printed. The other options may be used to restrict the output to a subset of the set of readonly names. The-poption causes output to be displayed in a format that may be reused as input. If a variable name is followed by =value, the value of the variable is set to value. The return status is zero unless an invalid option is supplied, one of the name arguments is not a valid shell variable or function name, or the-foption is supplied with a name that is not a shell function.
return
return [n]
Cause a shell function to stop executing and return the value n to its caller. If n is not supplied, the return value is the exit status of the last command executed in the function. If return
is executed by a trap handler, the last command used to determine the status is the last command executed before the trap handler. If return
is executed during a DEBUG
trap, the last command used to determine the status is the last command executed by the trap handler before return
was invoked. return
may also be used to terminate execution of a script being executed with the .
(source
) builtin, returning either n or the exit status of the last command executed within the script as the exit status of the script. If n is supplied, the return value is its least significant 8 bits. Any command associated with the RETURN
trap is executed before execution resumes after the function or script. The return status is non-zero if return
is supplied a non-numeric argument or is used outside a function and not during the execution of a script by .
or source
.
shift
shift [n]
Shift the positional parameters to the left by n. The positional parameters from n+1 … $#
are renamed to $1
… $#
-n. Parameters represented by the numbers $#
to $#
-n+1 are unset. n must be a non-negative number less than or equal to $#
. If n is zero or greater than $#
, the positional parameters are not changed. If n is not supplied, it is assumed to be 1. The return status is zero unless n is greater than $#
or less than zero, non-zero otherwise.
test
[
test expr
Evaluate a conditional expression expr and return a status of 0 (true) or 1 (false). Each operator and operand must be a separate argument. Expressions are composed of the primaries described below in Bash Conditional Expressions. test
does not accept any options, nor does it accept and ignore an argument of--as signifying the end of options.
When the [
form is used, the last argument to the command must be a ]
.
Expressions may be combined using the following operators, listed in decreasing order of precedence. The evaluation depends on the number of arguments; see below. Operator precedence is used when there are five or more arguments.
! expr
True if expr is false.
( expr )
Returns the value of expr. This may be used to override the normal precedence of operators.
expr1 -a expr2
True if both expr1 and expr2 are true.
expr1 -o expr2
True if either expr1 or expr2 is true.
The test
and [
builtins evaluate conditional expressions using a set of rules based on the number of arguments.
The expression is false.
The expression is true if, and only if, the argument is not null.
If the first argument is ‘!’, the expression is true if and only if the second argument is null. If the first argument is one of the unary conditional operators (see Bash Conditional Expressions), the expression is true if the unary test is true. If the first argument is not a valid unary operator, the expression is false.
The following conditions are applied in the order listed.
If the first argument is ‘!’, the result is the negation of the three-argument expression composed of the remaining arguments. Otherwise, the expression is parsed and evaluated according to precedence using the rules listed above.
The expression is parsed and evaluated according to precedence using the rules listed above.
When used with test
or ‘[’, the ‘<’ and ‘>’ operators sort lexicographically using ASCII ordering.
times
times
Print out the user and system times used by the shell and its children. The return status is zero.
trap
trap [-lp] [arg] [sigspec …]
The commands in arg are to be read and executed when the shell receives signal sigspec. If arg is absent (and there is a single sigspec) or equal to ‘-’, each specified signal’s disposition is reset to the value it had when the shell was started. If arg is the null string, then the signal specified by each sigspec is ignored by the shell and commands it invokes. If arg is not present and-phas been supplied, the shell displays the trap commands associated with each sigspec. If no arguments are supplied, or only-pis given, trap
prints the list of commands associated with each signal number in a form that may be reused as shell input. The-loption causes the shell to print a list of signal names and their corresponding numbers. Each sigspec is either a signal name or a signal number. Signal names are case insensitive and the SIG
prefix is optional.
If a sigspec is 0
or EXIT
, arg is executed when the shell exits. If a sigspec is DEBUG
, the command arg is executed before every simple command, for
command, case
command, select
command, every arithmetic for
command, and before the first command executes in a shell function. Refer to the description of the extdebug
option to the shopt
builtin (see The Shopt Builtin) for details of its effect on the DEBUG
trap. If a sigspec is RETURN
, the command arg is executed each time a shell function or a script executed with the .
or source
builtins finishes executing.
If a sigspec is ERR
, the command arg is executed whenever a pipeline (which may consist of a single simple command), a list, or a compound command returns a non-zero exit status, subject to the following conditions. The ERR
trap is not executed if the failed command is part of the command list immediately following an until
or while
keyword, part of the test following the if
or elif
reserved words, part of a command executed in a &&
or ||
list except the command following the final &&
or ||
, any command in a pipeline but the last, or if the command’s return status is being inverted using !
. These are the same conditions obeyed by the errexit
(-e) option.
Signals ignored upon entry to the shell cannot be trapped or reset. Trapped signals that are not being ignored are reset to their original values in a subshell or subshell environment when one is created.
The return status is zero unless a sigspec does not specify a valid signal.
umask
umask [-p] [-S] [mode]
Set the shell process’s file creation mask to mode. If mode begins with a digit, it is interpreted as an octal number; if not, it is interpreted as a symbolic mode mask similar to that accepted by the chmod
command. If mode is omitted, the current value of the mask is printed. If the-Soption is supplied without a mode argument, the mask is printed in a symbolic format. If the-poption is supplied, and mode is omitted, the output is in a form that may be reused as input. The return status is zero if the mode is successfully changed or if no mode argument is supplied, and non-zero otherwise.
Note that when the mode is interpreted as an octal number, each number of the umask is subtracted from 7
. Thus, a umask of 022
results in permissions of 755
.
unset
unset [-fnv] [name]
Remove each variable or function name. If the-voption is given, each name refers to a shell variable and that variable is removed. If the-foption is given, the names refer to shell functions, and the function definition is removed. If the-noption is supplied, and name is a variable with the nameref attribute, name will be unset rather than the variable it references.-nhas no effect if the-foption is supplied. If no options are supplied, each name refers to a variable; if there is no variable by that name, any function with that name is unset. Readonly variables and functions may not be unset. The return status is zero unless a name is readonly.
Next: Modifying Shell Behavior, Previous: Bourne Shell Builtins, Up: Shell Builtin Commands [Contents][Index]
This section describes builtin commands which are unique to or have been extended in Bash. Some of these commands are specified in the POSIX standard.
alias
alias [-p] [name[=value] …]
Without arguments or with the-poption, alias
prints the list of aliases on the standard output in a form that allows them to be reused as input. If arguments are supplied, an alias is defined for each name whose value is given. If no value is given, the name and value of the alias is printed. Aliases are described in Aliases.
bind
bind [-m keymap] [-lpsvPSVX] bind [-m keymap] [-q function] [-u function] [-r keyseq] bind [-m keymap] -f filename bind [-m keymap] -x keyseq:shell-command bind [-m keymap] keyseq:function-name bind [-m keymap] keyseq:readline-command
Display current Readline (see Command Line Editing) key and function bindings, bind a key sequence to a Readline function or macro, or set a Readline variable. Each non-option argument is a command as it would appear in a Readline initialization file (see Readline Init File), but each binding or command must be passed as a separate argument; e.g., ‘"\C-x\C-r":re-read-init-file’.
Options, if supplied, have the following meanings:
-m keymap
Use keymap as the keymap to be affected by the subsequent bindings. Acceptable keymap names are emacs
, emacs-standard
, emacs-meta
, emacs-ctlx
, vi
, vi-move
, vi-command
, and vi-insert
. vi
is equivalent to vi-command
(vi-move
is also a synonym); emacs
is equivalent to emacs-standard
.
-l
List the names of all Readline functions.
-p
Display Readline function names and bindings in such a way that they can be used as input or in a Readline initialization file.
-P
List current Readline function names and bindings.
-v
Display Readline variable names and values in such a way that they can be used as input or in a Readline initialization file.
-V
List current Readline variable names and values.
-s
Display Readline key sequences bound to macros and the strings they output in such a way that they can be used as input or in a Readline initialization file.
-S
Display Readline key sequences bound to macros and the strings they output.
-f filename
Read key bindings from filename.
-q function
Query about which keys invoke the named function.
-u function
Unbind all keys bound to the named function.
-r keyseq
Remove any current binding for keyseq.
-x keyseq:shell-command
Cause shell-command to be executed whenever keyseq is entered. When shell-command is executed, the shell sets the READLINE_LINE
variable to the contents of the Readline line buffer and the READLINE_POINT
variable to the current location of the insertion point. If the executed command changes the value of READLINE_LINE
or READLINE_POINT
, those new values will be reflected in the editing state.
-X
List all key sequences bound to shell commands and the associated commands in a format that can be reused as input.
The return status is zero unless an invalid option is supplied or an error occurs.
builtin
builtin [shell-builtin [args]]
Run a shell builtin, passing it args, and return its exit status. This is useful when defining a shell function with the same name as a shell builtin, retaining the functionality of the builtin within the function. The return status is non-zero if shell-builtin is not a shell builtin command.
caller
caller [expr]
Returns the context of any active subroutine call (a shell function or a script executed with the .
or source
builtins).
Without expr, caller
displays the line number and source filename of the current subroutine call. If a non-negative integer is supplied as expr, caller
displays the line number, subroutine name, and source file corresponding to that position in the current execution call stack. This extra information may be used, for example, to print a stack trace. The current frame is frame 0.
The return value is 0 unless the shell is not executing a subroutine call or expr does not correspond to a valid position in the call stack.
command
command [-pVv] command [arguments …]
Runs command with arguments ignoring any shell function named command. Only shell builtin commands or commands found by searching the PATH
are executed. If there is a shell function named ls
, running ‘command ls’ within the function will execute the external command ls
instead of calling the function recursively. The-poption means to use a default value for PATH
that is guaranteed to find all of the standard utilities. The return status in this case is 127 if command cannot be found or an error occurred, and the exit status of command otherwise.
If either the-Vor-voption is supplied, a description of command is printed. The-voption causes a single word indicating the command or file name used to invoke command to be displayed; the-Voption produces a more verbose description. In this case, the return status is zero if command is found, and non-zero if not.
declare
declare [-aAfFgilnrtux] [-p] [name[=value] …]
Declare variables and give them attributes. If no names are given, then display the values of variables instead.
The-poption will display the attributes and values of each name. When-pis used with name arguments, additional options, other than-fand-F, are ignored.
When-pis supplied without name arguments, declare
will display the attributes and values of all variables having the attributes specified by the additional options. If no other options are supplied with-p, declare
will display the attributes and values of all shell variables. The-foption will restrict the display to shell functions.
The-Foption inhibits the display of function definitions; only the function name and attributes are printed. If the extdebug
shell option is enabled using shopt
(see The Shopt Builtin), the source file name and line number where each name is defined are displayed as well.-Fimplies-f.
The-goption forces variables to be created or modified at the global scope, even when declare
is executed in a shell function. It is ignored in all other cases.
The following options can be used to restrict output to variables with the specified attributes or to give variables attributes:
-a
Each name is an indexed array variable (see Arrays).
-A
Each name is an associative array variable (see Arrays).
-f
Use function names only.
-i
The variable is to be treated as an integer; arithmetic evaluation (see Shell Arithmetic) is performed when the variable is assigned a value.
-l
When the variable is assigned a value, all upper-case characters are converted to lower-case. The upper-case attribute is disabled.
-n
Give each name the nameref attribute, making it a name reference to another variable. That other variable is defined by the value of name. All references, assignments, and attribute modifications to name, except for those using or changing the-nattribute itself, are performed on the variable referenced by name’s value. The nameref attribute cannot be applied to array variables.
-r
Make names readonly. These names cannot then be assigned values by subsequent assignment statements or unset.
-t
Give each name the trace
attribute. Traced functions inherit the DEBUG
and RETURN
traps from the calling shell. The trace attribute has no special meaning for variables.
-u
When the variable is assigned a value, all lower-case characters are converted to upper-case. The lower-case attribute is disabled.
-x
Mark each name for export to subsequent commands via the environment.
Using ‘+’ instead of ‘-’ turns off the attribute instead, with the exceptions that ‘+a’ and ‘+A’ may not be used to destroy array variables and ‘+r’ will not remove the readonly attribute. When used in a function, declare
makes each name local, as with the local
command, unless the-goption is used. If a variable name is followed by =value, the value of the variable is set to value.
When using-aor-Aand the compound assignment syntax to create array variables, additional attributes do not take effect until subsequent assignments.
The return status is zero unless an invalid option is encountered, an attempt is made to define a function using ‘-f foo=bar’, an attempt is made to assign a value to a readonly variable, an attempt is made to assign a value to an array variable without using the compound assignment syntax (see Arrays), one of the names is not a valid shell variable name, an attempt is made to turn off readonly status for a readonly variable, an attempt is made to turn off array status for an array variable, or an attempt is made to display a non-existent function with-f.
echo
echo [-neE] [arg …]
Output the args, separated by spaces, terminated with a newline. The return status is 0 unless a write error occurs. If-nis specified, the trailing newline is suppressed. If the-eoption is given, interpretation of the following backslash-escaped characters is enabled. The-Eoption disables the interpretation of these escape characters, even on systems where they are interpreted by default. The xpg_echo
shell option may be used to dynamically determine whether or not echo
expands these escape characters by default. echo
does not interpret--to mean the end of options.
echo
interprets the following escape sequences:
\a
alert (bell)
\b
backspace
\c
suppress further output
\e
\E
escape
\f
form feed
\n
new line
\r
carriage return
\t
horizontal tab
\v
vertical tab
\\
backslash
\0nnn
the eight-bit character whose value is the octal value nnn (zero to three octal digits)
\xHH
the eight-bit character whose value is the hexadecimal value HH (one or two hex digits)
\uHHHH
the Unicode (ISO/IEC 10646) character whose value is the hexadecimal value HHHH (one to four hex digits)
\UHHHHHHHH
the Unicode (ISO/IEC 10646) character whose value is the hexadecimal value HHHHHHHH (one to eight hex digits)
enable
enable [-a] [-dnps] [-f filename] [name …]
Enable and disable builtin shell commands. Disabling a builtin allows a disk command which has the same name as a shell builtin to be executed without specifying a full pathname, even though the shell normally searches for builtins before disk commands. If-nis used, the names become disabled. Otherwise names are enabled. For example, to use the test
binary found via $PATH
instead of the shell builtin version, type ‘enable -n test’.
If the-poption is supplied, or no name arguments appear, a list of shell builtins is printed. With no other arguments, the list consists of all enabled shell builtins. The-aoption means to list each builtin with an indication of whether or not it is enabled.
The-foption means to load the new builtin command name from shared object filename, on systems that support dynamic loading. The-doption will delete a builtin loaded with-f.
If there are no options, a list of the shell builtins is displayed. The-soption restricts enable
to the POSIX special builtins. If-sis used with-f, the new builtin becomes a special builtin (see Special Builtins).
The return status is zero unless a name is not a shell builtin or there is an error loading a new builtin from a shared object.
help
help [-dms] [pattern]
Display helpful information about builtin commands. If pattern is specified, help
gives detailed help on all commands matching pattern, otherwise a list of the builtins is printed.
Options, if supplied, have the following meanings:
-d
Display a short description of each pattern
-m
Display the description of each pattern in a manpage-like format
-s
Display only a short usage synopsis for each pattern
The return status is zero unless no command matches pattern.
let
let expression [expression …]
The let
builtin allows arithmetic to be performed on shell variables. Each expression is evaluated according to the rules given below in Shell Arithmetic. If the last expression evaluates to 0, let
returns 1; otherwise 0 is returned.
local
local [option] name[=value] …
For each argument, a local variable named name is created, and assigned value. The option can be any of the options accepted by declare
. local
can only be used within a function; it makes the variable name have a visible scope restricted to that function and its children. If name is ‘-’, the set of shell options is made local to the function in which local
is invoked: shell options changed using the set
builtin inside the function are restored to their original values when the function returns. The return status is zero unless local
is used outside a function, an invalid name is supplied, or name is a readonly variable.
logout
logout [n]
Exit a login shell, returning a status of n to the shell’s parent.
mapfile
mapfile [-d delim] [-n count] [-O origin] [-s count] [-t] [-u fd] [-C callback] [-c quantum] [array]
Read lines from the standard input into the indexed array variable array, or from file descriptor fd if the-uoption is supplied. The variable MAPFILE
is the default array. Options, if supplied, have the following meanings:
-d
The first character of delim is used to terminate each input line, rather than newline. If delim is the empty string, mapfile
will terminate a line when it reads a NUL character.
-n
Copy at most count lines. If count is 0, all lines are copied.
-O
Begin assigning to array at index origin. The default index is 0.
-s
Discard the first count lines read.
-t
Remove a trailing delim (default newline) from each line read.
-u
Read lines from file descriptor fd instead of the standard input.
-C
Evaluate callback each time quantum lines are read. The-coption specifies quantum.
-c
Specify the number of lines read between each call to callback.
If-Cis specified without-c, the default quantum is 5000. When callback is evaluated, it is supplied the index of the next array element to be assigned and the line to be assigned to that element as additional arguments. callback is evaluated after the line is read but before the array element is assigned.
If not supplied with an explicit origin, mapfile
will clear array before assigning to it.
mapfile
returns successfully unless an invalid option or option argument is supplied, array is invalid or unassignable, or array is not an indexed array.
printf
printf [-v var] format [arguments]
Write the formatted arguments to the standard output under the control of the format. The-voption causes the output to be assigned to the variable var rather than being printed to the standard output.
The format is a character string which contains three types of objects: plain characters, which are simply copied to standard output, character escape sequences, which are converted and copied to the standard output, and format specifications, each of which causes printing of the next successive argument. In addition to the standard printf(1)
formats, printf
interprets the following extensions:
%b
Causes printf
to expand backslash escape sequences in the corresponding argument in the same way as echo -e
(see Bash Builtins).
%q
Causes printf
to output the corresponding argument in a format that can be reused as shell input.
%(datefmt)T
Causes printf
to output the date-time string resulting from using datefmt as a format string for strftime
(3). The corresponding argument is an integer representing the number of seconds since the epoch. Two special argument values may be used: -1 represents the current time, and -2 represents the time the shell was invoked. If no argument is specified, conversion behaves as if -1 had been given. This is an exception to the usual printf
behavior.
Arguments to non-string format specifiers are treated as C language constants, except that a leading plus or minus sign is allowed, and if the leading character is a single or double quote, the value is the ASCII value of the following character.
The format is reused as necessary to consume all of the arguments. If the format requires more arguments than are supplied, the extra format specifications behave as if a zero value or null string, as appropriate, had been supplied. The return value is zero on success, non-zero on failure.
read
read [-ers] [-a aname] [-d delim] [-i text] [-n nchars] [-N nchars] [-p prompt] [-t timeout] [-u fd] [name …]
One line is read from the standard input, or from the file descriptor fd supplied as an argument to the-uoption, split into words as described above in Word Splitting, and the first word is assigned to the first name, the second word to the second name, and so on. If there are more words than names, the remaining words and their intervening delimiters are assigned to the last name. If there are fewer words read from the input stream than names, the remaining names are assigned empty values. The characters in the value of the IFS
variable are used to split the line into words using the same rules the shell uses for expansion (described above in Word Splitting). The backslash character ‘\’ may be used to remove any special meaning for the next character read and for line continuation. If no names are supplied, the line read is assigned to the variable REPLY
. The exit status is zero, unless end-of-file is encountered, read
times out (in which case the status is greater than 128), a variable assignment error (such as assigning to a readonly variable) occurs, or an invalid file descriptor is supplied as the argument to-u.
Options, if supplied, have the following meanings:
-a aname
The words are assigned to sequential indices of the array variable aname, starting at 0. All elements are removed from aname before the assignment. Other name arguments are ignored.
-d delim
The first character of delim is used to terminate the input line, rather than newline. If delim is the empty string, read
will terminate a line when it reads a NUL character.
-e
Readline (see Command Line Editing) is used to obtain the line. Readline uses the current (or default, if line editing was not previously active) editing settings, but uses Readline’s default filename completion.
-i text
If Readline is being used to read the line, text is placed into the editing buffer before editing begins.
-n nchars
read
returns after reading nchars characters rather than waiting for a complete line of input, but honors a delimiter if fewer than nchars characters are read before the delimiter.
-N nchars
read
returns after reading exactly nchars characters rather than waiting for a complete line of input, unless EOF is encountered or read
times out. Delimiter characters encountered in the input are not treated specially and do not cause read
to return until nchars characters are read. The result is not split on the characters in IFS
; the intent is that the variable is assigned exactly the characters read (with the exception of backslash; see the-roption below).
-p prompt
Display prompt, without a trailing newline, before attempting to read any input. The prompt is displayed only if input is coming from a terminal.
-r
If this option is given, backslash does not act as an escape character. The backslash is considered to be part of the line. In particular, a backslash-newline pair may not then be used as a line continuation.
-s
Silent mode. If input is coming from a terminal, characters are not echoed.
-t timeout
Cause read
to time out and return failure if a complete line of input (or a specified number of characters) is not read within timeout seconds. timeout may be a decimal number with a fractional portion following the decimal point. This option is only effective if read
is reading input from a terminal, pipe, or other special file; it has no effect when reading from regular files. If read
times out, read
saves any partial input read into the specified variable name. If timeout is 0, read
returns immediately, without trying to read and data. The exit status is 0 if input is available on the specified file descriptor, non-zero otherwise. The exit status is greater than 128 if the timeout is exceeded.
-u fd
Read input from file descriptor fd.
readarray
readarray [-d delim] [-n count] [-O origin] [-s count] [-t] [-u fd] [-C callback] [-c quantum] [array]
Read lines from the standard input into the indexed array variable array, or from file descriptor fd if the-uoption is supplied.
A synonym for mapfile
.
source
source filename
A synonym for .
(see Bourne Shell Builtins).
type
type [-afptP] [name …]
For each name, indicate how it would be interpreted if used as a command name.
If the-toption is used, type
prints a single word which is one of ‘alias’, ‘function’, ‘builtin’, ‘file’ or ‘keyword’, if name is an alias, shell function, shell builtin, disk file, or shell reserved word, respectively. If the name is not found, then nothing is printed, and type
returns a failure status.
If the-poption is used, type
either returns the name of the disk file that would be executed, or nothing if-twould not return ‘file’.
The-Poption forces a path search for each name, even if-twould not return ‘file’.
If a command is hashed,-pand-Pprint the hashed value, which is not necessarily the file that appears first in $PATH
.
If the-aoption is used, type
returns all of the places that contain an executable named file. This includes aliases and functions, if and only if the-poption is not also used.
If the-foption is used, type
does not attempt to find shell functions, as with the command
builtin.
The return status is zero if all of the names are found, non-zero if any are not found.
typeset
typeset [-afFgrxilnrtux] [-p] [name[=value] …]
The typeset
command is supplied for compatibility with the Korn shell. It is a synonym for the declare
builtin command.
ulimit
ulimit [-HSabcdefiklmnpqrstuvxPT] [limit]
ulimit
provides control over the resources available to processes started by the shell, on systems that allow such control. If an option is given, it is interpreted as follows:
-S
Change and report the soft limit associated with a resource.
-H
Change and report the hard limit associated with a resource.
-a
All current limits are reported.
-b
The maximum socket buffer size.
-c
The maximum size of core files created.
-d
The maximum size of a process’s data segment.
-e
The maximum scheduling priority ("nice").
-f
The maximum size of files written by the shell and its children.
-i
The maximum number of pending signals.
-k
The maximum number of kqueues that may be allocated.
-l
The maximum size that may be locked into memory.
-m
The maximum resident set size (many systems do not honor this limit).
-n
The maximum number of open file descriptors (most systems do not allow this value to be set).
-p
The pipe buffer size.
-q
The maximum number of bytes in POSIX message queues.
-r
The maximum real-time scheduling priority.
-s
The maximum stack size.
-t
The maximum amount of cpu time in seconds.
-u
The maximum number of processes available to a single user.
-v
The maximum amount of virtual memory available to the shell, and, on some systems, to its children.
-x
The maximum number of file locks.
-P
The maximum number of pseudoterminals.
-T
The maximum number of threads.
If limit is given, and the-aoption is not used, limit is the new value of the specified resource. The special limit values hard
, soft
, and unlimited
stand for the current hard limit, the current soft limit, and no limit, respectively. A hard limit cannot be increased by a non-root user once it is set; a soft limit may be increased up to the value of the hard limit. Otherwise, the current value of the soft limit for the specified resource is printed, unless the-Hoption is supplied. When setting new limits, if neither-Hnor-Sis supplied, both the hard and soft limits are set. If no option is given, then-fis assumed. Values are in 1024-byte increments, except for-t, which is in seconds;-p, which is in units of 512-byte blocks;-P,-T,-b,-k,-nand-u, which are unscaled values; and, when in POSIX Mode (see Bash POSIX Mode),-cand-f, which are in 512-byte increments.
The return status is zero unless an invalid option or argument is supplied, or an error occurs while setting a new limit.
unalias
unalias [-a] [name … ]
Remove each name from the list of aliases. If-ais supplied, all aliases are removed. Aliases are described in Aliases.
Next: Special Builtins, Previous: Bash Builtins, Up: Shell Builtin Commands [Contents][Index]
• The Set Builtin: | Change the values of shell attributes and positional parameters. | |
• The Shopt Builtin: | Modify shell optional behavior. |
Next: The Shopt Builtin, Up: Modifying Shell Behavior [Contents][Index]
This builtin is so complicated that it deserves its own section. set
allows you to change the values of shell options and set the positional parameters, or to display the names and values of shell variables.
set
set [--abefhkmnptuvxBCEHPT] [-o option-name] [argument …] set [+abefhkmnptuvxBCEHPT] [+o option-name] [argument …]
If no options or arguments are supplied, set
displays the names and values of all shell variables and functions, sorted according to the current locale, in a format that may be reused as input for setting or resetting the currently-set variables. Read-only variables cannot be reset. In POSIX mode, only shell variables are listed.
When options are supplied, they set or unset shell attributes. Options, if specified, have the following meanings:
-a
Each variable or function that is created or modified is given the export attribute and marked for export to the environment of subsequent commands.
-b
Cause the status of terminated background jobs to be reported immediately, rather than before printing the next primary prompt.
-e
Exit immediately if a pipeline (see Pipelines), which may consist of a single simple command (see Simple Commands), a list (see Lists), or a compound command (see Compound Commands) returns a non-zero status. The shell does not exit if the command that fails is part of the command list immediately following a while
or until
keyword, part of the test in an if
statement, part of any command executed in a &&
or ||
list except the command following the final &&
or ||
, any command in a pipeline but the last, or if the command’s return status is being inverted with !
. If a compound command other than a subshell returns a non-zero status because a command failed while-ewas being ignored, the shell does not exit. A trap on ERR
, if set, is executed before the shell exits.
This option applies to the shell environment and each subshell environment separately (see Command Execution Environment), and may cause subshells to exit before executing all the commands in the subshell.
If a compound command or shell function executes in a context where-eis being ignored, none of the commands executed within the compound command or function body will be affected by the-esetting, even if-eis set and a command returns a failure status. If a compound command or shell function sets-ewhile executing in a context where-eis ignored, that setting will not have any effect until the compound command or the command containing the function call completes.
-f
Disable filename expansion (globbing).
-h
Locate and remember (hash) commands as they are looked up for execution. This option is enabled by default.
-k
All arguments in the form of assignment statements are placed in the environment for a command, not just those that precede the command name.
-m
Job control is enabled (see Job Control). All processes run in a separate process group. When a background job completes, the shell prints a line containing its exit status.
-n
Read commands but do not execute them. This may be used to check a script for syntax errors. This option is ignored by interactive shells.
-o option-name
Set the option corresponding to option-name:
allexport
Same as -a
.
braceexpand
Same as -B
.
emacs
Use an emacs
-style line editing interface (see Command Line Editing). This also affects the editing interface used for read -e
.
errexit
Same as -e
.
errtrace
Same as -E
.
functrace
Same as -T
.
hashall
Same as -h
.
histexpand
Same as -H
.
history
Enable command history, as described in Bash History Facilities. This option is on by default in interactive shells.
ignoreeof
An interactive shell will not exit upon reading EOF.
keyword
Same as -k
.
monitor
Same as -m
.
noclobber
Same as -C
.
noexec
Same as -n
.
noglob
Same as -f
.
nolog
Currently ignored.
notify
Same as -b
.
nounset
Same as -u
.
onecmd
Same as -t
.
physical
Same as -P
.
pipefail
If set, the return value of a pipeline is the value of the last (rightmost) command to exit with a non-zero status, or zero if all commands in the pipeline exit successfully. This option is disabled by default.
posix
Change the behavior of Bash where the default operation differs from the POSIX standard to match the standard (see Bash POSIX Mode). This is intended to make Bash behave as a strict superset of that standard.
privileged
Same as -p
.
verbose
Same as -v
.
vi
Use a vi
-style line editing interface. This also affects the editing interface used for read -e
.
xtrace
Same as -x
.
-p
Turn on privileged mode. In this mode, the $BASH_ENV
and $ENV
files are not processed, shell functions are not inherited from the environment, and the SHELLOPTS
, BASHOPTS
, CDPATH
and GLOBIGNORE
variables, if they appear in the environment, are ignored. If the shell is started with the effective user (group) id not equal to the real user (group) id, and the-poption is not supplied, these actions are taken and the effective user id is set to the real user id. If the-poption is supplied at startup, the effective user id is not reset. Turning this option off causes the effective user and group ids to be set to the real user and group ids.
-t
Exit after reading and executing one command.
-u
Treat unset variables and parameters other than the special parameters ‘@’ or ‘*’ as an error when performing parameter expansion. An error message will be written to the standard error, and a non-interactive shell will exit.
-v
Print shell input lines as they are read.
-x
Print a trace of simple commands, for
commands, case
commands, select
commands, and arithmetic for
commands and their arguments or associated word lists after they are expanded and before they are executed. The value of the PS4
variable is expanded and the resultant value is printed before the command and its expanded arguments.
-B
The shell will perform brace expansion (see Brace Expansion). This option is on by default.
-C
Prevent output redirection using ‘>’, ‘>&’, and ‘<>’ from overwriting existing files.
-E
If set, any trap on ERR
is inherited by shell functions, command substitutions, and commands executed in a subshell environment. The ERR
trap is normally not inherited in such cases.
-H
Enable ‘!’ style history substitution (see History Interaction). This option is on by default for interactive shells.
-P
If set, do not resolve symbolic links when performing commands such as cd
which change the current directory. The physical directory is used instead. By default, Bash follows the logical chain of directories when performing commands which change the current directory.
For example, if/usr/sysis a symbolic link to/usr/local/systhen:
$ cd /usr/sys; echo $PWD /usr/sys $ cd ..; pwd /usr
If set -P
is on, then:
$ cd /usr/sys; echo $PWD /usr/local/sys $ cd ..; pwd /usr/local
-T
If set, any trap on DEBUG
and RETURN
are inherited by shell functions, command substitutions, and commands executed in a subshell environment. The DEBUG
and RETURN
traps are normally not inherited in such cases.
--
If no arguments follow this option, then the positional parameters are unset. Otherwise, the positional parameters are set to the arguments, even if some of them begin with a ‘-’.
-
Signal the end of options, cause all remaining arguments to be assigned to the positional parameters. The-xand-voptions are turned off. If there are no arguments, the positional parameters remain unchanged.
Using ‘+’ rather than ‘-’ causes these options to be turned off. The options can also be used upon invocation of the shell. The current set of options may be found in $-
.
The remaining N arguments are positional parameters and are assigned, in order, to $1
, $2
, … $N
. The special parameter #
is set to N.
The return status is always zero unless an invalid option is supplied.
Previous: The Set Builtin, Up: Modifying Shell Behavior [Contents][Index]
This builtin allows you to change additional shell optional behavior.
shopt
shopt [-pqsu] [-o] [optname …]
Toggle the values of settings controlling optional shell behavior. The settings can be either those listed below, or, if the-ooption is used, those available with the-ooption to the set
builtin command (see The Set Builtin). With no options, or with the-poption, a list of all settable options is displayed, with an indication of whether or not each is set; if optnames are supplied, the output is restricted to those options. The-poption causes output to be displayed in a form that may be reused as input. Other options have the following meanings:
-s
Enable (set) each optname.
-u
Disable (unset) each optname.
-q
Suppresses normal output; the return status indicates whether the optname is set or unset. If multiple optname arguments are given with-q, the return status is zero if all optnames are enabled; non-zero otherwise.
-o
Restricts the values of optname to be those defined for the-ooption to the set
builtin (see The Set Builtin).
If either-sor-uis used with no optname arguments, shopt
shows only those options which are set or unset, respectively.
Unless otherwise noted, the shopt
options are disabled (off) by default.
The return status when listing options is zero if all optnames are enabled, non-zero otherwise. When setting or unsetting options, the return status is zero unless an optname is not a valid shell option.
The list of shopt
options is:
assoc_expand_once
If set, the shell suppresses multiple evaluation of associative array subscripts during arithmetic expression evaluation, while executing builtins that can perform variable assignments, and while executing builtins that perform array dereferencing.
autocd
If set, a command name that is the name of a directory is executed as if it were the argument to the cd
command. This option is only used by interactive shells.
cdable_vars
If this is set, an argument to the cd
builtin command that is not a directory is assumed to be the name of a variable whose value is the directory to change to.
cdspell
If set, minor errors in the spelling of a directory component in a cd
command will be corrected. The errors checked for are transposed characters, a missing character, and a character too many. If a correction is found, the corrected path is printed, and the command proceeds. This option is only used by interactive shells.
checkhash
If this is set, Bash checks that a command found in the hash table exists before trying to execute it. If a hashed command no longer exists, a normal path search is performed.
checkjobs
If set, Bash lists the status of any stopped and running jobs before exiting an interactive shell. If any jobs are running, this causes the exit to be deferred until a second exit is attempted without an intervening command (see Job Control). The shell always postpones exiting if any jobs are stopped.
checkwinsize
If set, Bash checks the window size after each external (non-builtin) command and, if necessary, updates the values of LINES
and COLUMNS
. This option is enabled by default.
cmdhist
If set, Bash attempts to save all lines of a multiple-line command in the same history entry. This allows easy re-editing of multi-line commands. This option is enabled by default, but only has an effect if command history is enabled (see Bash History Facilities).
compat31
If set, Bash changes its behavior to that of version 3.1 with respect to quoted arguments to the conditional command’s ‘=~’ operator and with respect to locale-specific string comparison when using the [[
conditional command’s ‘<’ and ‘>’ operators. Bash versions prior to bash-4.1 use ASCII collation and strcmp(3); bash-4.1 and later use the current locale’s collation sequence and strcoll(3).
compat32
If set, Bash changes its behavior to that of version 3.2 with respect to locale-specific string comparison when using the [[
conditional command’s ‘<’ and ‘>’ operators (see previous item) and the effect of interrupting a command list. Bash versions 3.2 and earlier continue with the next command in the list after one terminates due to an interrupt.
compat40
If set, Bash changes its behavior to that of version 4.0 with respect to locale-specific string comparison when using the [[
conditional command’s ‘<’ and ‘>’ operators (see description of compat31
) and the effect of interrupting a command list. Bash versions 4.0 and later interrupt the list as if the shell received the interrupt; previous versions continue with the next command in the list.
compat41
If set, Bash, when in POSIX mode, treats a single quote in a double-quoted parameter expansion as a special character. The single quotes must match (an even number) and the characters between the single quotes are considered quoted. This is the behavior of POSIX mode through version 4.1. The default Bash behavior remains as in previous versions.
compat42
If set, Bash does not process the replacement string in the pattern substitution word expansion using quote removal.
compat43
If set, Bash does not print a warning message if an attempt is made to use a quoted compound array assignment as an argument to declare
, makes word expansion errors non-fatal errors that cause the current command to fail (the default behavior is to make them fatal errors that cause the shell to exit), and does not reset the loop state when a shell function is executed (this allows break
or continue
in a shell function to affect loops in the caller’s context).
compat44
If set, Bash saves the positional parameters to BASH_ARGV and BASH_ARGC before they are used, regardless of whether or not extended debugging mode is enabled.
complete_fullquote
If set, Bash quotes all shell metacharacters in filenames and directory names when performing completion. If not set, Bash removes metacharacters such as the dollar sign from the set of characters that will be quoted in completed filenames when these metacharacters appear in shell variable references in words to be completed. This means that dollar signs in variable names that expand to directories will not be quoted; however, any dollar signs appearing in filenames will not be quoted, either. This is active only when bash is using backslashes to quote completed filenames. This variable is set by default, which is the default Bash behavior in versions through 4.2.
direxpand
If set, Bash replaces directory names with the results of word expansion when performing filename completion. This changes the contents of the readline editing buffer. If not set, Bash attempts to preserve what the user typed.
dirspell
If set, Bash attempts spelling correction on directory names during word completion if the directory name initially supplied does not exist.
dotglob
If set, Bash includes filenames beginning with a ‘.’ in the results of filename expansion. The filenames ‘.’ and ‘..’ must always be matched explicitly, even if dotglob
is set.
execfail
If this is set, a non-interactive shell will not exit if it cannot execute the file specified as an argument to the exec
builtin command. An interactive shell does not exit if exec
fails.
expand_aliases
If set, aliases are expanded as described below under Aliases, Aliases. This option is enabled by default for interactive shells.
extdebug
If set at shell invocation, arrange to execute the debugger profile before the shell starts, identical to the--debuggeroption. If set after invocation, behavior intended for use by debuggers is enabled:
declare
builtin (see Bash Builtins) displays the source file name and line number corresponding to each function name supplied as an argument.DEBUG
trap returns a non-zero value, the next command is skipped and not executed.DEBUG
trap returns a value of 2, and the shell is executing in a subroutine (a shell function or a shell script executed by the .
or source
builtins), the shell simulates a call to return
.BASH_ARGC
and BASH_ARGV
are updated as described in their descriptions (see Bash Variables).( command )
inherit the DEBUG
and RETURN
traps.( command )
inherit the ERR
trap.extglob
If set, the extended pattern matching features described above (see Pattern Matching) are enabled.
extquote
If set, $'string'
and $"string"
quoting is performed within ${parameter}
expansions enclosed in double quotes. This option is enabled by default.
failglob
If set, patterns which fail to match filenames during filename expansion result in an expansion error.
force_fignore
If set, the suffixes specified by the FIGNORE
shell variable cause words to be ignored when performing word completion even if the ignored words are the only possible completions. See Bash Variables, for a description of FIGNORE
. This option is enabled by default.
globasciiranges
If set, range expressions used in pattern matching bracket expressions (see Pattern Matching) behave as if in the traditional C locale when performing comparisons. That is, the current locale’s collating sequence is not taken into account, so ‘b’ will not collate between ‘A’ and ‘B’, and upper-case and lower-case ASCII characters will collate together.
globstar
If set, the pattern ‘**’ used in a filename expansion context will match all files and zero or more directories and subdirectories. If the pattern is followed by a ‘/’, only directories and subdirectories match.
gnu_errfmt
If set, shell error messages are written in the standard GNU error message format.
histappend
If set, the history list is appended to the file named by the value of the HISTFILE
variable when the shell exits, rather than overwriting the file.
histreedit
If set, and Readline is being used, a user is given the opportunity to re-edit a failed history substitution.
histverify
If set, and Readline is being used, the results of history substitution are not immediately passed to the shell parser. Instead, the resulting line is loaded into the Readline editing buffer, allowing further modification.
hostcomplete
If set, and Readline is being used, Bash will attempt to perform hostname completion when a word containing a ‘@’ is being completed (see Commands For Completion). This option is enabled by default.
huponexit
If set, Bash will send SIGHUP
to all jobs when an interactive login shell exits (see Signals).
inherit_errexit
If set, command substitution inherits the value of the errexit
option, instead of unsetting it in the subshell environment. This option is enabled when POSIX mode is enabled.
interactive_comments
Allow a word beginning with ‘#’ to cause that word and all remaining characters on that line to be ignored in an interactive shell. This option is enabled by default.
lastpipe
If set, and job control is not active, the shell runs the last command of a pipeline not executed in the background in the current shell environment.
lithist
If enabled, and the cmdhist
option is enabled, multi-line commands are saved to the history with embedded newlines rather than using semicolon separators where possible.
localvar_inherit
If set, local variables inherit the value and attributes of a variable of the same name that exists at a previous scope before any new value is assigned. The nameref attribute is not inherited.
localvar_unset
If set, calling unset
on local variables in previous function scopes marks them so subsequent lookups find them unset until that function returns. This is identical to the behavior of unsetting local variables at the current function scope.
login_shell
The shell sets this option if it is started as a login shell (see Invoking Bash). The value may not be changed.
mailwarn
If set, and a file that Bash is checking for mail has been accessed since the last time it was checked, the message "The mail in mailfile has been read"
is displayed.
no_empty_cmd_completion
If set, and Readline is being used, Bash will not attempt to search the PATH
for possible completions when completion is attempted on an empty line.
nocaseglob
If set, Bash matches filenames in a case-insensitive fashion when performing filename expansion.
nocasematch
If set, Bash matches patterns in a case-insensitive fashion when performing matching while executing case
or [[
conditional commands, when performing pattern substitution word expansions, or when filtering possible completions as part of programmable completion.
nullglob
If set, Bash allows filename patterns which match no files to expand to a null string, rather than themselves.
progcomp
If set, the programmable completion facilities (see Programmable Completion) are enabled. This option is enabled by default.
progcomp_alias
If set, and programmable completion is enabled, Bash treats a command name that doesn’t have any completions as a possible alias and attempts alias expansion. If it has an alias, Bash attempts programmable completion using the command word resulting from the expanded alias.
promptvars
If set, prompt strings undergo parameter expansion, command substitution, arithmetic expansion, and quote removal after being expanded as described below (see Controlling the Prompt). This option is enabled by default.
restricted_shell
The shell sets this option if it is started in restricted mode (see The Restricted Shell). The value may not be changed. This is not reset when the startup files are executed, allowing the startup files to discover whether or not a shell is restricted.
shift_verbose
If this is set, the shift
builtin prints an error message when the shift count exceeds the number of positional parameters.
sourcepath
If set, the source
builtin uses the value of PATH
to find the directory containing the file supplied as an argument. This option is enabled by default.
xpg_echo
If set, the echo
builtin expands backslash-escape sequences by default.
The return status when listing options is zero if all optnames are enabled, non-zero otherwise. When setting or unsetting options, the return status is zero unless an optname is not a valid shell option.
Previous: Modifying Shell Behavior, Up: Shell Builtin Commands [Contents][Index]
For historical reasons, the POSIX standard has classified several builtin commands as special. When Bash is executing in POSIX mode, the special builtins differ from other builtin commands in three respects:
When Bash is not executing in POSIX mode, these builtins behave no differently than the rest of the Bash builtin commands. The Bash POSIX mode is described in Bash POSIX Mode.
These are the POSIX special builtins:
break : . continue eval exec exit export readonly return set shift trap unset
Next: Bash Features, Previous: Shell Builtin Commands, Up: Top [Contents][Index]
• Bourne Shell Variables: | Variables which Bash uses in the same way as the Bourne Shell. | |
• Bash Variables: | List of variables that exist in Bash. |
This chapter describes the shell variables that Bash uses. Bash automatically assigns default values to a number of variables.
Next: Bash Variables, Up: Shell Variables [Contents][Index]
Bash uses certain shell variables in the same way as the Bourne shell. In some cases, Bash assigns a default value to the variable.
CDPATH
A colon-separated list of directories used as a search path for the cd
builtin command.
HOME
The current user’s home directory; the default for the cd
builtin command. The value of this variable is also used by tilde expansion (see Tilde Expansion).
IFS
A list of characters that separate fields; used when the shell splits words as part of expansion.
MAIL
If this parameter is set to a filename or directory name and the MAILPATH
variable is not set, Bash informs the user of the arrival of mail in the specified file or Maildir-format directory.
MAILPATH
A colon-separated list of filenames which the shell periodically checks for new mail. Each list entry can specify the message that is printed when new mail arrives in the mail file by separating the filename from the message with a ‘?’. When used in the text of the message, $_
expands to the name of the current mail file.
OPTARG
The value of the last option argument processed by the getopts
builtin.
OPTIND
The index of the last option argument processed by the getopts
builtin.
PATH
A colon-separated list of directories in which the shell looks for commands. A zero-length (null) directory name in the value of PATH
indicates the current directory. A null directory name may appear as two adjacent colons, or as an initial or trailing colon.
PS1
The primary prompt string. The default value is ‘\s-\v\$’. See Controlling the Prompt, for the complete list of escape sequences that are expanded before PS1
is displayed.
PS2
The secondary prompt string. The default value is ‘>’. PS2
is expanded in the same way as PS1
before being displayed.
Previous: Bourne Shell Variables, Up: Shell Variables [Contents][Index]
These variables are set or used by Bash, but other shells do not normally treat them specially.
A few variables used by Bash are described in different chapters: variables for controlling the job control facilities (see Job Control Variables).
BASH
The full pathname used to execute the current instance of Bash.
BASHOPTS
A colon-separated list of enabled shell options. Each word in the list is a valid argument for the-soption to the shopt
builtin command (see The Shopt Builtin). The options appearing in BASHOPTS
are those reported as ‘on’ by ‘shopt’. If this variable is in the environment when Bash starts up, each shell option in the list will be enabled before reading any startup files. This variable is readonly.
BASHPID
Expands to the process ID of the current Bash process. This differs from $$
under certain circumstances, such as subshells that do not require Bash to be re-initialized. Assignments to BASHPID
have no effect. If BASHPID
is unset, it loses its special properties, even if it is subsequently reset.
BASH_ALIASES
An associative array variable whose members correspond to the internal list of aliases as maintained by the alias
builtin. (see Bourne Shell Builtins). Elements added to this array appear in the alias list; however, unsetting array elements currently does not cause aliases to be removed from the alias list. If BASH_ALIASES
is unset, it loses its special properties, even if it is subsequently reset.
BASH_ARGC
An array variable whose values are the number of parameters in each frame of the current bash execution call stack. The number of parameters to the current subroutine (shell function or script executed with .
or source
) is at the top of the stack. When a subroutine is executed, the number of parameters passed is pushed onto BASH_ARGC
. The shell sets BASH_ARGC
only when in extended debugging mode (see The Shopt Builtin for a description of the extdebug
option to the shopt
builtin). Setting extdebug
after the shell has started to execute a script, or referencing this variable when extdebug
is not set, may result in inconsistent values.
BASH_ARGV
An array variable containing all of the parameters in the current bash execution call stack. The final parameter of the last subroutine call is at the top of the stack; the first parameter of the initial call is at the bottom. When a subroutine is executed, the parameters supplied are pushed onto BASH_ARGV
. The shell sets BASH_ARGV
only when in extended debugging mode (see The Shopt Builtin for a description of the extdebug
option to the shopt
builtin). Setting extdebug
after the shell has started to execute a script, or referencing this variable when extdebug
is not set, may result in inconsistent values.
BASH_ARGV0
When referenced, this variable expands to the name of the shell or shell script (identical to $0
; See Special Parameters, for the description of special parameter 0). Assignment to BASH_ARGV0
causes the value assigned to also be assigned to $0
. If BASH_ARGV0
is unset, it loses its special properties, even if it is subsequently reset.
BASH_CMDS
An associative array variable whose members correspond to the internal hash table of commands as maintained by the hash
builtin (see Bourne Shell Builtins). Elements added to this array appear in the hash table; however, unsetting array elements currently does not cause command names to be removed from the hash table. If BASH_CMDS
is unset, it loses its special properties, even if it is subsequently reset.
BASH_COMMAND
The command currently being executed or about to be executed, unless the shell is executing a command as the result of a trap, in which case it is the command executing at the time of the trap.
BASH_COMPAT
The value is used to set the shell’s compatibility level. See The Shopt Builtin, for a description of the various compatibility levels and their effects. The value may be a decimal number (e.g., 4.2) or an integer (e.g., 42) corresponding to the desired compatibility level. If BASH_COMPAT
is unset or set to the empty string, the compatibility level is set to the default for the current version. If BASH_COMPAT
is set to a value that is not one of the valid compatibility levels, the shell prints an error message and sets the compatibility level to the default for the current version. The valid compatibility levels correspond to the compatibility options accepted by the shopt
builtin described above (for example, compat42 means that 4.2 and 42 are valid values). The current version is also a valid value.
BASH_ENV
If this variable is set when Bash is invoked to execute a shell script, its value is expanded and used as the name of a startup file to read before executing the script. See Bash Startup Files.
BASH_EXECUTION_STRING
The command argument to the-cinvocation option.
BASH_LINENO
An array variable whose members are the line numbers in source files where each corresponding member of FUNCNAME was invoked. ${BASH_LINENO[$i]}
is the line number in the source file (${BASH_SOURCE[$i+1]}
) where ${FUNCNAME[$i]}
was called (or ${BASH_LINENO[$i-1]}
if referenced within another shell function). Use LINENO
to obtain the current line number.
BASH_LOADABLES_PATH
A colon-separated list of directories in which the shell looks for dynamically loadable builtins specified by the enable
command.
BASH_REMATCH
An array variable whose members are assigned by the ‘=~’ binary operator to the [[
conditional command (see Conditional Constructs). The element with index 0 is the portion of the string matching the entire regular expression. The element with index n is the portion of the string matching the nth parenthesized subexpression. This variable is read-only.
BASH_SOURCE
An array variable whose members are the source filenames where the corresponding shell function names in the FUNCNAME
array variable are defined. The shell function ${FUNCNAME[$i]}
is defined in the file ${BASH_SOURCE[$i]}
and called from ${BASH_SOURCE[$i+1]}
BASH_SUBSHELL
Incremented by one within each subshell or subshell environment when the shell begins executing in that environment. The initial value is 0.
BASH_VERSINFO
A readonly array variable (see Arrays) whose members hold version information for this instance of Bash. The values assigned to the array members are as follows:
BASH_VERSINFO[0]
The major version number (the release).
BASH_VERSINFO[1]
The minor version number (the version).
BASH_VERSINFO[2]
The patch level.
BASH_VERSINFO[3]
The build version.
BASH_VERSINFO[4]
The release status (e.g., beta1).
BASH_VERSINFO[5]
The value of MACHTYPE
.
BASH_VERSION
The version number of the current instance of Bash.
BASH_XTRACEFD
If set to an integer corresponding to a valid file descriptor, Bash will write the trace output generated when ‘set -x’ is enabled to that file descriptor. This allows tracing output to be separated from diagnostic and error messages. The file descriptor is closed when BASH_XTRACEFD
is unset or assigned a new value. Unsetting BASH_XTRACEFD
or assigning it the empty string causes the trace output to be sent to the standard error. Note that setting BASH_XTRACEFD
to 2 (the standard error file descriptor) and then unsetting it will result in the standard error being closed.
CHILD_MAX
Set the number of exited child status values for the shell to remember. Bash will not allow this value to be decreased below a POSIX-mandated minimum, and there is a maximum value (currently 8192) that this may not exceed. The minimum value is system-dependent.
COLUMNS
Used by the select
command to determine the terminal width when printing selection lists. Automatically set if the checkwinsize
option is enabled (see The Shopt Builtin), or in an interactive shell upon receipt of a SIGWINCH
.
COMP_CWORD
An index into ${COMP_WORDS}
of the word containing the current cursor position. This variable is available only in shell functions invoked by the programmable completion facilities (see Programmable Completion).
COMP_LINE
The current command line. This variable is available only in shell functions and external commands invoked by the programmable completion facilities (see Programmable Completion).
COMP_POINT
The index of the current cursor position relative to the beginning of the current command. If the current cursor position is at the end of the current command, the value of this variable is equal to ${#COMP_LINE}
. This variable is available only in shell functions and external commands invoked by the programmable completion facilities (see Programmable Completion).
COMP_TYPE
Set to an integer value corresponding to the type of completion attempted that caused a completion function to be called: TAB, for normal completion, ‘?’, for listing completions after successive tabs, ‘!’, for listing alternatives on partial word completion, ‘@’, to list completions if the word is not unmodified, or ‘%’, for menu completion. This variable is available only in shell functions and external commands invoked by the programmable completion facilities (see Programmable Completion).
COMP_KEY
The key (or final key of a key sequence) used to invoke the current completion function.
COMP_WORDBREAKS
The set of characters that the Readline library treats as word separators when performing word completion. If COMP_WORDBREAKS
is unset, it loses its special properties, even if it is subsequently reset.
COMP_WORDS
An array variable consisting of the individual words in the current command line. The line is split into words as Readline would split it, using COMP_WORDBREAKS
as described above. This variable is available only in shell functions invoked by the programmable completion facilities (see Programmable Completion).
COMPREPLY
An array variable from which Bash reads the possible completions generated by a shell function invoked by the programmable completion facility (see Programmable Completion). Each array element contains one possible completion.
COPROC
An array variable created to hold the file descriptors for output from and input to an unnamed coprocess (see Coprocesses).
DIRSTACK
An array variable containing the current contents of the directory stack. Directories appear in the stack in the order they are displayed by the dirs
builtin. Assigning to members of this array variable may be used to modify directories already in the stack, but the pushd
and popd
builtins must be used to add and remove directories. Assignment to this variable will not change the current directory. If DIRSTACK
is unset, it loses its special properties, even if it is subsequently reset.
EMACS
If Bash finds this variable in the environment when the shell starts with value ‘t’, it assumes that the shell is running in an Emacs shell buffer and disables line editing.
ENV
Similar to BASH_ENV
; used when the shell is invoked in POSIX Mode (see Bash POSIX Mode).
EPOCHREALTIME
Each time this parameter is referenced, it expands to the number of seconds since the Unix Epoch as a floating point value with micro-second granularity (see the documentation for the C library function time for the definition of Epoch). Assignments to EPOCHREALTIME
are ignored. If EPOCHREALTIME
is unset, it loses its special properties, even if it is subsequently reset.
EPOCHSECONDS
Each time this parameter is referenced, it expands to the number of seconds since the Unix Epoch (see the documentation for the C library function time for the definition of Epoch). Assignments to EPOCHSECONDS
are ignored. If EPOCHSECONDS
is unset, it loses its special properties, even if it is subsequently reset.
EUID
The numeric effective user id of the current user. This variable is readonly.
EXECIGNORE
A colon-separated list of shell patterns (see Pattern Matching) defining the list of filenames to be ignored by command search using PATH
. Files whose full pathnames match one of these patterns are not considered executable files for the purposes of completion and command execution via PATH
lookup. This does not affect the behavior of the [
, test
, and [[
commands. Full pathnames in the command hash table are not subject to EXECIGNORE
. Use this variable to ignore shared library files that have the executable bit set, but are not executable files. The pattern matching honors the setting of the extglob
shell option.
FCEDIT
The editor used as a default by the-eoption to the fc
builtin command.
FIGNORE
A colon-separated list of suffixes to ignore when performing filename completion. A filename whose suffix matches one of the entries in FIGNORE
is excluded from the list of matched filenames. A sample value is ‘.o:~’
FUNCNAME
An array variable containing the names of all shell functions currently in the execution call stack. The element with index 0 is the name of any currently-executing shell function. The bottom-most element (the one with the highest index) is "main"
. This variable exists only when a shell function is executing. Assignments to FUNCNAME
have no effect. If FUNCNAME
is unset, it loses its special properties, even if it is subsequently reset.
This variable can be used with BASH_LINENO
and BASH_SOURCE
. Each element of FUNCNAME
has corresponding elements in BASH_LINENO
and BASH_SOURCE
to describe the call stack. For instance, ${FUNCNAME[$i]}
was called from the file ${BASH_SOURCE[$i+1]}
at line number ${BASH_LINENO[$i]}
. The caller
builtin displays the current call stack using this information.
FUNCNEST
If set to a numeric value greater than 0, defines a maximum function nesting level. Function invocations that exceed this nesting level will cause the current command to abort.
GLOBIGNORE
A colon-separated list of patterns defining the set of file names to be ignored by filename expansion. If a file name matched by a filename expansion pattern also matches one of the patterns in GLOBIGNORE
, it is removed from the list of matches. The pattern matching honors the setting of the extglob
shell option.
GROUPS
An array variable containing the list of groups of which the current user is a member. Assignments to GROUPS
have no effect. If GROUPS
is unset, it loses its special properties, even if it is subsequently reset.
histchars
Up to three characters which control history expansion, quick substitution, and tokenization (see History Interaction). The first character is the history expansion character, that is, the character which signifies the start of a history expansion, normally ‘!’. The second character is the character which signifies ‘quick substitution’ when seen as the first character on a line, normally ‘^’. The optional third character is the character which indicates that the remainder of the line is a comment when found as the first character of a word, usually ‘#’. The history comment character causes history substitution to be skipped for the remaining words on the line. It does not necessarily cause the shell parser to treat the rest of the line as a comment.
HISTCMD
The history number, or index in the history list, of the current command. If HISTCMD
is unset, it loses its special properties, even if it is subsequently reset.
HISTCONTROL
A colon-separated list of values controlling how commands are saved on the history list. If the list of values includes ‘ignorespace’, lines which begin with a space character are not saved in the history list. A value of ‘ignoredups’ causes lines which match the previous history entry to not be saved. A value of ‘ignoreboth’ is shorthand for ‘ignorespace’ and ‘ignoredups’. A value of ‘erasedups’ causes all previous lines matching the current line to be removed from the history list before that line is saved. Any value not in the above list is ignored. If HISTCONTROL
is unset, or does not include a valid value, all lines read by the shell parser are saved on the history list, subject to the value of HISTIGNORE
. The second and subsequent lines of a multi-line compound command are not tested, and are added to the history regardless of the value of HISTCONTROL
.
HISTFILE
The name of the file to which the command history is saved. The default value is~/.bash_history.
HISTFILESIZE
The maximum number of lines contained in the history file. When this variable is assigned a value, the history file is truncated, if necessary, to contain no more than that number of lines by removing the oldest entries. The history file is also truncated to this size after writing it when a shell exits. If the value is 0, the history file is truncated to zero size. Non-numeric values and numeric values less than zero inhibit truncation. The shell sets the default value to the value of HISTSIZE
after reading any startup files.
HISTIGNORE
A colon-separated list of patterns used to decide which command lines should be saved on the history list. Each pattern is anchored at the beginning of the line and must match the complete line (no implicit ‘*’ is appended). Each pattern is tested against the line after the checks specified by HISTCONTROL
are applied. In addition to the normal shell pattern matching characters, ‘&’ matches the previous history line. ‘&’ may be escaped using a backslash; the backslash is removed before attempting a match. The second and subsequent lines of a multi-line compound command are not tested, and are added to the history regardless of the value of HISTIGNORE
. The pattern matching honors the setting of the extglob
shell option.
HISTIGNORE
subsumes the function of HISTCONTROL
. A pattern of ‘&’ is identical to ignoredups
, and a pattern of ‘[ ]*’ is identical to ignorespace
. Combining these two patterns, separating them with a colon, provides the functionality of ignoreboth
.
HISTSIZE
The maximum number of commands to remember on the history list. If the value is 0, commands are not saved in the history list. Numeric values less than zero result in every command being saved on the history list (there is no limit). The shell sets the default value to 500 after reading any startup files.
HISTTIMEFORMAT
If this variable is set and not null, its value is used as a format string for strftime to print the time stamp associated with each history entry displayed by the history
builtin. If this variable is set, time stamps are written to the history file so they may be preserved across shell sessions. This uses the history comment character to distinguish timestamps from other history lines.
HOSTFILE
Contains the name of a file in the same format as/etc/hoststhat should be read when the shell needs to complete a hostname. The list of possible hostname completions may be changed while the shell is running; the next time hostname completion is attempted after the value is changed, Bash adds the contents of the new file to the existing list. If HOSTFILE
is set, but has no value, or does not name a readable file, Bash attempts to read/etc/hoststo obtain the list of possible hostname completions. When HOSTFILE
is unset, the hostname list is cleared.
HOSTNAME
The name of the current host.
HOSTTYPE
A string describing the machine Bash is running on.
IGNOREEOF
Controls the action of the shell on receipt of an EOF
character as the sole input. If set, the value denotes the number of consecutive EOF
characters that can be read as the first character on an input line before the shell will exit. If the variable exists but does not have a numeric value, or has no value, then the default is 10. If the variable does not exist, then EOF
signifies the end of input to the shell. This is only in effect for interactive shells.
INPUTRC
The name of the Readline initialization file, overriding the default of~/.inputrc.
INSIDE_EMACS
If Bash finds this variable in the environment when the shell starts, it assumes that the shell is running in an Emacs shell buffer and may disable line editing depending on the value of TERM
.
LANG
Used to determine the locale category for any category not specifically selected with a variable starting with LC_
.
LC_ALL
This variable overrides the value of LANG
and any other LC_
variable specifying a locale category.
LC_COLLATE
This variable determines the collation order used when sorting the results of filename expansion, and determines the behavior of range expressions, equivalence classes, and collating sequences within filename expansion and pattern matching (see Filename Expansion).
LC_CTYPE
This variable determines the interpretation of characters and the behavior of character classes within filename expansion and pattern matching (see Filename Expansion).
LC_MESSAGES
This variable determines the locale used to translate double-quoted strings preceded by a ‘$’ (see Locale Translation).
LC_NUMERIC
This variable determines the locale category used for number formatting.
LC_TIME
This variable determines the locale category used for data and time formatting.
LINENO
The line number in the script or shell function currently executing.
LINES
Used by the select
command to determine the column length for printing selection lists. Automatically set if the checkwinsize
option is enabled (see The Shopt Builtin), or in an interactive shell upon receipt of a SIGWINCH
.
MACHTYPE
A string that fully describes the system type on which Bash is executing, in the standard GNU cpu-company-system format.
MAILCHECK
How often (in seconds) that the shell should check for mail in the files specified in the MAILPATH
or MAIL
variables. The default is 60 seconds. When it is time to check for mail, the shell does so before displaying the primary prompt. If this variable is unset, or set to a value that is not a number greater than or equal to zero, the shell disables mail checking.
MAPFILE
An array variable created to hold the text read by the mapfile
builtin when no variable name is supplied.
OLDPWD
The previous working directory as set by the cd
builtin.
OPTERR
If set to the value 1, Bash displays error messages generated by the getopts
builtin command.
OSTYPE
A string describing the operating system Bash is running on.
PIPESTATUS
An array variable (see Arrays) containing a list of exit status values from the processes in the most-recently-executed foreground pipeline (which may contain only a single command).
POSIXLY_CORRECT
If this variable is in the environment when Bash starts, the shell enters POSIX mode (see Bash POSIX Mode) before reading the startup files, as if the--posixinvocation option had been supplied. If it is set while the shell is running, Bash enables POSIX mode, as if the command
set -o posix
had been executed. When the shell enters POSIX mode, it sets this variable if it was not already set.
PPID
The process ID of the shell’s parent process. This variable is readonly.
PROMPT_COMMAND
If set, the value is interpreted as a command to execute before the printing of each primary prompt ($PS1
).
PROMPT_DIRTRIM
If set to a number greater than zero, the value is used as the number of trailing directory components to retain when expanding the \w
and \W
prompt string escapes (see Controlling the Prompt). Characters removed are replaced with an ellipsis.
PS0
The value of this parameter is expanded like PS1 and displayed by interactive shells after reading a command and before the command is executed.
PS3
The value of this variable is used as the prompt for the select
command. If this variable is not set, the select
command prompts with ‘#?’
PS4
The value of this parameter is expanded like PS1 and the expanded value is the prompt printed before the command line is echoed when the-xoption is set (see The Set Builtin). The first character of the expanded value is replicated multiple times, as necessary, to indicate multiple levels of indirection. The default is ‘+’.
PWD
The current working directory as set by the cd
builtin.
RANDOM
Each time this parameter is referenced, a random integer between 0 and 32767 is generated. Assigning a value to this variable seeds the random number generator.
READLINE_LINE
The contents of the Readline line buffer, for use with ‘bind -x’ (see Bash Builtins).
READLINE_POINT
The position of the insertion point in the Readline line buffer, for use with ‘bind -x’ (see Bash Builtins).
REPLY
The default variable for the read
builtin.
SECONDS
This variable expands to the number of seconds since the shell was started. Assignment to this variable resets the count to the value assigned, and the expanded value becomes the value assigned plus the number of seconds since the assignment.
SHELL
The full pathname to the shell is kept in this environment variable. If it is not set when the shell starts, Bash assigns to it the full pathname of the current user’s login shell.
SHELLOPTS
A colon-separated list of enabled shell options. Each word in the list is a valid argument for the-ooption to the set
builtin command (see The Set Builtin). The options appearing in SHELLOPTS
are those reported as ‘on’ by ‘set -o’. If this variable is in the environment when Bash starts up, each shell option in the list will be enabled before reading any startup files. This variable is readonly.
SHLVL
Incremented by one each time a new instance of Bash is started. This is intended to be a count of how deeply your Bash shells are nested.
TIMEFORMAT
The value of this parameter is used as a format string specifying how the timing information for pipelines prefixed with the time
reserved word should be displayed. The ‘%’ character introduces an escape sequence that is expanded to a time value or other information. The escape sequences and their meanings are as follows; the braces denote optional portions.
%%
A literal ‘%’.
%[p][l]R
The elapsed time in seconds.
%[p][l]U
The number of CPU seconds spent in user mode.
%[p][l]S
The number of CPU seconds spent in system mode.
%P
The CPU percentage, computed as (%U + %S) / %R.
The optional p is a digit specifying the precision, the number of fractional digits after a decimal point. A value of 0 causes no decimal point or fraction to be output. At most three places after the decimal point may be specified; values of p greater than 3 are changed to 3. If p is not specified, the value 3 is used.
The optional l
specifies a longer format, including minutes, of the form MMmSS.FFs. The value of p determines whether or not the fraction is included.
If this variable is not set, Bash acts as if it had the value
$'\nreal\t%3lR\nuser\t%3lU\nsys\t%3lS'
If the value is null, no timing information is displayed. A trailing newline is added when the format string is displayed.
TMOUT
If set to a value greater than zero, TMOUT
is treated as the default timeout for the read
builtin (see Bash Builtins). The select
command (see Conditional Constructs) terminates if input does not arrive after TMOUT
seconds when input is coming from a terminal.
In an interactive shell, the value is interpreted as the number of seconds to wait for a line of input after issuing the primary prompt. Bash terminates after waiting for that number of seconds if a complete line of input does not arrive.
TMPDIR
If set, Bash uses its value as the name of a directory in which Bash creates temporary files for the shell’s use.
UID
The numeric real user id of the current user. This variable is readonly.
Next: Job Control, Previous: Shell Variables, Up: Top [Contents][Index]
This chapter describes features unique to Bash.
• Invoking Bash: | Command line options that you can give to Bash. | |
• Bash Startup Files: | When and how Bash executes scripts. | |
• Interactive Shells: | What an interactive shell is. | |
• Bash Conditional Expressions: | Primitives used in composing expressions for the test builtin. |
|
• Shell Arithmetic: | Arithmetic on shell variables. | |
• Aliases: | Substituting one command for another. | |
• Arrays: | Array Variables. | |
• The Directory Stack: | History of visited directories. | |
• Controlling the Prompt: | Customizing the various prompt strings. | |
• The Restricted Shell: | A more controlled mode of shell execution. | |
• Bash POSIX Mode: | Making Bash behave more closely to what the POSIX standard specifies. |
Next: Bash Startup Files, Up: Bash Features [Contents][Index]
bash [long-opt] [-ir] [-abefhkmnptuvxdBCDHP] [-o option] [-O shopt_option] [argument …] bash [long-opt] [-abefhkmnptuvxdBCDHP] [-o option] [-O shopt_option] -c string [argument …] bash [long-opt] -s [-abefhkmnptuvxdBCDHP] [-o option] [-O shopt_option] [argument …]
All of the single-character options used with the set
builtin (see The Set Builtin) can be used as options when the shell is invoked. In addition, there are several multi-character options that you can use. These options must appear on the command line before the single-character options to be recognized.
--debugger
Arrange for the debugger profile to be executed before the shell starts. Turns on extended debugging mode (see