Bash-Beginners-Guide

immediately with an exit status greater than 128, immediately after which the trap is executed.
12.2.3. More examples
12.2.3.1. Detecting when a variable is used
Bash Guide for Beginners
When debugging longer scripts, you might want to give a variable the trace attribute and trap DEBUG
messages for that variable. Normally you would just declare a variable using an assignment like
VARIABLE=value. Replacing the declaration of the variable with the following lines might provide valuable
information about what your script is doing:
declare -t VARIABLE=value
trap "echo VARIABLE is being used here." DEBUG
# rest of the script
12.2.3.2. Removing rubbish upon exit
The whatis command relies on a database which is regularly built using the makewhatis.cron script with
cron:
#!/bin/bash
LOCKFILE=/var/lock/makewhatis.lock
# Previous makewhatis should execute successfully:
[ -f $LOCKFILE ] && exit 0
# Upon exit, remove lockfile.
trap "{ rm -f $LOCKFILE ; exit 255; }" EXIT
touch $LOCKFILE
makewhatis -u -w
exit 0
12.3. Summary
Signals can be sent to your programs using the kill command or keyboard shortcuts. These signals can be
caught, upon which action can be performed, using the trap statement.
Some programs ignore signals. The only signal that no program can ignore is the KILL signal.
12.4. Exercises
A couple of practical examples:
1. Create a script that writes a boot image to a diskette using the dd utility. If the user tries to interrupt
the script using Ctrl+C, display a message that this action will make the diskette unusable.
Chapter 12. Catching signals 140