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 -- ksh does not need the shopt
   shopt -s extglob
   if [[ $var = @(foo|bar|more) ]]; then
      ...
   fi

Alternatively, the "inarray" function could be used: 

   # usage: inarray NEEDLE HAYSTACK ...
   # returns 0 if NEEDLE is in HAYSTACK, otherwise 1.
   inarray() {
     local n=$1 h
     shift

     for n; do
       [[ $n = "$h" ]] && return
     done
     return 1
   }

   if inarray $var foo bar more; then
     ...
   fi

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