bashstdoutstderrpiping

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 (&>> 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.


Solution

  • (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