separte pipe for stderr and stout in a bash script

ask4info · 03-19-2007, 03:58 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, 04:09 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, 01:07 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, 03:58 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, 08:04 PM

Code:

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

ta0kira

ta0kira · 03-24-2007, 08:09 PM

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

Code:

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

ta0kira

i_am_sorry · 09-05-2012, 12:57 PM

I am sorry for necroing this thread, but I found it while searching for a very similar problem and in case someone else finds this after me I want to provide an even better answer.

First,

Code:

( cmd | sed 's|out|OUT|g' ) 3>&1 1>&2 2>&3 | sed 's|err|ERR|g'

is really all you need, the swapping at the end is superfluous, because nothing is done with it after the second sed and the descriptors get reset to their previous state automatically.

Now to the better solution:

Code:

cmd 1> >(sed 's|out|OUT|g') 2> >(sed 's|err|ERR|g')

It's that simple

As a little extra *g*, you can use this to play all files found under the current directory with mplayer without losing control (because /dev/stdin doesn't get redirected from /dev/null) and without using a pipe made by mkfifo:

Code:

(xargs -a /proc/self/fd/3 -0 mplayer -vo null -shuffle) 3< <(find -type f -print0)

my source: h++p://stupefydeveloper.blogspot.de/2008/10/bash-file-descriptors_20.html

I hope this necro was not too annoying and instead relevant and "timely" enough

David the H. · 09-07-2012, 01:39 AM

Reopening an old thread is usually ok as long as you're adding something new and useful to the original conversation, and particularly if you also make it clear that you are re-opening it. You've done both, so I say good work.

I just want to point out clearly that what you've posted uses the bash-specific process substitution, and is therefore not portable across shells, unlike the standardized redirections and fifos. But they are very convenient.

As the above link points out, ksh appears to also have process substitution built in, but apparently it can't be combined with shell redirections in the way shown above. Perhaps with dedicated file descriptors?