[SOLVED] About Traps in bash script

ddenial · 05-13-2020, 08:31 AM

Hello All

I'm testing traps in Linux, and hence I've some doubts. Following is script for testing traps.

Code:

#!/bin/bash
set -euo pipefail

echo "Script PID: $$"

function CleanUp {
        echo 
        echo Oops I just got killed
        exit 2
}
trap 'CleanUp' SIGHUP SIGINT SIGQUIT SIGPIPE SIGTERM EXIT 

while true ; do 
        sleep 1
done

Output:

Code:

$ ./test.sh 
Script PID: 16467
^C
Oops I just got killed

Oops I just got killed

My doubts are:

What is 'set -euo pipefail'? I've seen most bash scripts start with this line. I know 'set -u', which makes mandatory to declare a variable before using it. But what about -e and -o pipefail? Read man pages, but couldn't understand.
Why I'm getting multiple trap echo lines?
How do I get exit code based on the type of interrupt signal rather than the exit code which I have specified explicitly (like 2 here)?

Thanks

wpeckham · 05-13-2020, 08:44 AM

Is this homework?

Quote:

My doubts are:
What is 'set -euo pipefail'? I've seen most bash scripts start with this line.
I know 'set -u', which makes mandatory to declare a variable before using it. But what about -e and -o pipefail?
Read man pages, but couldn't understand.
Why I'm getting multiple trap echo lines?
How do I get exit code based on the type of interrupt signal rather than the exit code which I have specified explicitly (like 2 here)?

#1 This set is actually fairly rare, EXCEPT when the script needs to trap events.
The man pages are clear enough. Look up the individual options for set -e -u -o and consider the interaction between them mentioned in the pipefail parameter to the -o option.

#2 You get multiple trap exit lines because you have trapped two events in that script. Think carefully about the flow.

#3 To treat different events differently, you might want to trap different events or signals to different managing functions. There is no directive that "there can be only one"! Consider carefully the syntax, and test your options and code to be sure that you understand the expected outcomes.

shruggy · 05-13-2020, 08:46 AM

to 1. If you don't know what set -e does, don't use it. See BashFAQ #105. About pipefail, see the last paragraph here.
to 2. The first one on SIGINT, the second one on EXIT.
to 3. Then trap each signal separately.

Turbocapitalist · 05-13-2020, 08:51 AM

1. The -e means that the script fails if any step fails. The -o pipefail means that if any one of a series of piped programs fails, then the pipe exits with the exit code of the failed program. See "man bash" and scroll down to "Pipelines".

2. You get two lines of output because the routine is called twice. Once from SIGINT caused by ctrl-C, then again by EXIT called when the script finally exits. You might want to remove EXIT
See "man 7 signal", except that signal 0, the EXIT, is not mentioned there.

3. Make separate trap routines for each signal you wish to identify or else look at the contents of the $_ variable within a single routine. A case statement might be a good choice for that.

ddenial · 05-13-2020, 09:54 AM

Thanks everybody.

scasey · 05-13-2020, 07:29 PM

Quote:

Originally Posted by ddenial

Thanks everybody.

Would you please share what worked for you?

ddenial · 05-14-2020, 12:47 AM

Quote:

Originally Posted by scasey

Would you please share what worked for you?

Sorry about this. https://www.linuxquestions.org/quest...rd-4175675084/