How to check if a string contains a substring in Bash

Question

I have a string in Bash:

string=\"My string\"

How can I test if it contains another string?

if [ $string ?? \'foo\' ]; then         


        

Answer 1

You should remember that shell scripting is less of a language and more of a collection of commands. Instinctively you think that this "language" requires you to follow an if with a [ or a [[. Both of those are just commands that return an exit status indicating success or failure (just like every other command). For that reason I'd use grep, and not the [ command.

Just do:

if grep -q foo <<<"$string"; then
    echo "It's there"
fi

Now that you are thinking of if as testing the exit status of the command that follows it (complete with semi-colon), why not reconsider the source of the string you are testing?

## Instead of this
filetype="$(file -b "$1")"
if grep -q "tar archive" <<<"$filetype"; then
#...

## Simply do this
if file -b "$1" | grep -q "tar archive"; then
#...

The -q option makes grep not output anything, as we only want the return code. <<< makes the shell expand the next word and use it as the input to the command, a one-line version of the << here document (I'm not sure whether this is standard or a Bashism).