I want to check if [[ $var == foo || $var == bar || $var == more ]] without repeating $var n times.

The portable solution uses case:

   # Bourne
   case $var in
      foo|bar|more) ... ;;
   esac

In Bash and ksh, Extended globs can also do this within a [[ command:

   # bash/ksh
   if [[ $var == @(foo|bar|more) ]]; then
      ...
   fi

Extended globs are turned on by default inside the [[ command in bash 4.1 and newer. If you need to target an older version of bash, you will need to turn them on in your script (shopt -s extglob outside of all functions or compound commands).

Alternatively, you may loop over a list of patterns, checking each individually.

# bash/ksh93

[[ -v BASH_VERSION ]] && shopt -s extglob

# usage: pmatch string pattern [ pattern ... ]
function any {
    [[ -n $1 ]] || return
    typeset pat match=$1
    shift

    for pat; do
        [[ $match == $pat ]] && return
    done

    return 1
}

var='foo bar'
if any "$var" '@(bar|baz)' foo\* blarg; then
    echo 'var matched at least one of the patterns!'
fi

For logical conjunction (return true if $var matches all patterns), ksh93 can use the & pattern delimiter.

    # ksh93 only
    [[ $var == @(foo&bar&more) ]] && ...

For shells that support only the ksh88 subset (extglob patterns), you may DeMorganify the logic using the negation sub-pattern operator.

    # bash/ksh88/etc...
    [[ $var == !(!(foo)|!(bar)|!(more)) ]] && ...

But this is quite unclear and not much shorter than just writing out separate expressions for each pattern.

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