Using grep to filter output of a process

I'm using command | grep -v "JavaScript strict warning" to filter the output of a command (hiding warnings). The grep command works fine when I test it like this:

$ printf "JavaScript strict warning: warning\nNot a warning\nJavaScript strict warning: warning\nJavaScript strict warning: warning\n" | grep -v "JavaScript strict warning"

The output is:

Not a warning

However, when I filter the running output, lines with JavaScript strict warning still come through. I believe that it's happening because the original command is spawning another process. Is there a way to keep filtering the output of the new process?

nathancahill

Posted 2014-03-12T21:03:39.480

Reputation: 152

1the output you see may be stdout and stderr, but a pipe only passes on stdout. – MaQleod – 2014-03-12T21:40:10.990

@MaQleod is there a way to know if it's stderr? Is there a way to pass stderr to another pipe? – nathancahill – 2014-03-13T01:10:56.847

Actually, just figured it out. I added 3>&1 1>&2 2>&3 before the pipe, and everything works. Thanks for the correct diagnosis! If you add an answer I'll accept it. – nathancahill – 2014-03-13T01:17:17.700

Using grep to filter output of a process

Answers