grep, else print message for no matches

Question

In a bash script, I have a list of lines in a file I wish to grep and then display on standard out, which is easiest done with a while read:

grep "regex" "filepath" | while read line; do
    printf "$line\n"
done

However, I would like to inform the user if no lines were matched by the grep. I know that one can do this by updating a variable inside the loop but it seems like a much more elegant approach (if possible) would be to try to read a line in an until loop, and if there were no output, an error message could be displayed.

This was my first attempt:

grep "regex" "filepath" | until [[ -z ${read line} ]]; do
    if [[ -z $input ]]; then
       printf "No matches found\n"
       break
    fi
    printf "$line\n"
done

But in this instance the read command is malformed, and I wasn't sure of another way the phrase the query. Is this approach possible, and if not, is there a more suitable solution to the problem?

Answer 1

You don't need a loop at all if you simply want to display a message when there's no match. Instead you can use grep's return code. A simple if statement will suffice:

if ! grep "regex" "filepath"; then
    echo "no match" >&2
fi

This will display the results of grep matches (since that's grep's default behavior), and will display the error message if it doesn't.

A popular alternative to if ! is to use the || operator. foo || bar can be read as "do foo or else do bar", or "if not foo then bar".

grep "regex" "filepath" || echo "no match" >&2

Answer 2

John Kugelman's answer is the correct and succinct one and you should accept it. I am addressing your question about syntax here just for completeness.

You cannot use ${read line} to execute read -- the brace syntax actually means (vaguely) that you want the value of a variable whose name contains a space. Perhaps you were shooting for $(read line) but really, the proper way to write your until loop would be more along the lines of

grep "regex" "filepath" | until read line; [[ -z "$line" ]]; do

... but of course, when there is no output, the pipeline will receive no lines, so while and until are both wrong here.

It is worth amphasizing that the reason you need a separate do is that you can have multiple commands in there. Even something like

while output=$(grep "regex filepath"); echo "grep done, please wait ...";
    count=$(echo "$output" | wc -l); [[ $count -gt 0 ]]
do ...

although again, that is much more arcane than you would ever really need. (And in this particular case, you would want probably actually want if , not while.)

As others already noted, there is no reason to use a loop like that here, but I wanted to sort out the question about how to write a loop like this for whenever you actually do want one.