/dev/null 2>&1 ???????????

waelaltaqi · 05-17-2007, 04:10 PM

i know that /dev/null is used redirect output of commands in shell scripts to a null device so the user won't see a lot of output when running certain scripts.
here is my question: what is the difference between these two command?

Code:

ls -l /etc | grep resolv.conf > /dev/null

and

Code:

ls -l /etc/ | grep resolv.conf > /dev/null 2>&1

i just want to understand what 2>&1 means.

acid_kewpie · 05-17-2007, 04:23 PM

> redirects the stdout stream, same as 1> as 1 is the number of that stream. 2 is the stderr stream. 2>&1 causes stderr to be redirected into stdout and then sunk into /dev/null, otherwise whilst all normal output disappears, any errors (e.g ls command not found, resolv.conf not allowd to be opened) would still show up. in the longest form you could write....

ls -l /etc/ | grep resolv.conf 1> /dev/null 2> /dev/null

but oviosuly, that's longer hence shortcuts and such.

waelaltaqi · 05-17-2007, 10:22 PM

after reading your reply and http://en.wikipedia.org/wiki/Redirection_%28Unix%29 i understand now why it's very commonly used. Otherwise, a lot of scripts will show a whole load of errors.
thanks.