Piping both stdout and stderr in bash?

Question

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

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

For example, in this line:

cmd-doesnt-respect-difference-between-stdout-and-stderr | grep -i SomeError

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

Note: this question is asking about piping, not redirecting - so it is not a duplicate of the question it's currently marked as a duplicate of.

Answer 1

(Note that &>>file appends to a file while &> would redirect and overwrite a previously existing file.)

To combine stdout and stderr you would redirect the latter to the former using 1>&2. This redirects stdout (file descriptor 1) to stderr (file descriptor 2), e.g.:

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

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

On the other hand:

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

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

You can read more about redirection here.

Regarding your example (POSIX):

cmd-doesnt-respect-difference-between-stdout-and-stderr 2>&1 | grep -i SomeError

or, using >=bash-4:

cmd-doesnt-respect-difference-between-stdout-and-stderr |& grep -i SomeError

Answer 2

Bash has a shorthand for 2>&1 |, namely |&, which pipes both stdout and stderr (see the manual):

cmd-doesnt-respect-difference-between-stdout-and-stderr |& grep -i SomeError

This was introduced in Bash 4.0, see the release notes.

Answer 3

If you want to pipe STDOUT and STDERR to different subshells instead of both to the same one (as |& / 2>&1 does), I use this:

producer \
  >  >( stdout_pipeline... ) \
  2> >( stderr_pipeline... ) \
  ...

While this uses the redirection operator, it uses pipes under the hood to connect the producer to the subshells.

You can process multiple output streams (2 is not the limit) this way, and also pipe (potentially multiple) input streams from subshells in a similar way with the penguin operator ([n]< <(...)).