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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 was How do I check whether one of my parameters was -v? then please see FAQ #35 instead. Otherwise, read on....

First of all, let's get the terminology straight. Bash has no notion of "lists" or "sets" or any such. Bash has strings and arrays. Strings are a "list" of characters, arrays are a "list" of strings.

NOTE: In the general case, a string cannot possibly contain a list of other strings because there is no reliable way to tell where each substring begins and ends.

Given a traditional array, the only proper way to do this is to loop over all elements in your array and check them for the element you are looking for. Say what we are looking for is in bar and our list is in the array foo:

If you need to perform this several times in your script, you might want to extract the logic into a function:

Or, if you want your function to return the index at which the element was found:

If your "list" is contained in a string, and for some half-witted reason you choose not to heed the warnings above, you can use the following code to search through "words" in a string. (The only real excuse for this would be that you're stuck in Bourne shell, which has no arrays.)

Here, a "word" is defined as any substring that is delimited by whitespace (or more specifically, the characters currently in IFS). The set -f prevents glob expansion of the words in the list. Turning glob expansions back on (set +f) is optional.

If you're working in bash 4 or ksh93, you have access to associative arrays. These will allow you to restructure the problem -- instead of making a list of words that are allowed, you can make an associative array whose keys are the words you want to allow. Their values could be meaningful, or not -- depending on the nature of the problem.

Here's a hack that you shouldn't use, but which is presented for the sake of completeness:

(The problem here is that is assumes space can be used as a delimiter between words. Your elements might contain spaces, which would break this!)

That same hack, for Bourne shells:

You can also use extended glob with printf to search for a word in an array. I haven't tested it enough, so it might break in some cases --sn18

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

Since this "feature" of GNU grep is both non-portable and poorly defined, we recommend not using it. It is simply mentioned here for the sake of completeness.

Bulk comparison

This method tries to compare the desired string to the entire contents of the array. It can potentially be very efficient, but it depends on a delimiter that must not be in the sought value or the array. Here we use $'\a', the BEL character, because it's extremely uncommon.


2012-07-01 04:05