How can I pipe stderr, and not stdout?

Question

I have a program that writes information to stdout and stderr, and I need to process the stderr with grep, leaving stdout aside.

Using a temporary file, one could do it in two steps:

command > /dev/null 2> temp.file
grep 'something' temp.file

But how can this be achieved without temp files, using one command and pipes?

Answer 1

First redirect stderr to stdout — the pipe; then redirect stdout to /dev/null (without changing where stderr is going):

command 2>&1 >/dev/null | grep 'something'

For the details of I/O redirection in all its variety, see the chapter on Redirections in the Bash reference manual.

Note that the sequence of I/O redirections is interpreted left-to-right, but pipes are set up before the I/O redirections are interpreted. File descriptors such as 1 and 2 are references to open file descriptions. The operation 2>&1 makes file descriptor 2 aka stderr refer to the same open file description as file descriptor 1 aka stdout is currently referring to (see dup2() and open()). The operation >/dev/null then changes file descriptor 1 so that it refers to an open file description for /dev/null, but that doesn't change the fact that file descriptor 2 refers to the open file description which file descriptor 1 was originally pointing to — namely, the pipe.