Answer by derobert for Can grep return true/false or are there alternative methods

grep returns a different exit code if it found something (zero) vs. if it hasn't found anything (non-zero). In an if statement, a zero exit code is mapped to "true" and a non-zero exit code is mapped to false. In addition, grep has a -q argument to not output the matched text (but only return the exit status code)

So, you can use grep like this:

if grep -q PATTERN file.txt; then    echo foundelse    echo not foundfi

As a quick note, when you do something like if [ -z "$var" ]…, it turns out that [ is actually a command you're running, just like grep. On my system, it's /usr/bin/[. (Well, technically, your shell probably has it built-in, but that's an optimization. It behaves as if it were a command). It works the same way, [ returns a zero exit code for true, a non-zero exit code for false. (test is the same thing as [, except for the closing ])