The case statement

case <WORD> in
  [(] <PATTERN1> ) <LIST1> ;; # or ;& or ;;& in Bash 4
  [(] <PATTERN2> ) <LIST2> ;;
  [(] <PATTERN3> | <PATTERN4> ) <LIST3-4> ;;
  ...
  [(] <PATTERNn>) <LISTn> [;;]
esac

The case-statement can execute commands based on a pattern matching decision. The word <WORD> is matched against every pattern <PATTERNn> and on a match, the associated list <LISTn> is executed. Every commandlist is terminated by ;;. This rule is optional for the very last commandlist (i.e., you can omit the ;; before the esac). Every <PATTERNn> is separated from it's associated <LISTn> by a ), and is optionally preceded by a (.

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:

var="test word"

case $var in
  ...
esac

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.

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.

Another one of my stupid examples…

printf '%s ' 'Which fruit do you like most?'
read -${BASH_VERSION+e}r fruit

case $fruit in
    apple)
        echo 'Mmmmh... I like those!'
        ;;
    banana)
        echo 'Hm, a bit awry, no?'
        ;;
    orange|tangerine)
        echo $'Eeeks! I don\'t like those!\nGo away!'
        exit 1
        ;;
    *)
        echo "Unknown fruit - sure it isn't toxic?"
esac

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.

# 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
}

A template for experiments with case logic, showing shared code between blocks using ;&, and the non-short-circuiting ;;& operator:

#!/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

• 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 for loops, but Bash doesn't support this syntax for case..esac.

• POSIX case conditional construct