How can I redirect stderr to a pipe?

A pipe can only carry standard output (stdout) of a program. To pipe standard error (stderr) through it, you need to redirect stderr to the same destination as stdout. Optionally you can close stdout or redirect it to /dev/null to only get stderr. Some sample code:

# Bourne
# Assume 'myprog' is a program that writes to both stdout and stderr.

# version 1: redirect stderr to the pipe while stdout survives (both come
# mixed)
myprog 2>&1 | grep ...

# version 2: redirect stderr to the pipe without getting stdout (it's
# redirected to /dev/null)
myprog 2>&1 >/dev/null | grep ...

# same idea, this time storing stdout in a file
myprog 2>&1 >file | grep ...

Another simple example of redirection stdout and stderr:

# Bourne
{ command | stdout_reader; } 2>&1 | stderr_reader

For further explanation of how redirections and pipes interact, see FAQ #55.

This has an obvious application with programs like dialog, which draws (using ncurses) windows onto the screen (stdout), and returns results on stderr. One way to deal with this would be to redirect stderr to a temporary file. But this is not necessary -- see FAQ #40 for examples of using dialog specifically!

In the examples above (as well as FAQ #40), we either discarded stdout altogether, or sent it to a known device (/dev/tty for the user's terminal). One may also pipe stderr only but keep stdout intact (without a priori knowledge of where the script's output is going). This is a bit trickier.

# Bourne
# Redirect stderr to a pipe, keeping stdout unaffected.

exec 3>&1                       # Save current "value" of stdout.
myprog 2>&1 >&3 | grep ...      # Send stdout to FD 3.
exec 3>&-                       # Now close it for the remainder of the script.

# Thanks to http://www.tldp.org/LDP/abs/html/io-redirection.html

The same can be done without exec:

# POSIX
$ myfunc () { echo "I'm stdout"; echo "I'm stderr" >&2; }
$ { myfunc 2>&1 1>&3 3>&- | cat  > stderr.file 3>&-; } 3>&1
I'm stdout
$ cat stderr.file
I'm stderr

The fd 3 is closed (3>&-) so that the commands do not inherit it. Note bash allows to duplicate and close in one redirection: 1>&3- You can check the difference on linux trying the following:

# Bash
{ bash <<< 'lsof -a -p $$ -d1,2,3'   ;} 3>&1
{ bash <<< 'lsof -a -p $$ -d1,2,3' 3>&-  ;} 3>&1

To show a dialog one-liner:

# Bourne
exec 3>&1
dialog --menu Title 0 0 0 FirstItem FirstDescription 2>&1 >&3 | sed 's/First/Only/'
exec 3>&-

This will have the dialog window working properly, yet it will be the output of dialog (returned to stderr) being altered by the sed.

A similar effect can be achieved with ProcessSubstitution:

# Bash
perl -e 'print "stdout\n"; warn "stderr\n"' 2> >(tr '[:lower:]' '[:upper:]')

This will pipe standard error through the tr command.

See this redirection tutorial (with an example that redirects stdout to one pipe and stderr to another pipe).

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