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syntax:ccmd:case [2012/07/31 10:56] ormaaj [Portability considerations] |
syntax:ccmd:case [2013/04/14 12:33] thebonsai Don't tread version 4 special - it has been around for a long time now |
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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''). | 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''). | ||
- | :V4: 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. | + | 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. |
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: | 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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This is similar to the behavior of the [[syntax:ccmd:conditional_expression | conditional expression command ("new test command")]] (also no word splitting for expansions). | This is similar to the behavior of the [[syntax:ccmd:conditional_expression | conditional expression command ("new test command")]] (also no word splitting for expansions). | ||
- | 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 :V4: Bash v4 changes above.) | + | 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.) |
Multiple ''|''-delimited patterns can be specified for a single block. This is a POSIX-compatable equivalent to the ''@(pattern-list)'' extglob construct. | 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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Another one of my stupid examples... | Another one of my stupid examples... | ||
<code> | <code> | ||
- | read -rp "Which fruit do you like most? " fruit | + | printf '%s ' 'Which fruit do you like most?' |
+ | read -${BASH_VERSION+e}r fruit | ||
- | case "$fruit" in | + | case $fruit in |
- | apple) echo "Mmmmh... I like those!" | + | apple) |
- | ;; | + | echo 'Mmmmh... I like those!' |
- | banana) echo "Hm, a bit awry, no?" | + | ;; |
- | ;; | + | banana) |
- | orange|tangerine) echo "Eeeks! I don't like those!" | + | echo 'Hm, a bit awry, no?' |
- | echo "Go away!" | + | ;; |
- | exit 1 | + | orange|tangerine) |
- | ;; | + | echo $'Eeeks! I don\'t like those!\nGo away!' |
- | *) echo "Unknown fruit - sure it isn't toxic?" | + | exit 1 |
- | ;; | + | ;; |
+ | *) | ||
+ | echo "Unknown fruit - sure it isn't toxic?" | ||
esac | esac | ||
+ | </code> | ||
+ | |||
+ | 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. | ||
+ | <code bash> | ||
+ | # Set radeon power management | ||
+ | function clk { | ||
+ | typeset base=/sys/class/drm/card0/device | ||
+ | [[ -r ${base}/hwmon/hwmon0/temp1_input && -r ${base}/power_profile ]] || return 1 | ||
+ | |||
+ | case $1 in | ||
+ | low|high|default) | ||
+ | printf '%s\n' "temp: $(<${base}/hwmon/hwmon0/temp1_input)C" "old profile: $(<${base}/power_profile)" | ||
+ | echo "$1" >${base}/power_profile | ||
+ | echo "new profile: $(<${base}/power_profile)" | ||
+ | ;; | ||
+ | *) | ||
+ | echo "Usage: $FUNCNAME [ low | high | default ]" | ||
+ | printf '%s\n' "temp: $(<${base}/hwmon/hwmon0/temp1_input)C" "current profile: $(<${base}/power_profile)" | ||
+ | esac | ||
+ | } | ||
</code> | </code> | ||
A template for experiments with ''case'' logic, showing shared code between blocks using '';&'', and the non-short-circuiting '';;&'' operator: | A template for experiments with ''case'' logic, showing shared code between blocks using '';&'', and the non-short-circuiting '';;&'' operator: | ||
- | <code> | + | <code bash> |
#!/usr/bin/env bash | #!/usr/bin/env bash | ||