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OSXFAQ Mac OS X UNIX Tip-of-the-Day
Redirection - Combine Redirections
Because of the order in which Bash processes a command line and redirection syntax, you must be careful combining the merging of standard out and standard err (1>&2 or 2>&1) with redirection.
This example uses an 'ls' command line that lists 'file' (which fails) and the directory 'test' that contains 'file':
$ ls test file
ls: file: No such file or directory
test:
file
We merge standard out into standard error and redirect standard error (hopefully this is now both out and error) to the file 'err'.
Our first attempt fails:
$ ls test file 1>&2 2>err
test:
file
$
Reversing the order of the redirect and the merge will yield the desired behaviour:
$ ls test file 2>err 1>&2
$ cat err
ls: file: No such file or directory
test:
file
$
The reason the first attempt fails is that out is merged with error at the time error is still going to the screen. Only later is error redirected to the file, too late for out to be redirected too. (I've always though that the two syntaxes work the opposite way around to what seems natural.)
To throw away either or both of standard out and standard error, redirect to the special file /dev/null. For example, if we like only good news:
$ ls test file 2>/dev/null
test:
file
$
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