Shell: redirect stdout to /dev/null and stderr to stdout [duplicate]

Question

I saw this interesting question at a comment on cyberciti.biz.

That I found I even can't find a flexible way to do this in one-line command with sh.

As far my thought for the solution is:

tmp_file=`mktemp` (./script 2>$tmp_file >/dev/null; cat $tmp_file) | ./other-script rm tmp_file 

But you see, this is not synchronous, and fatally, it's so ugly.

Welcome to share you mind about this. :)

Answer 1

You want

./script 2>&1 1>/dev/null | ./other-script 

The order here is important. Let's assume stdin (fd 0), stdout (fd 1) and stderr (fd 2) are all connected to a tty initially, so

0: /dev/tty, 1: /dev/tty, 2: /dev/tty 

The first thing that gets set up is the pipe. other-script's stdin gets connected to the pipe, and script's stdout gets connected to the pipe, so script's file descriptors so far look like:

0: /dev/tty, 1: pipe, 2: /dev/tty 

Next, the redirections occur, from left to right. 2>&1 makes fd 2 go wherever fd 1 is currently going, which is the pipe.

0: /dev/tty, 1: pipe, 2: pipe 

Lastly, 1>/dev/null redirects fd1 to /dev/null

0: /dev/tty, 1: /dev/null, 2: pipe 

End result, script's stdout is silenced, and its stderr is sent through the pipe, which ends up in other-script's stdin.

Also see http://bash-hackers.org/wiki/doku.php/howto/redirection_tutorial

Also note that 1>/dev/null is synonymous to, but more explicit than >/dev/null

Answer 2

How about this:

./script 3>&1 1>/dev/null 2>&3 | ./other-script 

The idea is to "backup" stdout descriptor, close the original stdout and then redirect strerr to saved stdout.

Its much similar to the solution provided by geirha, but its more explicit (bash coding can easily become very obscured).