Sending and trapping signals
In some circumstances you will wish that accidentally typing
ctrl-C will not kill the current process. This would be
true, for example, in the execution of a complex script that makes
non-trivial changes to files, where your filespace would be left in
a mess if the script died when only half-completed. There is a
mechanism, known as trapping signals, whereby a
shell takes action specified by you when it receives a signal,
rather than taking the default action. The command used to
intercept a signal is trap, and it is used in the
following manner:
trap 'action' signal
The action is either null or a string containing a
command, and the signal is one of the signal names. Create
a script interrupts containing:
trap 'echo Ouch' INT
echo Beginning
sleep 10
echo ten seconds
sleep 10
echo twenty seconds
sleep 10
echo thirty seconds and ended
and execute it using sh. Try to interrupt it by
typing ctrl-C at your terminal a couple of times and see
what happens. You should see
$ sh interrupts
Beginning
ctrl-C
Ouch
ten seconds
ctrl-C
Ouch
twenty seconds
ctrl-C
Ouch
thirty seconds and ended
Similar to SIGINT is SIGQUIT. This
signal can usually be generated from your terminal by typing
ctrl-\. The difference between the two is that
SIGQUIT will on many systems generate a
coredump, that is a file named core
which contains information about the interrupted command when it
received the signal. The file core can be used to
examine the state of the program when the signal was received. A
core file can be interrogated by an experienced UNIX
programmer using utilities such as dbx, but at this
stage you will not be interested in its contents. A coredump is
usually a big file and should be removed unless you intend to use
it. Try the following:
$ sleep 1000 &
[1] 17465
$ kill -s QUIT %1
[1]+ Quit (core dumped) sleep 1000
Check which files you have using ls and you should
find that one named core has now been created.
When a shell script exits, a signal is sent to it called
EXIT, which can be trapped. To see which signals you
have trapped, use trap with no arguments. Be careful
which signals you trap - in particular, don't try KILL
(or you would have difficulty using kill to destroy
the process) or HUP (or unpredictable things would
happen if you tried to suspend the process). If you have set a trap
on a signal, you can remove it by giving it the action
- (minus symbol), so:
trap - INT
will restore ctrl-C to its normal function.
Worked example
By setting a trap on your login shell, arrange to be given the
message Goodbye when you logout.
Solution: The signal EXIT is sent to
your login shell when you logout, so use trap to
perform an echo when this signal is intercepted:
$ trap 'echo Goodbye'
EXIT
There are various other signals, although most UNIX systems
support many more.
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