[SOLVED] $?

RaptorX · 07-26-2009, 07:55 AM

I was searching for a little if...fi command to check if a fs was already mounted and i stumbled on this:

Code:

cat /proc/mounts | grep /mnt/mymountpoint > /dev/null
if [ $? -eq 0 ] ; then
  echo The path /mnt/mymountpoint is mounted
fi

now what i dont understand is what does $? means in this case?
i have seen $(bash command) or $1, $VARIABLE...

Im fresh in this stuff so please bear with me here.

thanks!

chigurh8 · 07-26-2009, 08:14 AM

$? is the exit status of the last executed program.

chigurh8 · 07-26-2009, 08:19 AM

You'll get a 0 if grep finds it, and a 1 if it doesn't.

druuna · 07-26-2009, 08:35 AM

Just a minor addition to the above post:

Grep can also have an exit status that is 2......

Exit status grep:
0 -> found something,
1 -> found nothing,
2 -> an error occured.

chigurh8 · 07-26-2009, 09:01 AM

Thanks. Apologies for double posting, also ... it won't be happening again.

RaptorX · 07-26-2009, 09:16 AM

thanks for the info guys, does all programs have the same exit status? or is it dependent on the program itself

chigurh8 · 07-26-2009, 09:39 AM

As far as I know, a little of both.

druuna · 07-26-2009, 09:48 AM

Hi,

In general:
If the exit status is 0 (zero) all is ok. This is standard for all the progs/commands that I know of.
If the exit status is anything else, something is up. You need to look in the manpage of the specific command to see what has happened.

By the way, in some cases a none 0 exit status can be ok, grep being an example of that. It's nice that the developers of grep included an exit status of 1 to indicate that nothing is found, but technically this is not an error. It could even be the result you are looking for.

Hope this clears things up a bit.

David the H. · 07-26-2009, 10:26 AM

AIUI, there are some standardized error/exit status codes that are used for common situations, but in the end it's mostly up to the programmer(s) to define what events produce what codes. It may also depend on what programming language you're using.

For bash scripting, see here and here for more information.

RaptorX · 07-26-2009, 12:05 PM

thanks to all for the infos, everything is clear now.