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

by

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

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