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--- syntax:ccmd:case [2012/02/16 03:25]
felipe1982 [Examples] ;; is suggested for every case statement
+++ syntax:ccmd:case [2013/02/01 13:27]
ormaaj [Examples] A practical example to show off my video power function.
@@ Line 15: / Line 15: @@
 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 new action terminators. The classic behaviour using '';;'' is to execute the matching block and then terminate. The '';&'' terminator causes ''case'' to also execute the next block, the '';;&'' operator makes it checking the pattern of the next block for a match. Using these terminators, a ''case'' statement can be configured to "run through" all matches, for example.
+: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.
-The word ''<WORD>'' is expanded using //tilde//, //parameter// and //variable expansion//, //arithmetic//, //command// and //process substitution// and //quote removal//, **no word splitting is done**, which means, you can leave expansions unquoted without problems, like:
+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:
 <code>
 var="test word"
@@ Line 25: / Line 25: @@
 esac
 </code>
-This is similar to the behaviour 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 :V4: Bash v4 changes above.)
-More patterns to match for one list to execute are separated by ''|'' (pipe symbol).
+Multiple ''|''-delimited patterns can be specified for a single block. This is a POSIX-compatable equivalent to the ''@(pattern-list)'' extglob construct.
+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.
 ===== Examples =====
 Another one of my stupid examples...
 <code>
-read -p "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
 </code>
-===== Portability considerations =====
+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>
-===== See also =====
+A template for experiments with ''case'' logic, showing shared code between blocks using '';&'', and the non-short-circuiting '';;&'' operator:
+<code bash>
+#!/usr/bin/env bash
+f() {
+    local -a "$@"
+    local x
+    for x; do
+        case $x in
+            $1)
+                local "$x"'+=(1)' ;;&
+            $2)
+                local "$x"'+=(2)' ;&
+            $3)
+                local "$x"'+=(3)' ;;
+            $1|$2)
+                local "$x"'+=(4)'
+        esac
+        IFS=, local -a "$x"'=("${x}: ${'"$x"'[*]}")'
+    done
+    for x; do
+        echo "${!x}"
+    done
+}
+f a b c
+# output:
+# a: 1,4
+# b: 2,3
+# c: 3
+</code>
+===== Portability considerations =====
+  * Only the '';;'' delimiter is specified by POSIX.
+  * zsh and mksh use the '';|'' control operator instead of Bash's '';;&''. Mksh has '';;&'' for Bash compatability (undocumented).
+  * 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 =====
+  * [[http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_09_04_05|POSIX case conditional construct]]