[SOLVED] Piping both stdout and stderr in bash?

Piping both stdout and stderr in bash?

It seems that newer versions of bash have the &> operator, which (if I understand correctly) redirects both stdout and stderr to a file (or &>> appends to the file).

What's the simplest way to achieve the same thing, but instead piping to another command?

For example, in this line:

some-command | grep -i SomeError

I'd like the grep to match on content both in stdout and stderr (effectively, have them combined into one stream).

Solution

To combine stdout and stderr you would redirect stderr to stdout using 2>&1:

$ { echo "stdout"; echo "stderr" 1>&2; } 2>&1 | grep -v std
$

You can read more about redirection in the Bash manual: Redirections

To elaborate

This command group prints to stdout and stderr:

$ { echo "stdout"; echo "stderr" 1>&2; }
stdout
stderr
$

1>&2 redirects stdout (file descriptor 1) to stderr (file descriptor 2)

When we pipe to grep, the pipe only redirects stdout to grep's stdin, thus grep only filters stdout.

$ { echo "stdout"; echo "stderr" 1>&2; } | grep -v std
stderr
$

"stdout" goes to stdout while "stderr" goes to stderr. grep only sees "stdout", hence "stderr" prints to the terminal.

2>&1 redirects stderr back to stdout and grep sees both strings on stdin, thus filters out both.

Regarding your example

POSIX:

some-command 2>&1 | grep -i SomeError

or, using >=bash-4:

some-command |& grep -i SomeError