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
   ${BASH_VERSION+shopt -s extglob}
   if [[ $var == @(foo|bar|more) ]]; then
      ...
   fi

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

    # bash/ksh93/zsh (w/ emulate ksh)

    # usage: pmatch string pattern [ pattern ... ]
    function pmatch {
        ${1+typeset x=}"${1-false}" &&
        while command shift; do
            [[ $x == $1 ]] && return
        done 2>/dev/null
        return 1
    }

    var='foo bar'
    if pmatch "$var" foo bar baz foo\* blarg; then
        : ...
    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...
    ${BASH_VERSION+shopt -s extglob}
    [[ $var == !(!(foo)|!(bar)|!(more)) ]] && ...

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

CategoryShell