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syntax:ccmd:case [2012/05/17 12:21] ormaaj [See also] Backwards link |
syntax:ccmd:case [2017/02/08 15:04] (current) fgrose [Description] describe the separation of PATTERNn from surroundings |
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| ===== Description ===== | ===== Description ===== | ||
| - | 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''). Every ''<PATTERNn>'' is separated from it's associated ''<LISTn>'' by a '')'', and is optionally preceded by a ''(''. |
| - | :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 | ||
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| ===== Portability considerations ===== | ===== Portability considerations ===== | ||
| - | * Only the '';;'' delimiter is specified by POSIX | + | * Only the '';;'' delimiter is specified by POSIX. |
| * zsh and mksh use the '';|'' control operator instead of Bash's '';;&''. Mksh has '';;&'' for Bash compatability (undocumented). | * zsh and mksh use the '';|'' control operator instead of Bash's '';;&''. Mksh has '';;&'' for Bash compatability (undocumented). | ||
| - | * ksh93 has the '';&'' operator, but not '';;&''. | + | * ksh93 has the '';&'' operator, but no '';;&'' or equivalent. |
| + | * 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''. | ||
| ===== See also ===== | ===== See also ===== | ||
| * [[http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_09_04_05|POSIX case conditional construct]] | * [[http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_09_04_05|POSIX case conditional construct]] | ||