BashFAQ/046 - Greg's Wiki

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

The safest way to do this would be to loop over all elements in your set/list and check them for the element/word you are looking for. Say we are looking for the content of bar in the array foo:

   for element in "${foo[@]}"; do
      [[ $element = $bar ]] && echo "Found $bar."
   done

Or, to stop searching when you find it:

   for element in "${foo[@]}"; do
      [[ $element = $bar ]] && { echo "Found $bar."; break; }
   done

If for some reason your list/set is not in an array, but is a string of words, and the element you are searching for is also a word, you can use this:

   for element in $foo; do
      [[ $element = $bar ]] && echo "Found $bar."
   done

A less safe, but more clever version:

   if [[ " $foo " = *\ "$bar"\ * ]]; then
      echo "Found $bar."
   fi

And, if for some reason you don't know the syntax of for well enough, here's how to check your script's parameters for an element. For example, '-v':

   for element; do
      [[ $element = '-v' ]] && echo "Switching to verbose mode."
   done

GNU's grep has a \b feature which allegedly matches the edges of words. Using that, one may attempt to replicate the "clever" approach used above, but it is fraught with peril:

   # Is 'foo' one of the positional parameters? 
   egrep '\bfoo\b' <<<"$@" >/dev/null && echo yes
   # This is where it fails: is '-v' one of the positional parameters?
   egrep '\b-v\b' <<<"$@" >/dev/null && echo yes
   # Unfortunately, \b sees "v" as a separate word.
   # Nobody knows what the hell it's doing with the "-".

   # Is "someword" in the array 'array'?
   egrep '\bsomeword\b' <<<"${array[@]}"
   # Obviously, you can't use this if someword is '-v'!

Since this "feature" of GNU grep is both non-portable and poorly defined, we don't recommend using it.