Shell – Replacing stdout with stderr

file-descriptorsio-redirectionshell

Is it possible to redirect the output from a command to replace the text sent to stdout with the text from stderr?

$ ./script
this line is redirected to stdout
this line is redirected to stderr

$ ./script (insert redirections here)
this line is redirected to stderr

I know I could just use 1>/dev/null, but I want to know if there's a way to do this with the text from stderr redirected to stdout.

Best Answer

You can do it like this:

./script 2>&1 1>/dev/null

This redirects fd 2 to what fd 1 points to (i.e. stdout), then redirects fd 1 to /dev/null. The second redirect doesn't affect the first one, output to fd 2 will be sent to stdout.

Order does matter though. With this:

./script 1>/dev/null 2>&1

will send all output to /dev/null.

Related Solutions

Bash – Use a Variable to Store stderr|stdout Redirection

Another solution could be the following:

#!/bin/bash

verbose=0

exec 3>&1
exec 4>&2

if ((verbose)); then
  echo "verbose=1"
else
  echo "verbose=0"
  exec 1>/dev/null
  exec 2>/dev/null
fi

echo "this should be seen if verbose"
echo "this should always be seen" 1>&3 2>&4

Then add 1>&3 2>&4 only to commands of which you want to see the output.

Linux Shell – How to Redirect stdout & stderr to Logger

You need to combine the output of STDERR and STDOUT prior to piping it to logger. Try this instead:

/home/dirname/application_name -v 2>&1 | logger &

Example

$ echo "hi" 2>&1 | logger &
[1] 26818
[1]+  Done                    echo "hi" 2>&1 | logger

$ sudo tail /var/log/messages
Apr 12 17:53:57 greeneggs saml: hi

You can use the abbreviated notation here as well, if used cautiously in a actual Bash shell (not to be confused with Dash):

$ echo "hi" |& logger &

NOTE: This is equivalent to <cmd1> 2>&1 | <cmd2>. Again only use the above when making use of an actual Bash shell interactively, would be a good way to approach it.

excerpt from ABSG

# |& was added to Bash 4 as an abbreviation for 2>&1 |.

References

Advanced Bash Scripting Guide - Chapter 20. I/O Redirection