This article introduces the GNU Bash in detail. And the most of the content is from Bash Reference Manual v4.3. I try to understand all features of GNU Bash and add some examples in this article.

Introduction

What is a shell?

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.

What is Bash?

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.

Bash Releases

Here is GNU Bash official site, and here is Bash Reference Manual. The GNU Bash releases following versions:

Bash Repository

The GNU Bash Git repository is located here. Run the following command to get a copy of the GNU Bash source code from repository:

$ git clone git://git.savannah.gnu.org/bash.git

or, browse sources repository online here.

Definitions

Metacharacter

A metacharacter is a character that, when unquoted, separates words. It is a blank or one of the following characters: | & ; ( ) < >

Control Operator

A token that performs a control function. It is a newline or one of the following: || && & ; ;; | |& ( )

Basic Shell Features

Shell Syntax

Shell Operation

The following is a brief description of the shell’s operation when it reads and executes a command. Basically, the shell does the following:

1. Reads its input from a shell script, from a string supplied as an argument to the -c invocation option, or from the user’s terminal.
2. Breaks the input into words and operators, obeying the quoting rules described in Quoting. These tokens are separated by metacharacters. Alias expansion is performed by this step.
3. Parses the tokens into simple commands and compound commands.
4. Performs the various shell expansions, breaking the expanded tokens into lists of filenames, commands and arguments.
5. Performs any necessary redirections and removes the redirection operators and their operands from the argument list.
6. Executes the command.
7. Optionally waits for the command to complete and collects its exit status.

Quoting

Escape Character

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’s, it is removed from the input stream and effectively ignored.

Examples:

chenwx@chenwx ~ $ echo abc\
> def
abcdef

chenwx@chenwx ~ $ echo abc\\
abc\

Single Quotes

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.

Examples:

chenwx@chenwx ~ $ echo 'abc123~`!@#$%^&*()_-+=|\<,>.?/'
abc123~`!@#$%^&*()_-+=|\<,>.?/

Double Quotes

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, !. A double quote may be quoted within double quotes by preceding it with a backslash.

ANSI-C Quoting

Format:

$'string'

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 following table. The expanded result is single-quoted, as if the dollar sign had not been present.

Examples:

chenwx@chenwx ~ $ echo abc$'\a'def
abcdef
chenwx@chenwx ~ $ echo abc$'\b'def
abdef

chenwx@chenwx ~ $ echo abc$'\f'def
abc
   def

chenwx@chenwx ~ $ echo abc$'\n'def
abc
def
chenwx@chenwx ~ $ echo abc$'\r'def
def
chenwx@chenwx ~ $ echo abc$'\n'def$'\r'ghi
abc
ghi

chenwx@chenwx ~ $ echo abc$'\t'def
abc	def
chenwx@chenwx ~ $ echo abc$'\v'def
abc
   def
chenwx@chenwx ~ $ echo abc$'\v'def$'\v'hgi
abc
   def
      hgi

chenwx@chenwx ~ $ echo abc$'\\'def
abc\def

chenwx@chenwx ~ $ echo abc$'\60'def
abc0def
chenwx@chenwx ~ $ echo abc$'\71'def
abc9def
chenwx@chenwx ~ $ echo abc$'\101'def
abcAdef
chenwx@chenwx ~ $ echo abc$'\132'def
abcZdef

chenwx@chenwx ~ $ echo abc$'\x30'def
abc0def
chenwx@chenwx ~ $ echo abc$'\x39'def
abc9def
chenwx@chenwx ~ $ echo abc$'\x41'def
abcAdef
chenwx@chenwx ~ $ echo abc$'\x5A'def
abcZdef

chenwx@chenwx ~ $ echo abc$'\u30'def
abc0def
chenwx@chenwx ~ $ echo abc$'\u39'def
abc9def
chenwx@chenwx ~ $ echo abc$'\u41'def
abcAdef
chenwx@chenwx ~ $ echo abc$'\u5A'def
abcZdef

chenwx@chenwx ~ $ echo abc$'\cx'def
abc#def

Locale-Specific Translation

Format:

$"string"

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.

Refer to How to add localization support to your bash scripts.

Example:

chenwx@chenwx ~ $ echo $"abcdef"
abcdef

Comments

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.

Examples:

chenwx@chenwx ~ $ shopt -p interactive_comments
shopt -s interactive_comments
chenwx@chenwx ~ $ echo abc #print first three characters
abc
chenwx@chenwx ~ $ shopt -u interactive_comments
chenwx@chenwx ~ $ echo abc #print first three characters
abc #print first three characters

Shell Commands

Simple Commands

Formats:

command
command ;
command &

A simple command is just a sequence of words separated by blanks, terminated by one of the shell’s control operators. The first word generally specifies a command to be executed, with the rest of the words being that command’s arguments.

Pipelines

A pipeline is a sequence of simple commands separated by one of the control operators | or |&. The output of each command in the pipeline is connected via a pipe to the input of the next command.

Format #1:

[time [-p]] [!] command1 [ | command2 ] ...

Format #2:

[time [-p]] [!] command1 [ |& command2 ] ...
[time [-p]] [!] command1 [ 2>&1 | command2 ] ...

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 |.

If the reserved word ! precedes the pipeline, the exit status is the logical negation of the exit status.

Lists of Commands

A list is a sequence of one or more pipelines separated by one of the operators && || ; &, 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.

Format #1:

pipeline-command1 ; pipeline-command2
pipeline-command1 ; pipeline-command2 ;
pipeline-command1 ; pipeline-command2 &

Format #2:

pipeline-command1 & pipeline-command2
pipeline-command1 & pipeline-command2 ;
pipeline-command1 & pipeline-command2 &

Format #3:

pipeline-command1 && pipeline-command2
pipeline-command1 && pipeline-command2 ;
pipeline-command1 && pipeline-command2 &

Format #4:

pipeline-command1 || pipeline-command2
pipeline-command1 || pipeline-command2 ;
pipeline-command1 || pipeline-command2 &

Format #5:

pipeline-command1 && pipeline-command2 ; pipeline-command3 || pipeline-command4
pipeline-command1 && pipeline-command2 ; pipeline-command3 || pipeline-command4 &

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.

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. 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.

An AND list has the form command1 && command2, the command2 is executed if, and only if, command1 returns an exit status of zero.

An OR list has the form command1 || command2, the 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.

Examples:

chenwx@chenwx ~ $ echo abc ; echo def
abc
def
chenwx@chenwx ~ $ echo abc ; echo def ;
abc
def
chenwx@chenwx ~ $ echo abc ; echo def &
abc
[1] 4928
chenwx@chenwx ~ $ def

[1]+  Done                    echo def
chenwx@chenwx ~ $ echo abc & echo def  
[1] 4942
abc
def
chenwx@chenwx ~ $ echo abc & echo def ;
[2] 4943
def
[1]   Done                    echo abc
chenwx@chenwx ~ $ abc

[2]+  Done                    echo abc
chenwx@chenwx ~ $ echo abc & echo def &
[1] 4944
abc
[2] 4945
[1]-  Done                    echo abc
def
chenwx@chenwx ~ $
[2]+  Done                    echo def
chenwx@chenwx ~ $ echo abc && echo def  
abc
def
chenwx@chenwx ~ $ echo abc && echo def ;
abc
def
chenwx@chenwx ~ $ echo abc && echo def &
[1] 4956
chenwx@chenwx ~ $ abc
def

[1]+  Done                    echo abc && echo def
chenwx@chenwx ~ $ echo abc || echo def  
abc
chenwx@chenwx ~ $ echo abc || echo def ;
abc
chenwx@chenwx ~ $ echo abc || echo def &
[1] 4959
abc
chenwx@chenwx ~ $
[1]+  Done                    echo abc || echo def

Compound Commands

Compound commands are the shell programming 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.

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

Format #1:

until test-commands; do consequent-commands; done

Format #2:

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.

Example: save the following code into ex.sh:

#!/bin/bash

cnt=5

until (( cnt == 0 ))
do
    echo ${cnt}
    let cnt--
done

Then, execute the shell script:

chenwx@chenwx ~/ex $ chmod u+x ex.sh
chenwx@chenwx ~/ex $ ./ex.sh
5
4
3
2
1

Or, execute the code in command line directly:

chenwx@chenwx ~/ex $ cnt=5
chenwx@chenwx ~/ex $ until (( cnt == 0 ))
> do
>     echo ${cnt}
>     let cnt--
> done
5
4
3
2
1

while

Format #1:

while test-commands; do consequent-commands; done

Format #2:

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.

Example: save the following code into ex.sh:

#!/bin/bash

cnt=5

while (( cnt <= 10 ))
do
    echo ${cnt}
    let cnt++
done

Then, execute the shell script:

chenwx@chenwx ~/ex $ chmod u+x ex.sh
chenwx@chenwx ~/ex $ ./ex.sh
5
6
7
8
9
10

Or, execute the code in command line directly:

chenwx@chenwx ~/ex $ cnt=5
chenwx@chenwx ~/ex $ while (( cnt <= 10 ))
> do
>     echo ${cnt}
>     let cnt++
> done
5
6
7
8
9
10

for

Format #1:

for name [ [in [words ...] ] ; ] do commands; done

Format #2:

for name [in [words ...] ]
do
    commands
done

Expand words, 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. Refer to 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.

Format #3:

for (( expr1 ; expr2 ; expr3 )) ; do commands ; done

Format #4:

for (( expr1 ; expr2 ; expr3 ))
do
    commands
done

First, the arithmetic expression expr1 is evaluated. 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.

Example #1: save the following code into ex.sh:

#!/bin/bash

# print $0
echo '$0' is $0

# print $1 ~ $N
cnt=1
for i
do
    echo "\$${cnt} is $i"
    let cnt++
done

Then, execute the shell script:

chenwx@chenwx ~/ex $ chmod u+x ex.sh
chenwx@chenwx ~/ex $ ./ex.sh a b c
$0 is ./ex.sh
$1 is a
$2 is b
$3 is c

Example #2: save the following code into ex.sh:

#!/bin/bash

# print $0
echo '$0' is $0

# print $1 ~ $N
cnt=1
for i
do
    echo "\$${cnt} is $i"
    let cnt++
done

Then, execute the shell script:

chenwx@chenwx ~/ex $ chmod u+x ex.sh
chenwx@chenwx ~/ex $ ./ex.sh a b c
$0 is ./ex.sh
$1 is a
$2 is b
$3 is c

chenwx@chenwx ~/ex $ . ./ex.sh a b c
$0 is bash
$1 is a
$2 is b
$3 is c

Example #3: save the following code into ex.sh:

#!/bin/bash

# print $0
echo '$0' is $0

# print $1 ~ $N
cnt=1
for i in $@
do
    echo "\$${cnt} is $i"
    let cnt++
done

Then, execute the shell script:

chenwx@chenwx ~/ex $ chmod u+x ex.sh
chenwx@chenwx ~/ex $ ./ex.sh a b c
$0 is ./ex.sh
$1 is a
$2 is b
$3 is c

chenwx@chenwx ~/ex $ . ./ex.sh a b c
$0 is bash
$1 is a
$2 is b
$3 is c

Example #4: save the following code into ex.sh:

#!/bin/bash

for (( cnt=5 ; cnt <= 10 ; cnt++ ))
do
    echo ${cnt}
done

then, execute the shell script:

chenwx@chenwx ~/ex $ chmod u+x ex.sh
chenwx@chenwx ~/ex $./ex.sh
5
6
7
8
9
10

break

Refer to Bourne Shell Builtins for details.

Example: save the following code into ex.sh:

#!/bin/bash

for (( cnt=5 ; cnt <= 10 ; cnt++ ))
do
    echo ${cnt}

    if [ "${cnt}" == 8 ]; then
        break
    fi
done

Then, execute the shell script:

chenwx@chenwx ~/ex $ chmod u+x ex.sh
chenwx@chenwx ~/ex $ ./ex.sh
5
6
7
8

continue

Refer to Bourne Shell Builtins for details.

Example: save the following code into ex.sh:

#!/bin/bash

for (( cnt=5 ; cnt <= 10 ; cnt++ ))
do
    echo "${cnt} before"

    if [ "${cnt}" == 8 ]; then
        continue
    fi

    echo "${cnt} after"
done

Then, execute the shell script:

chenwx@chenwx ~/ex $ chmod u+x ex.sh
chenwx@chenwx ~/ex $ ./ex.sh
5 before
5 after
6 before
6 after
7 before
7 after
8 before
9 before
9 after
10 before
10 after

Conditional Constructs

if

Format:

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.

Example: save the following code into ex.sh:

#!/bin/bash

if [[ "$1" == 0 ]]; then
    echo "Enter if statement when \$1 is $1"
elif [[ "$1" == 1 ]]; then
    echo "Enter elif statement when \$1 is $1"
else
    echo "Enter else statement when \$1 is not 0 or 1"
fi

Then, execute the shell script:

chenwx@chenwx ~/ex $ ./ex.sh 0
Enter if statement when $1 is 0
chenwx@chenwx ~/ex $ ./ex.sh 1
Enter elif statement when $1 is 1
chenwx@chenwx ~/ex $ ./ex.sh 2
Enter else statement when $1 is not 0 or 1

case

Format:

case word in
    [ [(] pattern1 [| pattern2]... ) command-list ;; ]
    ...
    [ [(] pattern1 [| pattern2]... ) command-list ;& ]
    ...
    [ [(] pattern1 [| pattern2]... ) command-list ;;& ]
    ...
    [ [()] *) always-match-command-list ;; ]
esac

case will selectively execute the command-list corresponding to the first pattern that matches word. If the shell option nocasematch (see 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 ;; ;& ;;&. The word undergoes tilde expansion, parameter expansion, command substitution, arithmetic expansion, and quote removal 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.

• 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.

Example: save the following code to ex.sh:

#!/bin/sh

print ()
{
    echo Input $1;

    case $1 in
        a) echo "case a: ---match a---" ;;
        b) echo "case b: ---match b---" ;&
        c) echo "case c: ---match c---" ;;
        d) echo "case d: ---match d---" ;;&
        e) echo "case e: ---match e---" ;;
        f) echo "case f: ---match f---" ;;&
        *) echo "case *: ---match all---" ;;
    esac
}

print $1

Then, execute the shell script:

chenwx@chenwx ~/ex $ . ./ex.sh a
Input a
case a: ---match a---
chenwx@chenwx ~/ex $ . ./ex.sh b
Input b
case b: ---match b---
case c: ---match c---
chenwx@chenwx ~/ex $ . ./ex.sh c
Input c
case c: ---match c---
chenwx@chenwx ~/ex $ . ./ex.sh d
Input d
case d: ---match d---
case *: ---match all---
chenwx@chenwx ~/ex $ . ./ex.sh e
Input e
case e: ---match e---
chenwx@chenwx ~/ex $ . ./ex.sh f
Input f
case f: ---match f---
case *: ---match all---
chenwx@chenwx ~/ex $ . ./ex.sh x
Input x
case *: ---match all---

select

The select construct allows the easy generation of menus. It has almost the same syntax as the for command:

Format #1:

select name [in words ...]; do commands; done

Format #2:

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, refer to Special Parameters. 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.

Example: save the following code to ex.sh:

#!/bin/bash

print ()
{
    select name in a b
    do
        if [ "$name" == "a" ]; then
            echo "\$REPLY is $REPLY, \$name is $name, let\'s try again!"
        fi
        if [ "$name" == "b" ]; then
            echo "\$REPLY is $REPLY, \$name is $name, break now!"
            break
        fi
    done
}

print

Then, execute the shell script:

chenwx@chenwx ~/Downloads/ex $ . ./ex.sh
1) a
2) b
#? 1
$REPLY is 1, $name is a, let\'s try again!
#? 2
$REPLY is 2, $name is b, break now!

(( expression ))

Format:

(( expression ))

The arithmetic expression expression is evaluated. 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" for a full description of the let builtin, refer to Bash Builtin Commands.

Example:

chenwx@chenwx ~ $ echo $(( 3+4 ))
7

chenwx@chenwx ~ $ SUM1=$(( 3+4 ))
chenwx@chenwx ~ $ echo $SUM1
7

chenwx@chenwx ~ $ let SUM2="3+4"
chenwx@chenwx ~ $ echo $SUM2
7

chenwx@chenwx ~/blog $ if (( 1+1 )); then echo "if statement"; else echo "else statement"; fi
if statement

chenwx@chenwx ~/blog $ if (( 1-1 )); then echo "if statement"; else echo "else statement"; fi
else statement

[[ expression ]]

Format:

[[ expression ]]

Return a status of 0 or 1 depending on the evaluation of the conditional expression expression. Expressions are composed of the primaries described 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 in Pattern Matching, as if the extglob shell option were enabled (see The shopt Builtin). The = operator is identical to ==. If the shell option nocasematch is enabled (see The shopt Builtin), 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 an 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 shell option nocasematch is enabled (see The shopt Builtin), 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:

The && and || operators do not evaluate expression2 if the value of expression1 is sufficient to determine the return value of the entire conditional expression.

Grouping Commands

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 )

Format:

( 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; }

Format:

{ 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.

[NOTE] 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 blanks 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.

Coprocesses

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.

Format:

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; 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. 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.

GNU Parallel

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 -print0 option and parallel’s -0 option.

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 -X option:

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 -k option

{ 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 commnds to a shell. Parallel can speed this up. Assuming that file contains 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.

Shell Functions

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 ]

name ()
{
    compound-command
} [ redirections ]

or,

function name [()] compound-command [ redirections ]

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. 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 -f option to the unset builtin.

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 blanks 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.

Functions are invoked using this syntax:

func-name "param1" "param2" ...

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.

Function names and definitions may be listed with the -f option to the declare (typeset) builtin command (see Bash Builtins). The -F option 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 -f option to the export builtin (see Bourne Shell Builtins). Note that shell functions and variables with the same name may result in multiple identically-named entries in the environment passed to the shell’s children. Care should be taken in cases where this may cause a problem.

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.

Examples: save the following code into ex.sh:

#! /bin/bash

###################################
# Part 1: Definition of functions
###################################

func1 ()
{
    echo -e "Executing function $FUNCNAME with $# positional parameters...\n"

    local cnt=${1}
    until (( cnt == 0 ))
    do
        echo ${cnt}
        let cnt--
    done
}

function func2 ()
{
    echo -e "Executing function $FUNCNAME with $# positional parameters...\n"

    local cnt=${1}
    while (( cnt <= 10 ))
    do
        echo ${cnt}
        let cnt++
    done
} > ~/Downloads/ex/output.txt

###################################
# Part 2: Execution of functions
###################################

echo -e "== 1 == Call function func1() with 1 input parameter\n"
func1 5

echo -e "\n== 2 == Call function func2() with 1 input parameter\n"
func2 5
echo -e "Content of ~/Downloads/ex/output.txt:\n"
cat ~/Downloads/ex/output.txt

echo -e "\n== 3 == Local variable cnt in function func1() and func2() has value: >>${cnt}<<\n"

echo -e "== 4 == Unset function func1():\n"
unset func1

echo -e "== 5 == Call function func1() again after it's deleted\n"
func1 5

Then, execute the shell script ex.sh:

chenwx@chenwx ~/Downloads/ex $ chmod +x ./ex.sh
chenwx@chenwx ~/Downloads/ex $ ./ex.sh
== 1 == Call function func1() with 1 input parameter

Executing function func1 with 1 positional parameters...

5
4
3
2
1

== 2 == Call function func2() with 1 input parameter

Content of ~/Downloads/ex/output.txt:

Executing function func2 with 1 positional parameters...

5
6
7
8
9
10

== 3 == Local variable cnt in function func1() and func2() has value: >><<

== 4 == Unset function func1():

== 5 == Call function func1() again after it's deleted

./ex.sh: line 42: func1: command not found

Shell Parameters

Positional Parameters

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. When a positional parameter consisting of more than a single digit is expanded, it must be enclosed in braces, that’s ${N}.

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.

Special Parameters

The shell treats several parameters specially. These parameters may only be referenced; assignment to them is not allowed.

Shell Expansions

Expansion is performed on the command line after it has been split into tokens, refer to step 4 in Shell Operation. There are several kinds of expansion performed:

1. Brace expansion
2. Tilde expansion
3. Parameter and variable expansion
4. Arithmetic expansion
5. Command substitution
6. Process substitution (on systems that support named pipes (FIFOs) or the /dev/fd method of naming open files)
7. Word splitting
8. 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.

The order of expansions is:

Only brace expansion, word splitting, and filename expansion can change 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 $@ (see Special Parameters) and ${name[@]} (see Arrays).

After all expansions, quote removal (see Quote Removal) is performed.

Brace expansion

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.

Format #1:

[preamble]{str1,[str2,[str3,...]]}[postscript]

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.

Examples:

Format #2:

{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.

Examples:

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. To avoid conflicts with parameter expansion, the string ${ is not considered eligible for brace expansion.

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.

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}}

Tilde expansion

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:

Parameter and variable expansion

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.

Format #1

${parameter}

The value of parameter is substituted. The parameter is a shell parameter as described in 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 (!), it introduces a level of variable indirection. Bash uses the value of the variable formed from the rest of parameter as the name of the variable; this variable is then expanded and that value is used in the rest of the substitution, rather than the value of parameter itself. This is known as 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.

Examples:

$ p1=10

Format #2

${parameter:-word}

If parameter is unset or null, the expansion of word is substituted. Otherwise, the value of parameter is substituted.

Examples:

$ unset p1
$ p2=20

Examples:

$ p1=10
$ p2=20

Format #3

${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.

Examples:

$ unset p1
$ p2=20

Examples:

$ unset p1
$ p2=20

Examples:

$ unset p1
$ p2=20

Format #4

${parameter:?word}

If parameter is null or unset, the expansion of word is written to the standard error and the shell, if it is not interactive, exits. Otherwise, the value of parameter is substituted.

Examples:

$ unset p1
$ p2=20

Examples:

$ unset p1
$ p2=20

Examples:

$ p1=10
$ p2=20

Format #5

${parameter:+word}

If parameter is null or unset, nothing is substituted. Otherwise, the expansion of word is substituted.

Examples:

$ unset p1
$ p2=20

Examples:

$ p1=10
$ p2=20

Examples:

$ p1=10
$ p2=20

Format #6

${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 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.

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.

Examples:

$ string=01234567890abcdefgh

Examples:

$ set -- 01234567890abcdefgh

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.

Examples:

$ set -- 1 2 3 4 5 6 7 8 9 0 a b c d e f g h

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.

Examples:

$ array=(0 1 2 3 4 5 6 7 8 9 0 a b c d e f g h)

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.

Format #7

${!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.

Examples:

$ string1=01234567890abcdefgh
$ string2=abcdefghijk01234567

Format #8

${!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.

Examples:

$ array1=(0 1 2 3 4 5 6 7 8 9 0 a b c d e f g h)

Format #9

${#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.

Examples:

$ string=01234567890abcdefgh
$ set -- 0 1 2 3 4 5 6 7 8 9
$ array=(0 1 2 3 4 5 6 7 8 9 0 a b c d e f g h)

Format #10

${parameter#word}
${parameter##word}

The word is expanded to produce a pattern just as in filename expansion. 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.

Examples:

$ string=012345012345abcdeabcde
$ set -- 0 1 2 3 4 5 6 7 8 9
$ array=(0 1 2 3 4 5 6 7 8 9 0 a b c d e f g h)

Format #11

${parameter%word}
${parameter%%word}

The word is expanded to produce a pattern just as in filename expansion. 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.

Examples:

$ string=012345012345abcdeabcde
$ set -- 0 1 2 3 4 5 6 7 8 9
$ array=(0 1 2 3 4 5 6 7 8 9 0 a b c d e f g h)

Format #12

${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.

• 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 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.

Examples:

$ string=012345012345abcdeabcde
$ set -- 0 1 2 3 4 5 6 7 8 9
$ array=(0 1 2 3 4 5 6 7 8 9 0 a b c d e f g h)

Format #13

${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 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.

Examples:

$ string=abcdeABCDEabcdeABCDE
$ set -- ABCDEabcdeABCDEabcde
$ array=(A B C D E a b c d e A B C D E a b c d e)

Arithmetic expansion

Format:

$(( expression ))

Arithmetic expansion allows the evaluation of an arithmetic expression and the substitution of the result.

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 in Shell Arithmetic. If the expression is invalid, Bash prints a message indicating failure to the standard error and no substitution occurs.

Examples:

Command Substitution

Formats:

$(command)
`command`

Command substitution allows the output of a command to replace the command itself.

Bash performs the expansion by executing command 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.

Examples:

Process Substitution

Formats:

<(list)
>(list)

Process substitution is supported on systems that support named pipes (FIFOs) or the /dev/fd method of naming open files.

The process list is run with its input or output connected to a FIFO or some file in /dev/fd. The name of this file 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.

When available, process substitution is performed simultaneously with parameter and variable expansion, command substitution, and arithmetic expansion.

Word Splitting

The shell scans the results of parameter and variable 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 and tab 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. 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.

Note that if no expansion occurs, no splitting is performed.

Filename Expansion

After word splitting, unless the -f option has been set, 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 shell option failglob 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. When matching a filename, the slash character must always be matched explicitly. In other cases, the . character is not treated specially.

See the description of shopt in The shopt Builtin for a description of the nocaseglob, nullglob, failglob, and dotglob options.

The shell variable GLOBIGNORE may be used to restrict the set of filenames matching a pattern.

• If GLOBIGNORE is set, each matching filename that also matches one of the patterns in GLOBIGNORE is removed from the list of matches. 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.

Pattern Matching

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:

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:

Quote Removal

After the preceding expansions, all unquoted occurrences of the characters \, ', and " that did not result from one of the above expansions are removed.

Redirections

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.

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.

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.

Bash handles several filenames specially when they are used in redirections, as described in the following table:

Redirection operators:

Executing Commands

Simple Command Expansion

When a simple command is executed, the shell performs the following expansions, assignments, and redirections, from left to right.

1. The words that the parser has marked as variable assignments (those preceding the command name) and redirections are saved for later processing.

2. The words that are not variable assignments or redirections are expanded (see Shell Expansions). If any words remain after expansion, the first word is taken to be the name of the command and the remaining words are the arguments.

3. Redirections are performed as described in Redirections.

4. The text after the = in each variable assignment undergoes tilde expansion, parameter expansion, command substitution, arithmetic expansion, and quote removal before being assigned to the variable.

5. 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.

Command Search and Execution

The following figure shows the command search and execution:

Command Execution Environment

The shell has an execution environment, which consists of the following:

• open files inherited by the shell at invocation, as modified by redirections supplied to the exec builtin

• the current working directory as set by cd, pushd, or popd, or inherited by the shell at invocation

• the file creation mode mask as set by umask or inherited from the shell’s parent

• current traps set by trap

• shell parameters that are set by variable assignment or with set or inherited from the shell’s parent in the environment

• shell functions defined during execution or inherited from the shell’s parent in the environment

• options enabled at invocation (either by default or with command-line arguments) or by set

• options enabled by shopt (see The shopt Builtin)

• shell aliases defined with alias (see Aliases)

• various process IDs, including those of background jobs, the value of $$, 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.

• the shell’s open files, plus any modifications and additions specified by redirections to the command

• the current working directory

• the file creation mode mask

• shell variables and functions marked for export, along with variables exported for the command, passed in the environment (see Environment)

• traps caught by the shell are reset to the values inherited from the shell’s parent, and traps ignored by the shell are ignored

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 -e option from the parent shell. When not in posix mode, Bash clears the -e option 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.

Environment

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 -k option 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, see Special Parameters.

Exit Status

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.

• If a command is found but is not executable, the return status is 126.

• If a command is not found, the child process created to execute it returns a status of 127.

• If a command terminates on a fatal signal whose number is N, Bash uses the value 128+N as the exit status.

• 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 and some of the list constructs.

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.

Check the special parameter $? to get the exit status of the most recently executed foreground pipeline.

Signals

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.

Shell Scripts

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 -c nor -s option 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.

Example: save the following code to ex1.sh:

#! /bin/bash

echo Your input is "$1"

and save the following code to ex2.sh:

#! /bin/bash -v

echo Your input is "$1"

Then, execute the following commands:

chenwx@chenwx ~ $ chmod +x ex1.sh ex2.sh
chenwx@chenwx ~ $ ./ex1.sh 1
Your input is 1
chenwx@chenwx ~ $ ./ex2.sh 2
#! /bin/bash -v

echo Your input is "$1"
Your input is 2

Shell Builtin Commands

Bourne Shell Builtins

The following shell builtin commands are inherited from the Bourne Shell. These commands are implemented as specified by the POSIX standard, see Shell and Utilities, Issue 7 (XCU7).

Bash Builtin Commands

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, see Shell and Utilities, Issue 7 (XCU7).

Modifying Shell Behavior

The set Builtin

The shell builtin 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.

Formats:

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:

The -o option-name sets the option corresponding to option-name:

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.

Examples:

chenwx@chenwx ~ $ set | more
BASH=/bin/bash
BASHOPTS=checkwinsize:cmdhist:complete_fullquote:expand_aliases:extglob:extquote:force_fignore:interactive_comments:progcomp:promptvars:sourcepath
BASH_ALIASES=()
BASH_ARGC=()
BASH_ARGV=()
BASH_CMDS=()
BASH_COMPLETION_COMPAT_DIR=/etc/bash_completion.d
BASH_LINENO=()
BASH_REMATCH=()
BASH_SOURCE=()
BASH_VERSINFO=([0]="4" [1]="3" [2]="11" [3]="1" [4]="release" [5]="x86_64-pc-linux-gnu")
BASH_VERSION='4.3.11(1)-release'
CLUTTER_IM_MODULE=ibus
COLORTERM=gnome-terminal
...

chenwx@chenwx ~ $ echo $-
bhimBH

The shopt Builtin

Format:

shopt [-pqsu] [-o] [optname ...]

The shell builtin shopt allows you to change additional shell optional behavior. Toggle the values of settings controlling optional shell behavior. The settings can be either those listed below, or, if the -o option is used, those available with the -o option to the set builtin command (see The set Builtin).

If either -s or -u is 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, nonzero 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:

The return status when listing options is zero if all optnames are enabled, nonzero otherwise. When setting or unsetting options, the return status is zero unless an optname is not a valid shell option.

Examples:

chenwx@chenwx ~ $ shopt -p
shopt -u autocd
shopt -u cdable_vars
shopt -u cdspell
shopt -u checkhash
shopt -u checkjobs
shopt -s checkwinsize
shopt -s cmdhist
shopt -u compat31
shopt -u compat32
shopt -u compat40
shopt -u compat41
shopt -u compat42
shopt -s complete_fullquote
shopt -u direxpand
shopt -u dirspell
shopt -u dotglob
shopt -u execfail
shopt -s expand_aliases
shopt -u extdebug
shopt -s extglob
shopt -s extquote
shopt -u failglob
shopt -s force_fignore
shopt -u globstar
shopt -u globasciiranges
shopt -u gnu_errfmt
shopt -u histappend
shopt -u histreedit
shopt -u histverify
shopt -u hostcomplete
shopt -u huponexit
shopt -s interactive_comments
shopt -u lastpipe
shopt -u lithist
shopt -u login_shell
shopt -u mailwarn
shopt -u no_empty_cmd_completion
shopt -u nocaseglob
shopt -u nocasematch
shopt -u nullglob
shopt -s progcomp
shopt -s promptvars
shopt -u restricted_shell
shopt -u shift_verbose
shopt -s sourcepath
shopt -u xpg_echo

chenwx@chenwx ~ $ shopt -sp
shopt -s checkwinsize
shopt -s cmdhist
shopt -s complete_fullquote
shopt -s expand_aliases
shopt -s extglob
shopt -s extquote
shopt -s force_fignore
shopt -s interactive_comments
shopt -s progcomp
shopt -s promptvars
shopt -s sourcepath

chenwx@chenwx ~ $ shopt -p autocd cdspell
shopt -u autocd
shopt -u cdspell

chenwx@chenwx ~ $ shopt autocd cdspell
autocd         	off
cdspell        	off

Special Builtins

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:

1. Special builtins are found before shell functions during command lookup.
2. If a special builtin returns an error status, a non-interactive shell exits.
3. Assignment statements preceding the command stay in effect in the shell environment after the command completes.

When Bash is not executing in POSIX mode, these builtins behave no differently than the rest of the Bash builtin commands.

These are the POSIX special builtins:

break : . continue eval exec exit export readonly return set shift trap unset

Shell Variables

Bash automatically assigns default values to a number of variables.

Bourne Shell Variables

Bash uses certain shell variables in the same way as the Bourne shell. In some cases, Bash assigns a default value to the variable.

Bash Variables

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 Features

Invoking Bash

Format #1:

bash [long-opt] [-ir] [-abefhkmnptuvxdBCDHP] [-o option] [-O shopt_option] [argument ...]

Format #2:

bash [long-opt] -s [-abefhkmnptuvxdBCDHP] [-o option] [-O shopt_option] [argument ...]

Format #3:

bash [long-opt] [-abefhkmnptuvxdBCDHP] [-o option] [-O shopt_option] -c string [argument ...]

All of the single-character options used with the set builtin can be used as options when the shell is invoked.

These options must appear on the command line before the single-character options to be recognized:

There are several single-character options that may be supplied at invocation which are not available with the set builtin.

A login shell is one whose first character of argument zero is -, or one invoked with the --login option.

An interactive shell is one started without non-option arguments, unless -s is specified, without specifying the -c option, and whose input and output are both connected to terminals, or one started with the -i option.

If arguments remain after option processing, and neither the -c nor the -s option has been supplied, the first argument is assumed to be the name of a file containing shell commands. When Bash is invoked in this fashion, $0 is set to the name of the file, and the positional parameters are set to the remaining arguments. Bash reads and executes commands from this file, then exits. Bash’s exit status is the exit status of the last command executed in the script. If no commands are executed, the exit status is 0.

Bash Startup Files

This section describes how Bash executes its startup files. If any of the files exist but cannot be read, Bash reports an error. Tildes are expanded in filenames as described above under Tilde Expansion (see Tilde Expansion).

Invoked as an interactive login shell, or with --login

When Bash is invoked as an interactive login shell, or as a non-interactive shell with the --login option, it first reads and executes commands from the file /etc/profile, if that file exists. After reading that file, it looks for ~/.bash_profile, ~/.bash_login, and ~/.profile, in that order, and reads and executes commands from the first one that exists and is readable. The --noprofile option may be used when the shell is started to inhibit this behavior.

When a login shell exits, Bash reads and executes commands from the file ~/.bash_logout, if it exists.

Invoked as an interactive non-login shell

When an interactive shell that is not a login shell is started, Bash reads and executes commands from ~/.bashrc, if that file exists. This may be inhibited by using the --norc option. The --rcfile file option will force Bash to read and execute commands from file instead of ~/.bashrc.

So, typically, your ~/.bash_profile contains the following line after (or before) any login-specific initializations.

if [ -f ~/.bashrc ]; then . ~/.bashrc; fi

Invoked non-interactively

When Bash is started non-interactively, to run a shell script, for example, it looks for the variable BASH_ENV in the environment, expands its value if it appears there, and uses the expanded value as the name of a file to read and execute. Bash behaves as if the following command were executed:

if [ -n "$BASH_ENV" ]; then . "$BASH_ENV"; fi

but the value of the PATH variable is not used to search for the filename.

As noted above, if a non-interactive shell is invoked with the --login option, Bash attempts to read and execute commands from the login shell startup files.

Invoked with name sh

If Bash is invoked with the name sh, it tries to mimic the startup behavior of historical versions of sh as closely as possible, while conforming to the POSIX standard as well.

When invoked as an interactive login shell, or as a non-interactive shell with the --login option, it first attempts to read and execute commands from /etc/profile and ~/.profile, in that order. The --noprofile option may be used to inhibit this behavior. When invoked as an interactive shell with the name sh, Bash looks for the variable ENV, expands its value if it is defined, and uses the expanded value as the name of a file to read and execute. Since a shell invoked as sh does not attempt to read and execute commands from any other startup files, the --rcfile option has no effect. A non-interactive shell invoked with the name sh does not attempt to read any other startup files.

When invoked as sh, Bash enters POSIX mode after the startup files are read.

Invoked in POSIX mode

When Bash is started in POSIX mode, as with the --posix command line option, it follows the POSIX standard for startup files. In this mode, interactive shells expand the ENV variable and commands are read and executed from the file whose name is the expanded value. No other startup files are read.

Invoked by remote shell daemon

Bash attempts to determine when it is being run with its standard input connected to a network connection, as when executed by the remote shell daemon, usually rshd, or the secure shell daemon sshd. If Bash determines it is being run in this fashion, it reads and executes commands from ~/.bashrc, if that file exists and is readable. It will not do this if invoked as sh. The --norc option may be used to inhibit this behavior, and the --rcfile option may be used to force another file to be read, but neither rshd nor sshd generally invoke the shell with those options or allow them to be specified.

Invoked with unequal effective and real UID/GIDs

If Bash is started with the effective user (group) id not equal to the real user (group) id, and the -p option is not supplied, no startup files are read, shell functions are not inherited from the environment, the SHELLOPTS, BASHOPTS, CDPATH, and GLOBIGNORE variables, if they appear in the environment, are ignored, and the effective user id is set to the real user id. If the -p option is supplied at invocation, the startup behavior is the same, but the effective user id is not reset.

Interactive Shells

What is an Interactive Shell?

An interactive shell is one started without non-option arguments, unless -s is specified, without specifying the -c option, and whose input and error output are both connected to terminals (as determined by isatty(3)), or one started with the -i option.

An interactive shell generally reads from and writes to a user’s terminal.

The -s invocation option may be used to set the positional parameters when an interactive shell is started.

Is this Shell Interactive?

To determine within a startup script whether or not Bash is running interactively, test the value of the - special parameter. It contains i when the shell is interactive. For example:

case "$-" in
    *i*) echo This shell is interactive ;;
    *) echo This shell is not interactive ;;
esac

Alternatively, startup scripts may examine the variable PS1; it is unset in non-interactive shells, and set in interactive shells. Thus:

if [ -z "$PS1" ]; then
    echo This shell is not interactive
else
    echo This shell is interactive
fi

Interactive Shell Behavior

When the shell is running interactively, it changes its behavior in several ways.

1. Startup files are read and executed as described in Bash Startup Files.

2. Job Control is enabled by default. When job control is in effect, Bash ignores the keyboard-generated job control signals SIGTTIN, SIGTTOU, and SIGTSTP.

3. Bash expands and displays PS1 before reading the first line of a command, and expands and displays PS2 before reading the second and subsequent lines of a multi-line command.

4. Bash executes the value of the PROMPT_COMMAND variable as a command before printing the primary prompt, $PS1 (see Bash Variables).

5. Readline (see Command Line Editing) is used to read commands from the user’s terminal.

6. Bash inspects the value of the ignoreeof option to set -o instead of exiting immediately when it receives an EOF on its standard input when reading a command (see The set Builtin).

7. Command history (see Bash History Facilities) and history expansion (see History Interaction) are enabled by default. Bash will save the command history to the file named by $HISTFILE when a shell with history enabled exits.

8. Alias expansion (see Aliases) is performed by default.

9. In the absence of any traps, Bash ignores SIGTERM (see Signals).

10. In the absence of any traps, SIGINT is caught and handled (see Signals). SIGINT will interrupt some shell builtins.

11. An interactive login shell sends a SIGHUP to all jobs on exit if the huponexit shell option has been enabled (see Signals).

12. The -n invocation option is ignored, and set -n has no effect (see The set Builtin).

13. Bash will check for mail periodically, depending on the values of the MAIL, MAILPATH, and MAILCHECK shell variables (see Bash Variables).

14. Expansion errors due to references to unbound shell variables after set -u has been enabled will not cause the shell to exit (see The set Builtin).

15. The shell will not exit on expansion errors caused by var being unset or null in ${var:?word} expansions (see Shell Parameter Expansion).

16. Redirection errors encountered by shell builtins will not cause the shell to exit.

17. When running in POSIX mode, a special builtin returning an error status will not cause the shell to exit (see Bash POSIX Mode).

18. A failed exec will not cause the shell to exit (see Bourne Shell Builtins).

19. Parser syntax errors will not cause the shell to exit.

20. Simple spelling correction for directory arguments to the cd builtin is enabled by default (see the description of the cdspell option to the shopt builtin in The shopt Builtin).

21. The shell will check the value of the TMOUT variable and exit if a command is not read within the specified number of seconds after printing $PS1 (see Bash Variables).

Bash Conditional Expressions

Conditional expressions are used by the [[ compound command and the test and [ builtin commands.

Expressions may be unary or binary. Unary expressions are often used to examine the status of a file. There are string operators and numeric comparison operators as well. If the file argument to one of the primaries is of the form /dev/fd/N, then file descriptor N is checked. If the file argument to one of the primaries is one of /dev/stdin, /dev/stdout, or /dev/stderr, file descriptor 0, 1, or 2, respectively, is checked.

When used with [[, the < and > operators sort lexicographically using the current locale. The test command uses ASCII ordering.

Unless otherwise specified, primaries that operate on files follow symbolic links and operate on the target of the link, rather than the link itself.

Shell Arithmetic

The shell allows arithmetic expressions to be evaluated, as one of the shell expansions or by the let and the -i option to the declare builtins.

Evaluation is done in fixed-width integers with no check for overflow, though division by 0 is trapped and flagged as an error. The operators and their precedence, associativity, and values are the same as in the C language. The following list of operators is grouped into levels of equal-precedence operators. The levels are listed in order of decreasing precedence.

Shell variables are allowed as operands; parameter expansion is performed before the expression is evaluated. Within an expression, shell variables may also be referenced by name without using the parameter expansion syntax. A shell variable that is null or unset evaluates to 0 when referenced by name without using the parameter expansion syntax. The value of a variable is evaluated as an arithmetic expression when it is referenced, or when a variable which has been given the integer attribute using declare -i is assigned a value. A null value evaluates to 0. A shell variable need not have its integer attribute turned on to be used in an expression.

Constants with a leading 0 are interpreted as octal numbers. A leading 0x or 0X denotes hexadecimal. Otherwise, numbers take the form [base#]n, where the optional base is a decimal number between 2 and 64 representing the arithmetic base, and n is a number in that base. If base# is omitted, then base 10 is used. When specifying n, the digits greater than 9 are represented by the lowercase letters, the uppercase letters, @, and _, in that order. If base is less than or equal to 36, lowercase and uppercase letters may be used interchangeably to represent numbers between 10 and 35.

Operators are evaluated in order of precedence. Sub-expressions in parentheses are evaluated first and may override the precedence rules above.

Aliases

Aliases allow a string to be substituted for a word when it is used as the first word of a simple command. The shell maintains a list of aliases that may be set and unset with the alias and unalias builtin commands, refer to Bash Builtin Commands for details.

• The first word of each simple command, if unquoted, is checked to see if it has an alias. If so, that word is replaced by the text of the alias.

• The characters / $ ` = and any of the shell metacharacters or quoting characters may not appear in an alias name. The replacement text may contain any valid shell input, including shell metacharacters.

• The first word of the replacement text is tested for aliases, but a word that is identical to an alias being expanded is not expanded a second time.

• If the last character of the alias value is a blank, then the next command word following the alias is also checked for alias expansion.

• Aliases are not expanded when the shell is not interactive, unless the shell option expand_aliases is set using shopt, refer to The shopt Builtin:

$ shopt -s expand_aliases

Bash always reads at least one complete line of input before executing any of the commands on that line. Aliases are expanded when a command is read, not when it is executed.

• Therefore, an alias definition appearing on the same line as another command does not take effect until the next line of input is read. The commands following the alias definition on that line are not affected by the new alias.

• Aliases are expanded when a function definition is read, not when the function is executed, because a function definition is itself a compound command. As a consequence, aliases defined in a function are not available until after that function is executed. To be safe, always put alias definitions on a separate line, and do not use alias in compound commands.

For almost every purpose, shell functions are preferred over aliases.

Frequently Used Aliases

The following is some frequently used aliases:

# Basic
alias ll="ls --color=auto -lh"
alias lla="ls --color=auto -lha"
alias dusum="du -sh"

# Linux kernel
alias checkpatch="~/linux/scripts/checkpatch.pl -terse -file"

# find specified file in current directory
alias findf="find . -noignore_readdir_race -nowarn -name"

# find installed kernel, e.g.: findkernel "*3.15*"
alias findkernel="find /boot /var -ignore_readdir_race -nowarn -name"

# find specified keyword in header files (.h)
alias findh="find . -noignore_readdir_race -nowarn -name '*.h' | xargs grep --color -n -s"

# find specified keyword in source files (.c)
alias findc="find . -noignore_readdir_race -nowarn -name '*.c' | xargs grep --color -n -s"

# find specified keyword in header and source files (.h, .c)
alias findahc="find . -noignore_readdir_race -nowarn -name '*.[hc]' | xargs grep --color -n -s"

# find specified keyword in all kind of files
alias findaf="find . -noignore_readdir_race -nowarn -type f | xargs grep --color -n -s"

# remove tail spaces, used when "git commit" failed caused by pre-commit script
# usage: rmtailspace lib.c > lib.c.notailspace
alias rmtailspace="sed -e \"s/[ \t]*$//g\""

# clean git directory, build git, and install built git
alias cleangit="make distclean"
alias buildgit="make prefix=/usr all doc info"
alias installgit="sudo make prefix=/usr install install-doc install-html install-info"

Arrays

Bash provides one-dimensional indexed and associative array variables. Any variable may be used as an indexed array; the declare builtin will explicitly declare an array. There is no maximum limit on the size of an array, nor any requirement that members be indexed or assigned contiguously. Indexed arrays are referenced using integers (including arithmetic expressions, see Shell Arithmetic) and are zero-based; associative arrays use arbitrary strings. Unless otherwise noted, indexed array indices must be nonnegative integers.

An indexed array is created automatically if any variable is assigned to using the syntax name[subscript]=value. The subscript is treated as an arithmetic expression that must evaluate to a number. To explicitly declare an array, use declare -a name. The syntax declare -a name[subscript] is also accepted; the subscript is ignored. Associative arrays are created using declare -A name.

Attributes may be specified for an array variable using the declare and readonly builtins. Each attribute applies to all members of an array.

Arrays are assigned to using compound assignments of the form name=(value1 value2 ...) where each value is of the form [subscript]=string. Indexed array assignments do not require anything but string. When assigning to indexed arrays, if the optional subscript is supplied, that index is assigned to; otherwise the index of the element assigned is the last index assigned to by the statement plus one. Indexing starts at zero.

When assigning to an associative array, the subscript is required.

This syntax is also accepted by the declare builtin. Individual array elements may be assigned to using the name[subscript]=value syntax introduced above.

When assigning to an indexed array, if name is subscripted by a negative number, that number is interpreted as relative to one greater than the maximum index of name, so negative indices count back from the end of the array, and an index of -1 references the last element.

Any element of an array may be referenced using ${name[subscript]}. The braces are required to avoid conflicts with the shell’s filename expansion operators. If the subscript is @ or *, the word expands to all members of the array name. These subscripts differ only when the word appears within double quotes. If the word is double-quoted,

• ${name[*]} expands to a single word with the value of each array member separated by the first character of the IFS variable, and
• ${name[@]} expands each element of name to a separate word.

When there are no array members, ${name[@]} expands to nothing. 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. This is analogous to the expansion of the special parameters @ and *. ${#name[subscript]} expands to the length of ${name[subscript]}. If subscript is @ or *, the expansion is the number of elements in the array. Referencing an array variable without a subscript is equivalent to referencing with a subscript of 0. If the subscript used to reference an element of an indexed array evaluates to a number less than zero, it is interpreted as relative to one greater than the maximum index of the array, so negative indices count back from the end of the array, and an index of -1 refers to the last element.

An array variable is considered set if a subscript has been assigned a value. The null string is a valid value.

It is possible to obtain the keys (indices) of an array as well as the values. ${!name[@]} and ${!name[*]} expand to the indices assigned in array variable name. The treatment when in double quotes is similar to the expansion of the special parameters @ and * within double quotes.

The unset builtin is used to destroy arrays. unset name[subscript] destroys the array element at index subscript. Negative subscripts to indexed arrays are interpreted as described above. Care must be taken to avoid unwanted side effects caused by filename expansion. unset name, where name is an array, removes the entire array. A subscript of * or @ also removes the entire array.

The declare, local, and readonly builtins each accept a -a option to specify an indexed array and a -A option to specify an associative array. If both options are supplied, -A takes precedence. The read builtin accepts a -a option to assign a list of words read from the standard input to an array, and can read values from the standard input into individual array elements. The set and declare builtins display array values in a way that allows them to be reused as input.

Examples: indexed array

chenwx@chenwx ~ $ ia1[1]=10
chenwx@chenwx ~ $ echo ${ia1[0]}

chenwx@chenwx ~ $ echo ${ia1[1]}
10

chenwx@chenwx ~ $ declare -a ia2
chenwx@chenwx ~ $ echo ${ia1[*]}

chenwx@chenwx ~ $ echo ${ia1[@]}

chenwx@chenwx ~ $ ia2=([0]=0 [1]=100 [2]=200)
chenwx@chenwx ~ $ echo ${ia2}
0
chenwx@chenwx ~ $ echo ${ia2[0]}
0
chenwx@chenwx ~ $ echo ${ia2[1]}
100
chenwx@chenwx ~ $ echo ${ia2[2]}
200
chenwx@chenwx ~ $ echo ${ia2[*]}
0 100 200
chenwx@chenwx ~ $ echo ${ia2[@]}
0 100 200

chenwx@chenwx ~ $ echo ${#ia2}
1
chenwx@chenwx ~ $ echo ${#ia2[0]}
1
chenwx@chenwx ~ $ echo ${#ia2[1]}
2
chenwx@chenwx ~ $ echo ${#ia2[2]}
3

chenwx@chenwx ~ $ echo ${#ia2[*]}
3
chenwx@chenwx ~ $ echo ${#ia2[@]}
3

chenwx@chenwx ~ $ echo ${ia2[-1]}
200
chenwx@chenwx ~ $ echo ${ia2[-2]}
10
chenwx@chenwx ~ $ echo ${ia2[-3]}
0
chenwx@chenwx ~ $ echo ${ia2[-4]}
bash: ia2: bad array subscript

chenwx@chenwx ~ $ echo ${!ia2[@]}
0 1 2
chenwx@chenwx ~ $ echo ${!ia2[*]}
0 1 2

chenwx@chenwx ~ $ unset ia2[1]
chenwx@chenwx ~ $ echo ${!ia2[*]}
0 2
chenwx@chenwx ~ $ unset ia2   
chenwx@chenwx ~ $ echo ${!ia2[*]}

chenwx@chenwx ~ $ echo ${ia2[0]}

chenwx@chenwx ~ $ echo ${ia2[1]}

chenwx@chenwx ~ $ echo ${ia2[2]}

Examples: associative array

chenwx@chenwx ~ $ declare -A aa1
chenwx@chenwx ~ $ echo ${aa1[*]}

chenwx@chenwx ~ $ echo ${aa1[@]}

chenwx@chenwx ~ $ aa1=([0]=10 [1]=100 [2]=1000)
chenwx@chenwx ~ $ echo ${aa1}
10
chenwx@chenwx ~ $ echo ${aa1[0]}
10
chenwx@chenwx ~ $ echo ${aa1[1]}
100
chenwx@chenwx ~ $ echo ${aa1[2]}
1000
chenwx@chenwx ~ $ echo ${aa1[*]}
10 100 1000
chenwx@chenwx ~ $ echo ${aa1[@]}
10 100 1000

chenwx@chenwx ~ $ echo ${#aa1}
2
chenwx@chenwx ~ $ echo ${#aa1[0]}
2
chenwx@chenwx ~ $ echo ${#aa1[1]}
3
chenwx@chenwx ~ $ echo ${#aa1[2]}
4

chenwx@chenwx ~ $ echo ${#aa1[*]}
3
chenwx@chenwx ~ $ echo ${#aa1[@]}
3

chenwx@chenwx ~ $ echo ${aa1[-1]}

chenwx@chenwx ~ $ echo ${aa1[-2]}

chenwx@chenwx ~ $ echo ${aa1[-3]}

chenwx@chenwx ~ $ echo ${aa1[-4]}

chenwx@chenwx ~ $ echo ${!aa1[@]}
0 1 2
chenwx@chenwx ~ $ echo ${!aa1[*]}
0 1 2

chenwx@chenwx ~ $ unset aa1[1]
chenwx@chenwx ~ $ echo ${!aa1[*]}
0 2
chenwx@chenwx ~ $ unset aa1
chenwx@chenwx ~ $ echo ${!aa1[*]}

chenwx@chenwx ~ $ echo ${aa1[0]}

chenwx@chenwx ~ $ echo ${aa1[1]}

chenwx@chenwx ~ $ echo ${aa1[2]}

The Directory Stack

The directory stack is a list of recently-visited directories. The pushd builtin adds directories to the stack as it changes the current directory, and the popd builtin removes specified directories from the stack and changes the current directory to the directory removed. The dirs builtin displays the contents of the directory stack.

The contents of the directory stack are also visible as the value of the DIRSTACK shell variable.

Examples:

chenwx@chenwx ~/blog $ dirs
~/blog

chenwx@chenwx ~/blog $ pushd ~/Downloads/
~/Downloads ~/blog
chenwx@chenwx ~/Downloads $ pushd /etc/       
/etc ~/Downloads ~/blog
chenwx@chenwx /etc $ pushd /usr/lib
/usr/lib /etc ~/Downloads ~/blog

chenwx@chenwx /usr/lib $ dirs -l
/usr/lib /etc /home/chenwx/Downloads /home/chenwx/blog
chenwx@chenwx /usr/lib $ dirs -p
/usr/lib
/etc
~/Downloads
~/blog
chenwx@chenwx /usr/lib $ dirs -v
 0  /usr/lib
 1  /etc
 2  ~/Downloads
 3  ~/blog

chenwx@chenwx /usr/lib $ popd
/etc ~/Downloads ~/blog
chenwx@chenwx /etc $ popd
~/Downloads ~/blog
chenwx@chenwx ~/Downloads $ popd
~/blog

Controlling the Prompt

The value of the variable PROMPT_COMMAND is examined just before Bash prints each primary prompt. If PROMPT_COMMAND is set and has a non-null value, then the value is executed just as if it had been typed on the command line.

In addition, the following table describes the special characters which can appear in the prompt variables PS1 to PS4:

The command number and the history number are usually different: the history number of a command is its position in the history list, which may include commands restored from the history file (see Bash History Facilities), while the command number is the position in the sequence of commands executed during the current shell session.

After the string is decoded, it is expanded via parameter expansion, command substitution, arithmetic expansion, and quote removal, subject to the value of the promptvars shell option (see Bash Builtin Commands).

The Restricted Shell

If Bash is started with the name rbash, or the --restricted or -r option is supplied at invocation, the shell becomes restricted. A restricted shell is used to set up an environment more controlled than the standard shell. A restricted shell behaves identically to bash with the exception that the following are disallowed or not performed:

• Changing directories with the cd builtin.

• Setting or unsetting the values of the SHELL, PATH, ENV, or BASH_ENV variables.

• Specifying command names containing slashes.

• Specifying a filename containing a slash as an argument to the . builtin command.

• Specifying a filename containing a slash as an argument to the -p option to the hash builtin command.

• Importing function definitions from the shell environment at startup.

• Parsing the value of SHELLOPTS from the shell environment at startup.

• Redirecting output using the >, >|, <>, >&, &>, and >> redirection operators.

• Using the exec builtin to replace the shell with another command.

• Adding or deleting builtin commands with the -f and -d options to the enable builtin.

• Using the enable builtin command to enable disabled shell builtins.

• Specifying the -p option to the command builtin.

• Turning off restricted mode with set +r or set +o restricted.

These restrictions are enforced after any startup files are read.

When a command that is found to be a shell script is executed (see Shell Scripts), rbash turns off any restrictions in the shell spawned to execute the script.

Bash POSIX Mode

Starting Bash with the --posix command-line option or executing set -o posix while Bash is running will cause Bash to conform more closely to the POSIX standard by changing the behavior to match that specified by POSIX in areas where the Bash default differs.

When invoked as sh, Bash enters POSIX mode after reading the startup files.

The following list is what’s changed when POSIX mode is in effect:

1. When a command in the hash table no longer exists, Bash will re-search $PATH to find the new location. This is also available with shopt -s checkhash.

2. The message printed by the job control code and builtins when a job exits with a non-zero status is Done(status).

3. The message printed by the job control code and builtins when a job is stopped is Stopped(signame), where signame is, for example, SIGTSTP.

4. The bg builtin uses the required format to describe each job placed in the background, which does not include an indication of whether the job is the current or previous job.

5. Reserved words appearing in a context where reserved words are recognized do not undergo alias expansion.

6. The POSIX PS1 and PS2 expansions of ! to the history number and !! to ! are enabled, and parameter expansion is performed on the values of PS1 and PS2 regardless of the setting of the promptvars option.

7. The POSIX startup files are executed ($ENV) rather than the normal Bash files.

8. Tilde expansion is only performed on assignments preceding a command name, rather than on all assignment statements on the line.

9. The command builtin does not prevent builtins that take assignment statements as arguments from expanding them as assignment statements; when not in POSIX mode, assignment builtins lose their assignment statement expansion properties when preceded by command.

10. The default history file is ~/.sh_history (this is the default value of $HISTFILE).

11. The output of kill -l prints all the signal names on a single line, separated by spaces, without the SIG prefix.

12. The kill builtin does not accept signal names with a SIG prefix.

13. Non-interactive shells exit if filename in . filename is not found.

14. Non-interactive shells exit if a syntax error in an arithmetic expansion results in an invalid expression.

15. Non-interactive shells exit if there is a syntax error in a script read with the . or source builtins, or in a string processed by the eval builtin.

16. Redirection operators do not perform filename expansion on the word in the redirection unless the shell is interactive.

17. Redirection operators do not perform word splitting on the word in the redirection.

18. Function names must be valid shell names. That is, they may not contain characters other than letters, digits, and underscores, and may not start with a digit. Declaring a function with an invalid name causes a fatal syntax error in non-interactive shells.

19. Function names may not be the same as one of the POSIX special builtins.

20. POSIX special builtins are found before shell functions during command lookup.

21. The time reserved word may be used by itself as a command. When used in this way, it displays timing statistics for the shell and its completed children. The TIMEFORMAT variable controls the format of the timing information.

22. When parsing and expanding a ${. . .} expansion that appears within double quotes, single quotes are no longer special and cannot be used to quote a closing brace or other special character, unless the operator is one of those defined to perform pattern removal. In this case, they do not have to appear as matched pairs.

23. The parser does not recognize time as a reserved word if the next token begins with a -.

24. If a POSIX special builtin returns an error status, a non-interactive shell exits. The fatal errors are those listed in the POSIX standard, and include things like passing incorrect options, redirection errors, variable assignment errors for assignments preceding the command name, and so on.

25. A non-interactive shell exits with an error status if a variable assignment error occurs when no command name follows the assignment statements. A variable assignment error occurs, for example, when trying to assign a value to a readonly variable.

26. A non-interactive shell exits with an error status if a variable assignment error occurs in an assignment statement preceding a special builtin, but not with any other simple command.

27. A non-interactive shell exits with an error status if the iteration variable in a for statement or the selection variable in a select statement is a readonly variable.

28. Process substitution is not available.

29. While variable indirection is available, it may not be applied to the # and ? special parameters.

30. Assignment statements preceding POSIX special builtins persist in the shell environment after the builtin completes.

31. Assignment statements preceding shell function calls persist in the shell environment after the function returns, as if a POSIX special builtin command had been executed.

32. The export and readonly builtin commands display their output in the format required by POSIX.

33. The trap builtin displays signal names without the leading SIG.

34. The trap builtin doesn’t check the first argument for a possible signal specification and revert the signal handling to the original disposition if it is, unless that argument consists solely of digits and is a valid signal number. If users want to reset the handler for a given signal to the original disposition, they should use - as the first argument.

35. The . and source builtins do not search the current directory for the filename argument if it is not found by searching PATH.

36. Subshells spawned to execute command substitutions inherit the value of the -e option from the parent shell. When not in POSIX mode, Bash clears the -e option in such subshells.

37. Alias expansion is always enabled, even in non-interactive shells.

38. When the alias builtin displays alias definitions, it does not display them with a leading alias unless the -p option is supplied.

39. When the set builtin is invoked without options, it does not display shell function names and definitions.

40. When the set builtin is invoked without options, it displays variable values without quotes, unless they contain shell metacharacters, even if the result contains nonprinting characters.

41. When the cd builtin is invoked in logical mode, and the pathname constructed from $PWD and the directory name supplied as an argument does not refer to an existing directory, cd will fail instead of falling back to physical mode.

42. The pwd builtin verifies that the value it prints is the same as the current directory, even if it is not asked to check the file system with the -P option.

43. When listing the history, the fc builtin does not include an indication of whether or not a history entry has been modified.

44. The default editor used by fc is ed.

45. The type and command builtins will not report a non-executable file as having been found, though the shell will attempt to execute such a file if it is the only so-named file found in $PATH.

46. The vi editing mode will invoke the vi editor directly when the v command is run, instead of checking $VISUAL and $EDITOR.

47. When the xpg_echo option is enabled, Bash does not attempt to interpret any arguments to echo as options. Each argument is displayed, after escape characters are converted.

48. The ulimit builtin uses a block size of 512 bytes for the -c and -f options.

49. The arrival of SIGCHLD when a trap is set on SIGCHLD does not interrupt the wait builtin and cause it to return immediately. The trap command is run once for each child that exits.

50. The read builtin may be interrupted by a signal for which a trap has been set. If Bash receives a trapped signal while executing read, the trap handler executes and read returns an exit status greater than 128.

There is other POSIX behavior that Bash does not implement by default even when in POSIX mode. Specifically:

1. The fc builtin checks $EDITOR as a program to edit history entries if FCEDIT is unset, rather than defaulting directly to ed. fc uses ed if EDITOR is unset.

2. As noted above, Bash requires the xpg_echo option to be enabled for the echo builtin to be fully conformant.

Bash can be configured to be POSIX-conformant by default, by specifying the --enablestrict-posix-default to configure when building (see Optional Features).

Job Control

Job Control Basics

Job control refers to the ability to selectively stop (suspend) the execution of processes and continue (resume) their execution at a later point. A user typically employs this facility via an interactive interface supplied jointly by the operating system kernel’s terminal driver and Bash.

The shell associates a job with each pipeline. It keeps a table of currently executing jobs, which may be listed with the jobs command. When Bash starts a job asynchronously, it prints a line that looks like:

[1] 25647

indicating that this job is job number 1 and that the process ID of the last process in the pipeline associated with this job is 25647. All of the processes in a single pipeline are members of the same job. Bash uses the job abstraction as the basis for job control.

To facilitate the implementation of the user interface to job control, the operating system maintains the notion of a current terminal process group ID. Members of this process group (processes whose process group ID is equal to the current terminal process group ID) receive keyboard-generated signals such as SIGINT. These processes are said to be in the foreground. Background processes are those whose process group ID differs from the terminal’s; such processes are immune to keyboard-generated signals. Only foreground processes are allowed to read from or, if the user so specifies with stty tostop, write to the terminal. Background processes which attempt to read from (write to when stty tostop is in effect) the terminal are sent a SIGTTIN (SIGTTOU) signal by the kernel’s terminal driver, which, unless caught, suspends the process.

If the operating system on which Bash is running supports job control, Bash contains facilities to use it. Typing the suspend character (typically ^Z, Control-Z) while a process is running causes that process to be stopped and returns control to Bash. Typing the delayed suspend character (typically ^Y, Control-Y) causes the process to be stopped when it attempts to read input from the terminal, and control to be returned to Bash. The user then manipulates the state of this job, using the bg command to continue it in the background, the fg command to continue it in the foreground, or the kill command to kill it. A ^Z takes effect immediately, and has the additional side effect of causing pending output and typeahead to be discarded.

There are a number of ways to refer to a job in the shell. The character % introduces a job specification (jobspec).

Job number n may be referred to as %n. The symbols %% and %+ refer to the shell’s notion of the current job, which is the last job stopped while it was in the foreground or started in the background. A single % (with no accompanying job specification) also refers to the current job. The previous job may be referenced using %-. If there is only a single job, %+ and %- can both be used to refer to that job. In output pertaining to jobs (e.g., the output of the jobs command), the current job is always flagged with a +, and the previous job with a -.

A job may also be referred to using a prefix of the name used to start it, or using a substring that appears in its command line. For example, %ce refers to a stopped ce job. Using %?ce, on the other hand, refers to any job containing the string ce in its command line. If the prefix or substring matches more than one job, Bash reports an error.

Simply naming a job can be used to bring it into the foreground: %1 is a synonym for fg %1, bringing job 1 from the background into the foreground. Similarly, %1 & resumes job 1 in the background, equivalent to bg %1.

The shell learns immediately whenever a job changes state. Normally, Bash waits until it is about to print a prompt before reporting changes in a job’s status so as to not interrupt any other output. If the -b option to the set builtin is enabled, Bash reports such changes immediately. Any trap on SIGCHLD is executed for each child process that exits.

If an attempt to exit Bash is made while jobs are stopped, (or running, if the checkjobs option is enabled), the shell prints a warning message, and if the checkjobs option is enabled, lists the jobs and their statuses. The jobs command may then be used to inspect their status. If a second attempt to exit is made without an intervening command, Bash does not print another warning, and any stopped jobs are terminated.

Job Control Builtins

When job control is not active, the kill and wait builtins do not accept jobspec arguments. They must be supplied process IDs.

Job Control Variables

Command Line Editing

Command line editing is provided by the Readline library, which is used by several different programs, including Bash. Command line editing is enabled by default when using an interactive shell, unless the --noediting option is supplied at shell invocation. Line editing is also used when using the -e option to the read builtin command. By default, the line editing commands are similar to those of Emacs. A vi-style line editing interface is also available. Line editing can be enabled at any time using the -o emacs or -o vi options to the set builtin command (see The set Builtin), or disabled using the +o emacs or +o vi options to the set builtin.

Introduction to Line Editing