pipe stdout and stderr to two different processes in shell script?

I've a pipline doing just

 command1 | command2

So, stdout of command1 goes to command2 , while stderr of command1 go to the terminal (or wherever stdout of the shell is).

How can I pipe stderr of command1 to a third process (command3) while stdout is still going to command2 ?

Solution 1:

Use another file descriptor

{ command1 2>&3 | command2; } 3>&1 1>&2 | command3

You can use up to 7 other file descriptors: from 3 to 9.
If you want more explanation, please ask, I can explain ;-)

Test

{ { echo a; echo >&2 b; } 2>&3 | sed >&2 's/$/1/'; } 3>&1 1>&2 | sed 's/$/2/'

output:

b2
a1

Example

Produce two log files:
1. stderr only
2. stderr and stdout

{ { { command 2>&1 1>&3; } | tee err-only.log; } 3>&1; } > err-and-stdout.log

If command is echo "stdout"; echo "stderr" >&2 then we can test it like that:

$ { { { echo out>&3;echo err>&1;}| tee err-only.log;} 3>&1;} > err-and-stdout.log
$ head err-only.log err-and-stdout.log
==> err-only.log <==
err

==> err-and-stdout.log <==
out
err

Solution 2:

The accepted answer results in the reversing of stdout and stderr. Here's a method that preserves them (since Googling on that purpose brings up this post):

{ command 2>&1 1>&3 3>&- | stderr_command; } 3>&1 1>&2 | stdout_command

Notice:

• 3>&- is required to prevent fd 3 from being inherited by command. (As this can lead to unexpected results depending on what command does inside.)

Parts explained:

1. Outer part first:

   1. 3>&1 -- fd 3 for { ... } is set to what fd 1 was (i.e. stdout)
   2. 1>&2 -- fd 1 for { ... } is set to what fd 2 was (i.e. stderr)
   3. | stdout_command -- fd 1 (was stdout) is piped through stdout_command

2. Inner part inherits file descriptors from the outer part:

   1. 2>&1 -- fd 2 for command is set to what fd 1 was (i.e. stderr as per outer part)
   2. 1>&3 -- fd 1 for command is set to what fd 3 was (i.e. stdout as per outer part)
   3. 3>&- -- fd 3 for command is set to nothing (i.e. closed)
   4. | stderr_command -- fd 1 (was stderr) is piped through stderr_command

Example:

foo() {
    echo a
    echo b >&2
    echo c
    echo d >&2
}

{ foo 2>&1 1>&3 3>&- | sed -u 's/^/err: /'; } 3>&1 1>&2 | sed -u 's/^/out: /'

Output:

out: a
err: b
err: d
out: c

(Order of a -> c and b -> d will always be indeterminate because there's no form of synchronization between stderr_command and stdout_command.)