With bash, how can I pipe standard error into another process?

Question

It's well known how to pipe the standard ouput of a process into another processes standard input:

proc1 | proc2

But what if I want to send the standard error of proc1 to proc2 and leave the standard output going to its current location? You would think bash would have a command along the lines of:

proc1 2| proc2

But, alas, no. Is there any way to do this?

Answer 1

You can use the following trick to swap stdout and stderr. Then you just use the regular pipe functionality.

( proc1 3>&1 1>&2- 2>&3- ) | proc2

Provided stdout and stderr both pointed to the same place at the start, this will give you what you need.

What it does:

• 3>&1 creates a new file handle 3 which is set to the current 1 (original stdout) just to save it somewhere.
• 1>2- sets stdout to got to the current file handle 2 (original stderr) then closes 2.
• 2>3- sets stderr to got to the current file handle 3 (original stdout) then closes 3.

It's effectively the swap command you see in sorting:

temp   = value1;
value1 = value2;
value2 = temp;

Answer 2

Bash 4 has this feature:

If `|&' is used, the standard error of command1 is connected to command2's standard input through the pipe; it is shorthand for 2>&1 |. This implicit redirection of the standard error is performed after any redirections specified by the command.

zsh also has this feature.