How to redirect a part of stderr and stdout to /dev/null

Question

My program outputs to stderr and stdout. I want to grep for "pattern" either in stderr and stdout. And I want the rest to be sent to /dev/null.

If I pipe after redirect stderr : ./prog 2>/dev/null | grep "pattern" I don't get the lines of stderr that contain "pattern".

If I pipe before redirecting stderr : ./prog | grep "pattern" 2>/dev/null none of stderr is redirected to /dev/null

thank you for your help.

Answer 1

if you do not care whether the string match originates from stdout or stderr, then just merge the two streams by redirecting stderr to stdout, then do the grep:

$ your_program 2>&1 | grep "pattern"

the example works in sh, bash, ksh, zsh. csh should be:

$ your_program |& grep "pattern"

Answer 2

./prog 2>&1 | grep pattern

You must see only the lines containing "pattern"