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.
(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