How to signal the presence of a pattern via exit code, without altering STDOUT

Question

I need to run cmd1. If and only if PATTERN is missing from its STDOUT, i need to run cmd2 too:

cmd1 | grep "$PATTERN"  ||  cmd2

But I'd like to see all output from cmd1, not just the lines that match PATTERN.

I know I could do something like this:

OUTPUT=`cmd1`
echo "$OUTPUT"
echo "$OUTPUT" | grep -q "$PATTERN"  ||  cmd2

but this would separate STDOUT from STDERR. Also I'd rather have a handy one-liner. So I am looking for an option in grep, or an alternative to grep, saying "Don't filter, just set the exit code".

Answer 1

How'bout using tee to send the output to the tty like so:

cmd1 | tee /dev/tty | grep "$PATTERN"  ||  cmd2

Example with PATTERN matching cmd1's output:

% echo "cmd1 output" | tee /dev/tty | grep -q "output" || echo "nothing matches PATTERN"
cmd1 output

Example with PATTERN not matching cmd1's output:

% echo "cmd1 output" | tee /dev/tty | grep -q "outputttt" || echo "nothing matches PATTERN"
cmd1 output
nothing matches PATTERN