Traps and Signals
In this lesson you will learn what a signal is, how the `kill` command can be used to send signals, and how signals can be trapped. In addition, you'll cover the 'wait' bash builtin, and what a 'process group' is.
How Important is this Lesson?
Traps are an advanced concept. If you’re new to bash you might want to follow this lesson to be aware of it, and apply it as you get more knowledge of Linux or go deeper into bash scripting.
Triggering signals
Any easy way to trigger a signal is one you will likely already have used.
Follow the instructions here:
sleep 999
Now hit the CTRL key down, hold it, and then hit the c key (CTRL-c). Then get the exit code:
echo $?
You should have got the output of a number over 128. You will of course
remember that $? is a special variable that gives the exit code of the
last-run command.
What you are less likely to have remembered is that exit codes over 128 indicate that a signal triggered the exit, and that to determine the signal number you take 128 away from the number you saw.
Bonus points if you did remember!
Can you work out what the signal was that stopped the sleep command?
The signals are usually documented in the signal man page.
man signalman 7 signal
Note:
manpages have different sections.man manwill explain more if you’re interested, but to get a specific section, you put the section number in the middle, as above. Find out what section 7 is by readingman man. You might not have ...