BashFAQ/046

I want to check to see whether a word is in a list (or an element is a member of a set).

If your real question is How do I check whether one of my parameters was -v? then see FAQ #35 instead. Otherwise, read on…

Associative arrays

All we need to do is create an entry for each item and look it up by index. In this example, we test whether the user input x is a member of the set a:

# Bash etc.

function get_input {
        [[ -t 0 ]] || return
        printf 'hm? '
        IFS= read -r${BASH_VERSION+\e} -- "$1"
}

set -- Bigfoot UFOs Republicans
typeset -A a
for x; do
        a+=([$x]=)
done

get_input x

if [[ -v a[$x] ]]; then
        printf '%s exists!\n' "$x"
else
        printf $'%s doesn\'t exist.\\n' "$x"
fi

Indexed arrays

We can store a list of strings in an indexed array by looping over each element:

# Bash

typeset -a haystack
for x in "${haystack[@]}"; do
        [[ $x == "$needle" ]] && printf 'Found %q!\n' "$needle"
done

enum (ksh93)

In ksh93t or later, one may create enum types/variables/constants using the enum builtin. These work similarly to C enums (and the equivalent feature of other languages). These may be used to restrict which values may be assigned to a variable so as to avoid the need for an expensive test each time an array variable is set or referenced. Like types created using typeset -T, the result of an enum command is a new declaration command that can be used to instantiate objects of that type.

# ksh93

 $ enum colors=(red green blue)
 $ colors foo=green
 $ foo=yellow
ksh: foo:  invalid value yellow

typeset -a can also be used in combination with an enum type to allow enum constants as subscripts.

# ksh93

 $ typeset -a '[colors]' bar
 $ bar[blue]=test1
 $ typeset -p bar
typeset -a '[colors]' bar=([blue]=test)
 $ bar[orange]=test
ksh: colors:  invalid value orange

CategoryShell