Why doesn't grep using pipe work here?

Question

I have the following command:

find / -name libGL.so.1

Which returns lots of lines with "Permission denied". I want to exclude such lines, so I added the following:

find / -name libGL.so.1 | grep -v 'denied'

But the output is the same - my grep -v 'denied' is not filtering out the lines with Permission denied. I've tried many variations, looked over grep tutorials, but I cannot figure out the problem. Any suggestions?

Solution 1

That's nothing to do with grep - it's because the pipe | redirects the standard output stream stdout whereas the Permission denied messages are in the standard error stream stderr. You could achieve the result you want by combining the streams using 2>&1 (redirect the stream whose file descriptor is 2 to the stream whose file descriptor is 1) so that stderr as well as stdout gets piped to the input of the grep command

find / -name libGL.so.1 2>&1 | grep -v 'denied'

but it would be more usual to simply discard stderr altogether by redirecting it to /dev/null

find / -name libGL.so.1 2>/dev/null

Using |& instead of 2>&1 |

If you take a look at the Bash man page you'll likely notice this blurb:

If |& is used, the standard error of command is connected to command2's standard input through the pipe; it is shorthand for 2>&1 |.

So you can also use this construct as well if you want to join STDERR and STDOUT:

find / -name libGL.so.1 |& grep -v 'denied'

Solution 2

Your command should be:

find / -name libGL.so.1 2>/dev/null

Find is complaining about permissions on standard error (fd2). In order to eliminate those lines, redirect (>) standard out to the bit bucket (/dev/null).

Solution 3

The "permission denied" lines are going to the stderr (standard error) stream, but you are piping stdout (standard out) through grep.

You can redirect away stderr entirely with

find / -name libGL.so.1 2> /dev/null

Solution 4

Have you tried calling the command with sudo?

sudo find / -name libGL.so.1

If it still shows the message, use the already mentioned redirect of stderr (fd=2) to nirvana (/dev/null):

sudo find / -name libGL.so.1 2> /dev/null

More ideas here, good luck!

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Comments

[Izkata]

Additional note on combining them: If for some bizarre reason the file exists at /foo/bar/denied/libGL.so.1, combining stdout and stderr then filtering would hide the result