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 found
else
    echo not found
fi

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 ])

Another simple way is to use grep -c.

That outputs (not return as exit code), the number of lines that match the pattern, so 0 if there's no match or 1 or more if there's a match.

So, if you wanted to check that the pattern is matched 3 or more times, you would do:

if [ "$(grep -c "^$1" schemas.txt)" -ge 3 ]; then
  ...

I know I am late for this, but I love this short version:

grep -q ^$1 schemas.txt && echo "Schema already exists. Please try again" || echo "$@" >> schemas.txt