3.2 Shell Commands

A simple shell command such as echo a b c consists of the command itself followed by arguments, separated by spaces.

More complex shell commands are composed of simple commands arranged together in a variety of ways: in a pipeline in which the output of one command becomes the input of a second, in a loop or conditional construct, or in some other grouping.

3.2.1 Simple Commands

A simple command is the kind of command encountered most often. It's just a sequence of words separated by blanks, terminated by one of the shell's control operators (see section Definitions). The first word generally specifies a command to be executed, with the rest of the words being that command's arguments.

The return status (see section Exit Status) of a simple command is its exit status as provided by the POSIX 1003.1 waitpid function, or 128+n if the command was terminated by signal n.

3.2.2 Pipelines

A pipeline is a sequence of simple commands separated by `|'.

The format for a pipeline is

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

The output of each command in the pipeline is connected via a pipe to the input of the next command. That is, each command reads the previous command's output.

The reserved word time causes timing statistics to be printed for the pipeline once it finishes. The statistics currently consist of elapsed (wall-clock) time and user and system time consumed by the command's execution. The `-p' option changes the output format to that specified by POSIX. The TIMEFORMAT variable may be set to a format string that specifies how the timing information should be displayed. See section Bash Variables, for a description of the available formats. The use of time as a reserved word permits the timing of shell builtins, shell functions, and pipelines. An external time command cannot time these easily.

If the pipeline is not executed asynchronously (see section Lists of Commands), the shell waits for all commands in the pipeline to complete.

Each command in a pipeline is executed in its own subshell (see section Command Execution Environment). The exit status of a pipeline is the exit status of the last command in the pipeline, unless the pipefail option is enabled (see section The Set Builtin). If pipefail is enabled, the pipeline's return status is the value of the last (rightmost) command to exit with a non-zero status, or zero if all commands exit successfully. If the reserved word `!' precedes the pipeline, the exit status is the logical negation of the exit status as described above. The shell waits for all commands in the pipeline to terminate before returning a value.

3.2.3 Lists of Commands

A list is a sequence of one or more pipelines separated by one of the operators `;', `&', `&&', or `||', and optionally terminated by one of `;', `&', or a newline.

Of these list operators, `&&' and `||' have equal precedence, followed by `;' and `&', which have equal precedence.

A sequence of one or more newlines may appear in a list to delimit commands, equivalent to a semicolon.

If a command is terminated by the control operator `&', the shell executes the command asynchronously in a subshell. This is known as executing the command in the background. The shell does not wait for the command to finish, and the return status is 0 (true). When job control is not active (see section Job Control), the standard input for asynchronous commands, in the absence of any explicit redirections, is redirected from /dev/null.

Commands separated by a `;' are executed sequentially; the shell waits for each command to terminate in turn. The return status is the exit status of the last command executed.

The control operators `&&' and `||' denote AND lists and OR lists, respectively. An AND list has the form

command1 && command2

command2 is executed if, and only if, command1 returns an exit status of zero.

An OR list has the form

command1 || command2

command2 is executed if, and only if, command1 returns a non-zero exit status.

The return status of AND and OR lists is the exit status of the last command executed in the list.

3.2.4 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 section Redirections) associated with a compound command apply to all commands within that compound command unless explicitly overridden.

Bash provides looping constructs, conditional commands, and mechanisms to group commands and execute them as a unit.

3.2.4.1 Looping Constructs

Bash supports the following looping constructs.

Note that wherever a `;' appears in the description of a command's syntax, it may be replaced with one or more newlines.

until

The syntax of the until command is:

until test-commands; do consequent-commands; done

Execute consequent-commands as long as test-commands has an exit status which is not zero. The return status is the exit status of the last command executed in consequent-commands, or zero if none was executed.

while

The syntax of the while command is:

while test-commands; do consequent-commands; done

Execute consequent-commands as long as test-commands has an exit status of zero. The return status is the exit status of the last command executed in consequent-commands, or zero if none was executed.

for

The syntax of the for command is:

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

Expand words, and execute commands once for each member in the resultant list, with name bound to the current member. If `in words' is not present, the for command executes the commands once for each positional parameter that is set, as if `in "$@"' had been specified (see section Special Parameters). The return status is the exit status of the last command that executes. If there are no items in the expansion of words, no commands are executed, and the return status is zero.

An alternate form of the for command is also supported:

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

First, the arithmetic expression expr1 is evaluated according to the rules described below (see section Shell Arithmetic). The arithmetic expression expr2 is then evaluated repeatedly until it evaluates to zero. Each time expr2 evaluates to a non-zero value, commands are executed and the arithmetic expression expr3 is evaluated. If any expression is omitted, it behaves as if it evaluates to 1. The return value is the exit status of the last command in list that is executed, or false if any of the expressions is invalid.

The break and continue builtins (see section Bourne Shell Builtins) may be used to control loop execution.

3.2.4.2 Conditional Constructs

if

The syntax of the if command is:

if test-commands; then consequent-commands; [elif more-test-commands; then more-consequents;] [else alternate-consequents;] fi

The test-commands list is executed, and if its return status is zero, the consequent-commands list is executed. If test-commands returns a non-zero status, each elif list is executed in turn, and if its exit status is zero, the corresponding more-consequents is executed and the command completes. If `else alternate-consequents' is present, and the final command in the final if or elif clause has a non-zero exit status, then alternate-consequents is executed. The return status is the exit status of the last command executed, or zero if no condition tested true.

case

The syntax of the case command is:

case word in [ [(] pattern [| pattern]…) command-list ;;]… esac

case will selectively execute the command-list corresponding to the first pattern that matches word. If the shell option nocasematch (see the description of shopt in Bash Builtin Commands) 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 `;;'. The first pattern that matches determines the command-list that is executed.

Here is an example using case in a script that could be used to describe one interesting feature of an animal:

echo -n "Enter the name of an animal: " read ANIMAL echo -n "The $ANIMAL has " case $ANIMAL in horse | dog | cat) echo -n "four";; man | kangaroo ) echo -n "two";; *) echo -n "an unknown number of";; esac echo " legs."

The return status is zero if no pattern is matched. Otherwise, the return status is the exit status of the command-list executed.

select

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

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

The list of words following in is expanded, generating a list of items. The set of expanded words is printed on the standard error output stream, each preceded by a number. If the `in words' is omitted, the positional parameters are printed, as if `in "$@"' had been specifed. The PS3 prompt is then displayed and a line is read from the standard input. If the line consists of a number corresponding to one of the displayed words, then the value of name is set to that word. If the line is empty, the words and prompt are displayed again. If EOF is read, the select command completes. Any other value read causes name to be set to null. The line read is saved in the variable REPLY.

The commands are executed after each selection until a break command is executed, at which point the select command completes.

Here is an example that allows the user to pick a filename from the current directory, and displays the name and index of the file selected.

select fname in *; do echo you picked $fname \($REPLY\) break; done

((…))

(( expression ))

The arithmetic expression is evaluated according to the rules described below (see section Shell Arithmetic). If the value of the expression is non-zero, the return status is 0; otherwise the return status is 1. This is exactly equivalent to

let "expression"

See section Bash Builtin Commands, for a full description of the let builtin.

[[…]]

[[ expression ]]

Return a status of 0 or 1 depending on the evaluation of the conditional expression expression. Expressions are composed of the primaries described below in Bash Conditional Expressions. Word splitting and filename expansion are not performed on the words between the `[[' and `]]'; tilde expansion, parameter and variable expansion, arithmetic expansion, command substitution, process substitution, and quote removal are performed. Conditional operators such as `-f' must be unquoted to be recognized as primaries.

When the `==' and `!=' operators are used, the string to the right of the operator is considered a pattern and matched according to the rules described below in Pattern Matching. If the shell option nocasematch (see the description of shopt in Bash Builtin Commands) is enabled, the match is performed without regard to the case of alphabetic characters. The return value is 0 if the string matches (`==') or does not match (`!=')the pattern, and 1 otherwise. Any part of the pattern may be quoted to force it 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 (see the description of shopt in Bash Builtin Commands) is enabled, the match is performed without regard to the case of alphabetic characters. 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.

Expressions may be combined using the following operators, listed in decreasing order of precedence:

( expression )

Returns the value of expression. This may be used to override the normal precedence of operators.

! expression

True if expression is false.

expression1 && expression2

True if both expression1 and expression2 are true.

expression1 || expression2

True if either expression1 or expression2 is true.

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

3.2.4.3 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 )

Placing a list of commands between parentheses causes a subshell environment to be created (see section Command Execution Environment), and each of the commands in list to be executed in that subshell. Since the list is executed in a subshell, variable assignments do not remain in effect after the subshell completes.

{}

{ list; }

Placing a list of commands between curly braces causes the list to be executed in the current shell context. No subshell is created. The semicolon (or newline) following list is required.

In addition to the creation of a subshell, there is a subtle difference between these two constructs due to historical reasons. The braces are reserved words, so they must be separated from the list by blanks. 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.