Issue
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
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
Answered By - oHo Answer Checked By - David Marino (WPSolving Volunteer)