separte pipe for stderr and stout in a bash script

ask4info · 03-19-2007, 04:58 PM

hi all

what's the best way to output the stderr and stdout of a shell command thru different pipes?
I want to process the stderr different from stout, generated by a command. How to combine both and still have the same output descriptors? In other words, how
to combine ("a one liner"!) the two commands:

command 2>&1 | .. pipe 1 .. -> sterr (&2)
command | .. pipe 2 ... -> stdout (&1)

any ideas?

thanks

cfaj · 03-19-2007, 05:09 PM

Quote:

Originally Posted by ask4info

hi all

what's the best way to output the stderr and stdout of a shell command thru different pipes?
I want to process the stderr different from stout, generated by a command. How to combine both and still have the same output descriptors? In other words, how
to combine ("a one liner"!) the two commands:

command 2>&1 | .. pipe 1 .. -> sterr (&2)
command | .. pipe 2 ... -> stdout (&1)

any ideas?

Use a named pipe. This is a very basic example:
Code:
q()
{
  echo 1_$RANDOM
  echo 2_$RANDOM >&2
}

mkfifo stdo
cat stdo &
q 2>stdo | cat
rm stdo
Or save stderr to a regular file and have your command read from that.

ask4info · 03-20-2007, 02:07 AM

hi cfai

thanks for your sample.

thats what i did too however i was wondering wether there is a "smart" way to do it with e.g. the 'exec' command somehow. The sample below is now my final:

#!/bin/bash

q()
{
echo 1_stdout
echo 2_stderr >&2
}

p()
{
rm -fr /tmp/stdo
mkfifo /tmp/stdo
cat /tmp/stdo | sed 's|err|ERR|g' >&2 &
q 2>/tmp/stdo | sed 's|out|OUT|g'
rm -fr /tmp/stdo
}

echo -n "sterr: "
p 1>/dev/null
echo -n "stout: "
p 2>/dev/null

Note: what was missing was the redirection of the named pipe to &2

have a nice day

ask4info · 03-20-2007, 04:58 AM

hi cfai

i just found out a better way using the designator swapping technic:

#!/bin/bash

cmd()
{
echo 1_stdout
echo 2_stderr 1>&2
}

out1() # different sub shell
{
( ( cmd | sed 's|out|OUT|g' ) 3>&1 1>&2 2>&3 | sed 's|err|ERR|g' ) 3>&1 1>&2 2>&3
# \----swap----/ |-> stout=stderr \----swap----/
}

out2() # current sub shell
{
{ { cmd | sed 's|out|OUT|g'; } 3>&1 1>&2 2>&3 | sed 's|err|ERR|g'; } 3>&1 1>&2 2>&3
# \----swap----/ |-> stout=stderr \----swap----/
}

echo -n "sterr: "
out1 1>/dev/null
echo -n "stout: "
out1 2>/dev/null

until next question...!

ta0kira · 03-24-2007, 09:04 PM

Code:

#!/bin/bash
echo "NORMAL!" > /proc/self/fd/1
echo "ERROR!" > /proc/self/fd/2

ta0kira

ta0kira · 03-24-2007, 09:09 PM

Wait, maybe I'm misunderstanding. Will this work?

Code:

( ./your-script > ./stdout_pipe ) 2> ./stderr_pipe

ta0kira