Trap 'Ctrl + c' for bash script but not for process open in this script
Trap 'Ctrl + c' for bash script but not for process open in this script
I tried to have an interactive program in a bash script :
my_program
And I wish to be able to close it with 'Ctrl + c'.
But when I do it my script is closing as well.
I know about.
trap '' 2
my_program
trap 2
But in this case, I just can't close my_program with Ctrl + c.
my_program
Do you have any idea how to allow Ctrl + c on a program, but not closing the script running it ?
EDIT : add example
#!/bin/bash
my_program
my_program2
If i use Ctrl + c to close my_program, my_program2 is never executed because the whole script is exited.
my_program
my_program2
3 Answers
3
You should use trap true 2 or trap : 2 instead of trap '' 2. That's what "help trap" in a bash shell says about it:
trap true 2
trap : 2
trap '' 2
If ARG is the null string each SIGNAL_SPEC is ignored by the
shell and by the commands it invokes.
Example:
$ cat /tmp/test
#! /bin/sh
trap : INT
cat
echo first cat killed
cat
echo second cat killed
echo done
$ /tmp/test
<press control-C>
^Cfirst cat killed
<press control-C>
^Csecond cat killed
done
tail
You can reset a trap to its default by giving the trap command - as its action argument. If you do this in a subshell, it won't affect the trap in the parent shell. In your script, you can do this for each command that you need to be interruptible with Ctrl-C:
-
#!/bin/bash
# make the shell (and its children) ignore SIGINT
trap '' INT
.
.
.
# but this child won't ignore SIGINT
(trap - INT; my_program)
# the rest of the script is still ignoring SIGINT
.
.
.
While the accepted answer is probably better and more canonical for shell, this is a great answer in itself in that it introduces the general principle (not shell-specific) for how to do this sort of signal masking/ignoring safely.
– R..
Sep 10 '18 at 17:11
I guess you can
exec my_program in the subshell, to be slightly more efficient.– Toby Speight
Sep 10 '18 at 17:15
exec my_program
@R.. that's absolutely not shell specific -- if you set a signal to SIG_IGN (that's what trap with an empty string is doing), that state will be inherited through exec(), except for SIGCHLD. That's POSIX mandated behavior. See also the answer to stackoverflow.com/questions/32708086/…
– mosvy
Sep 10 '18 at 17:26
@mosvy: I don't see what part of my comment you're disagreeing with. My whole point was that "mask/ignore before fork, unmask/unignore in the child before exec" is a general principle that's useful to know outside the context of just shell programming.
– R..
Sep 10 '18 at 17:30
This is a very useful answer. As an experienced Linux user, I can say I've never heard of this before. It has quite a lot of uses... you got my upvote and my bookmark.
– Dev
Sep 11 '18 at 4:37
When you use Crtl+C, you interrupt the program ("kill" it).
What you are probably looking for is to suspend your program ("pause" it). For this, you can use Crtl+Z.
Once your program is paused, you can see it using jobs. For example:[1]+ Stopped ./foobar
Here I only have one job, job #1, but there can be more than one - each job has its own number.
You can control you suspended process using a number of commands, for example bg, fg and kill.bg %1 will restart job #1 in the backgroundfg %1 will restart job #1 in the foregroundkill %1 will kill job #1
Note that you can use bg and fg with no arguments if you only have one active job.
Hope this helps.
jobs
[1]+ Stopped ./foobar
bg
fg
kill
bg %1
fg %1
kill %1
bg
fg
Thank you but no, I wish to do Ctrl + c.
– bob dylan
Sep 10 '18 at 12:41
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How about killing
tails instead of killing cats next time?– kubanczyk
Sep 11 '18 at 7:13