How can I pipe stderr, and not stdout?

Question

I have a program that writes information to stdout and stderr, and I need to process the stderr with grep, leaving

Answer 1

Or to swap the output from standard error and standard output over, use:

command 3>&1 1>&2 2>&3

This creates a new file descriptor (3) and assigns it to the same place as 1 (standard output), then assigns fd 1 (standard output) to the same place as fd 2 (standard error) and finally assigns fd 2 (standard error) to the same place as fd 3 (standard output).

Standard error is now available as standard output and the old standard output is preserved in standard error. This may be overkill, but it hopefully gives more details on Bash file descriptors (there are nine available to each process).

Answer 2

It's much easier to visualize things if you think about what's really going on with "redirects" and "pipes." Redirects and pipes in bash do one thing: modify where the process file descriptors 0, 1, and 2 point to (see /proc/[pid]/fd/*).

When a pipe or "|" operator is present on the command line, the first thing to happen is that bash creates a fifo and points the left side command's FD 1 to this fifo, and points the right side command's FD 0 to the same fifo.

Next, the redirect operators for each side are evaluated from left to right, and the current settings are used whenever duplication of the descriptor occurs. This is important because since the pipe was set up first, the FD1 (left side) and FD0 (right side) are already changed from what they might normally have been, and any duplication of these will reflect that fact.

Therefore, when you type something like the following:

command 2>&1 >/dev/null | grep 'something'

Here is what happens, in order:

1. a pipe (fifo) is created. "command FD1" is pointed to this pipe. "grep FD0" also is pointed to this pipe
2. "command FD2" is pointed to where "command FD1" currently points (the pipe)
3. "command FD1" is pointed to /dev/null

So, all output that "command" writes to its FD 2 (stderr) makes its way to the pipe and is read by "grep" on the other side. All output that "command" writes to its FD 1 (stdout) makes its way to /dev/null.

If instead, you run the following:

command >/dev/null 2>&1 | grep 'something'

Here's what happens:

1. a pipe is created and "command FD 1" and "grep FD 0" are pointed to it
2. "command FD 1" is pointed to /dev/null
3. "command FD 2" is pointed to where FD 1 currently points (/dev/null)

So, all stdout and stderr from "command" go to /dev/null. Nothing goes to the pipe, and thus "grep" will close out without displaying anything on the screen.

Also note that redirects (file descriptors) can be read-only (<), write-only (>), or read-write (<>).

A final note. Whether a program writes something to FD1 or FD2, is entirely up to the programmer. Good programming practice dictates that error messages should go to FD 2 and normal output to FD 1, but you will often find sloppy programming that mixes the two or otherwise ignores the convention.

Answer 3

This will redirect command1 stderr to command2 stdin, while leaving command1 stdout as is.

exec 3>&1
command1 2>&1 >&3 3>&- | command2 3>&-
exec 3>&-

Taken from LDP

Answer 4

You can use the rc shell.

First install the package (it's less than 1 MB).

This an example of how you would discard standard output and pipe standard error to grep in rc:

find /proc/ >[1] /dev/null |[2] grep task

You can do it without leaving Bash:

rc -c 'find /proc/ >[1] /dev/null |[2] grep task'

As you may have noticed, you can specify which file descriptor you want piped by using brackets after the pipe.

Standard file descriptors are numerated as such:

• 0 : Standard input
• 1 : Standard output
• 2 : Standard error

Answer 5

I try follow, find it work as well,

command > /dev/null 2>&1 | grep 'something'