Bash command exit status

humbletech99 · 07-06-2006, 05:44 AM

When I do

Code:

somecommand | head -n 2

$? is always the exit status of the "head" command rather than that of "somecommand"

Is there a way to do this and to test the exit status of "somecommand" rather than "head" (which will always be success even if somecommand fails) ?

raskin · 07-06-2006, 05:55 AM

I guess it's rather hard: really, somecommand will very often get sigpipe after head exits. So its exit status is mangled. You can create a temporary file with mktemp, redirect somecommand to it, remember exit status, show two first lines of a file to user and then remove the file.

humbletech99 · 07-06-2006, 06:01 AM

that's what I've ended up doing but it is not elegent and if /tmp is full the script won't work as expected...

druuna · 07-06-2006, 06:02 AM

Hi,

Like raskin said, this is not easy to solve without some extra work. Maybe this will help:

somecommand 1>/dev/null 2>&1 && somecommand | head -n 2

If the first part (somecommand 1>/dev/null 2>&1) fails, the rest (somecommand | head -n 2) is not executed and accordingly the exit status is 1. If the first part works, the second part is executed (with always has exit status 0, as you stated).

BTW: The && stands for AND.

Hope this helps.

spirit receiver · 07-06-2006, 09:10 AM

This might be of interest (from man bash):

Quote:

The return status of a pipeline is the exit status of the last command, unless the pipefail option is enabled. If pipefail is enabled, the pipeline's return status is the value of the last (rightmost) command to exit with a non-zero status, or zero if all commands exit successfully.

druuna · 07-06-2006, 09:49 AM

Hi again,

Nice, did not know that.

But: The pipefail option depends on the bash version you are using. My version uses the ! to toggle the way the exit status is handled inside a pipe, pipefail is not recognized or mentioned in the manpage.

This:

! somecommand | head -n 2

Does the trick on my box.

$ bash --version
GNU bash, version 2.05b.0(1)-release (i686-pc-linux-gnu)
Copyright (C) 2002 Free Software Foundation, Inc.

humbletech99 · 07-06-2006, 09:54 AM

thank you guys, that was very helpful!

druuna: your suggestion does work and is quite clever, but unfortunately the head command only filters the second command after the && so I can't catch and limit the output.

spirit receiver: I read the man bash again and found the references for pipefail, then at the beginning of the script I did "set -o pipefail" and but later in the script it seems to always return status of 141 for the pipe command regarless of whether it works or not!

spirit receiver · 07-06-2006, 10:08 AM

Works fine here. Maybe you should post the code that causes your problems.

Edit: As an alternative, you could use process substitution:

Code:

cat file-does-not-exist > >(head -2); echo $?
cat: file-does-not-exist: Datei oder Verzeichnis nicht gefunden
1

druuna · 07-06-2006, 10:16 AM

Hi,

Quote:

druuna: your suggestion does work and is quite clever, but unfortunately the head command only filters the second command after the && so I can't catch and limit the output.

The first part (before &&) is there to check if the second part should be executed, the second part can be changed/edited or something else completely:

ls [0-9]* 1>dev/null 2>&1 && echo "Yep, present"
The echo part will only be executed if the first part is true, otherwise the exit status will be 1.

ls [0-9]* 1>/dev/null 2>&1 && ls [0-9]* | grep 31 | head -n 2
All after && will only be executed if the first part is true, otherwise the exit status will be 1.

But using pipetail (or !) is more elegant

humbletech99 · 07-06-2006, 10:22 AM

the problem is limited to the command. From the command line I do:

Code:

somecommand -options blah
echo $?
1

but then I do

Code:

somecommand -options same blah | head -n 2
echo $?
141

humbletech99 · 07-06-2006, 10:27 AM

Quote:

Originally Posted by druuna

Hi,

The first part (before &&) is there to check if the second part should be executed, the second part can be changed/edited or something else completely:

ls [0-9]* 1>dev/null 2>&1 && echo "Yep, present"
The echo part will only be executed if the first part is true, otherwise the exit status will be 1.

ls [0-9]* 1>/dev/null 2>&1 && ls [0-9]* | grep 31 | head -n 2
All after && will only be executed if the first part is true, otherwise the exit status will be 1.

But using pipetail (or !) is more elegant

I fully understand what you are saying but in practice the |head fails to catch the output from the first command which is the whole point. If that was what I wanted to do, I could leave the pipe off altogether and just collect the exit status as normal...

raskin · 07-06-2006, 12:10 PM

I told you! When you run like this, head gets two lines - and exits. Ah oh who will collect somecommand's output? Nothing? Then somecommand gets SIGPIPE - broken pipe (it is, as head is dead). And surely it exits, as it has no signal handler there. Exit status = 128 + signal nr = 128 + 13 = 141 . Everything as predicted.

humbletech99 · 07-06-2006, 12:20 PM

I find that the simplest and best way of doing this is:

Code:

output="`somecommand -options blah`"
result=$?
echo "$output" | head -n 2

This doesn't use a tempfile or anything which is better and is the clearest way I can see of doing this.