Monitoring external process: Exit when STDOUT matches pattern

Question

I'm running a system command and waiting for output matching a specific pattern, e.g:

open(my $fh, '-|', 'echo line 1; sleep 20; echo line 2');
while (<$fh>) {
    print && last if /1/;
}
close $fh;

This will print line 1 and leave the loop but won't exit until the external command has completed.

How can I allow the script to exit immediately after matching the required output?

Answer 1

You can send TERM signal to the PID and then close file handle without waiting,

my $pid = open(my $fh, '-|', 'echo line 1;sleep 5; echo line 2') or die $!;
while (<$fh>) {
    print && last if /1/;
}
kill TERM => $pid;
close $fh;

Answer 2

You could use awk:

echo hello; sleep 2; echo pattern |awk '{print}/pattern/{exit}' 

This will give you all the lines till pattern occurs, then exit.

Answer 3

If the process is to be left running after the match is found, append an & to the command to run it in the background:

open(my $fh, '-|', 'echo line 1 && sleep 20 && echo line 2 &');
while (<$fh>) {
    print && last if /1/;
}
close $fh;