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120 lines
5.5 KiB
Plaintext
120 lines
5.5 KiB
Plaintext
====== The case statement ======
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===== Synopsis =====
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<code>
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case <WORD> in
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[(] <PATTERN1> ) <LIST1> ;; # or ;& or ;;& in Bash 4
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[(] <PATTERN2> ) <LIST2> ;;
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[(] <PATTERN3> | <PATTERN4> ) <LIST3-4> ;;
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...
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[(] <PATTERNn>) <LISTn> [;;]
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esac
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</code>
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===== Description =====
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The ''case''-statement can execute commands based on a [[syntax:pattern | pattern matching]] decision. The word ''<WORD>'' is matched against every pattern ''<PATTERNn>'' and on a match, the associated [[syntax:basicgrammar#lists | list]] ''<LISTn>'' is executed. Every commandlist is terminated by ''<nowiki>;;</nowiki>''. This rule is optional for the very last commandlist (i.e., you can omit the ''<nowiki>;;</nowiki>'' before the ''esac''). Every ''<PATTERNn>'' is separated from it's associated ''<LISTn>'' by a '')'', and is optionally preceded by a ''(''.
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Bash 4 introduces two new action terminators. The classic behavior using '';;'' is to execute only the list associated with the first matching pattern, then break out of the ''case'' block. The '';&'' terminator causes ''case'' to also execute the next block without testing its pattern. The '';;&'' operator is like '';;'', except the case statement doesn't terminate after executing the associated list - Bash just continues testing the next pattern as though the previous pattern didn't match. Using these terminators, a ''case'' statement can be configured to test against all patterns, or to share code between blocks, for example.
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The word ''<WORD>'' is expanded using //tilde//, //parameter// and //variable expansion//; //arithmetic//, //command// and //process substitution//; and //quote removal//. **No word splitting, brace, or pathname expansion is done**, which means you can leave expansions unquoted without problems:
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<code>
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var="test word"
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case $var in
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...
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esac
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</code>
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This is similar to the behavior of the [[syntax:ccmd:conditional_expression | conditional expression command ("new test command")]] (also no word splitting for expansions).
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Unlike the C-case-statement, only the matching list and nothing else is executed. If more patterns match the word, only the first match is taken. (**Note** the comment about Bash v4 changes above.)
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Multiple ''|''-delimited patterns can be specified for a single block. This is a POSIX-compatable equivalent to the ''@(pattern-list)'' extglob construct.
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The ''case'' statement is one of the most difficult commands to indent clearly, and people frequently ask about the most "correct" style. Just do your best - there are many variations of indenting style for ''case'' and no real agreed-upon best practice.
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===== Examples =====
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Another one of my stupid examples...
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<code>
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printf '%s ' 'Which fruit do you like most?'
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read -${BASH_VERSION+e}r fruit
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case $fruit in
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apple)
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echo 'Mmmmh... I like those!'
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;;
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banana)
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echo 'Hm, a bit awry, no?'
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;;
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orange|tangerine)
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echo $'Eeeks! I don\'t like those!\nGo away!'
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exit 1
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;;
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*)
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echo "Unknown fruit - sure it isn't toxic?"
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esac
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</code>
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Here's a practical example showing a common pattern involving a ''case'' statement. If the first argument is one of a valid set of alternatives, then perform some sysfs operations under Linux to control a video card's power profile. Otherwise, show a usage synopsis, and print the current power profile and GPU temperature.
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<code bash>
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# Set radeon power management
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function clk {
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typeset base=/sys/class/drm/card0/device
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[[ -r ${base}/hwmon/hwmon0/temp1_input && -r ${base}/power_profile ]] || return 1
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case $1 in
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low|high|default)
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printf '%s\n' "temp: $(<${base}/hwmon/hwmon0/temp1_input)C" "old profile: $(<${base}/power_profile)"
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echo "$1" >${base}/power_profile
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echo "new profile: $(<${base}/power_profile)"
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;;
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*)
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echo "Usage: $FUNCNAME [ low | high | default ]"
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printf '%s\n' "temp: $(<${base}/hwmon/hwmon0/temp1_input)C" "current profile: $(<${base}/power_profile)"
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esac
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}
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</code>
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A template for experiments with ''case'' logic, showing shared code between blocks using '';&'', and the non-short-circuiting '';;&'' operator:
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<code bash>
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#!/usr/bin/env bash
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f() {
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local -a "$@"
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local x
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for x; do
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case $x in
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$1)
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local "$x"'+=(1)' ;;&
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$2)
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local "$x"'+=(2)' ;&
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$3)
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local "$x"'+=(3)' ;;
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$1|$2)
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local "$x"'+=(4)'
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esac
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IFS=, local -a "$x"'=("${x}: ${'"$x"'[*]}")'
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done
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for x; do
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echo "${!x}"
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done
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}
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f a b c
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# output:
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# a: 1,4
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# b: 2,3
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# c: 3
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</code>
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===== Portability considerations =====
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* Only the '';;'' delimiter is specified by POSIX.
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* zsh and mksh use the '';|'' control operator instead of Bash's '';;&''. Mksh has '';;&'' for Bash compatability (undocumented).
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* ksh93 has the '';&'' operator, but no '';;&'' or equivalent.
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* ksh93, mksh, zsh, and posh support a historical syntax where open and close braces may be used in place of ''in'' and ''esac'': ''case word { x) ...; };''. This is similar to the alternate form Bash supports for its [[syntax/ccmd/classic_for | for loops]], but Bash doesn't support this syntax for ''case..esac''.
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===== See also =====
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* [[http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_09_04_05|POSIX case conditional construct]]
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