Robust Bash Scripts - Part Four

This is part of a series of posts which should aid anyone writing Bash scripts to make them more robust.

This post describes how you can use traps to help your code fail more gracefully.

Sometimes you write a script, it fails, and leaves a shed load of temporary files over your file system.  Or perhaps (like in part two) some stuff gets set up, your script fails and leaves a network half set up or a naming service half completed.  Whatever the problem, we can trap them (well, almost all of them…).

For my example, I’m going to use a section of my dissertation.  I have a script which is called INSTALL.sh, this sets some stuff up itself, and also delegates some tasks to other scripts.  Here is the general gist:

1. Check for root permissions - If no permissions, exit with status 1
2. Move some files about
3. Create some directories and move files to those directories
4. Call a script to set up the network
5. Install NIS (A.K.A. YP)
6. Set up NIS
7. Set up ability to share home directories
8. Call a script to set up a Solaris Zone (virtual machine)
9. Move a customised file over a system file
10. Call a cleanup function

Now, if any part of that script goes wrong, the script will fail (it will because I’ve used the set -e option).  If the script exists half of the configuration is done, and if you try to run the script again, it will fail because some of the setup is already complete.

To get out of this horrible situation, I have used a trap.  If the script exists with an exit status of 1 or greater, or if it is interrupted by a HUP, INT, QUIT or TERM signal, it will be trapped and run a function called “abort”.  Here is my trap statement:

trap 'abort' 1 2 3 15 

My “abort” method looks something like this:

abort(){
	echo "ABORTING!  Please take note of any warnings!"
	# If pkg-manage exists, rename it to pkg
	if [[ -a /usr/bin/pkg-manage ]]; then
	rm /usr/bin/pkg
	mv /usr/bin/pkg-manage /usr/bin/pkg
	fi
	# Call cleanup to clear away temp files
	cleanup
	# Remove contents of /var/vaes
	rm -rf /var/vaes
	# Remove config file
	rm $CONFIG_FILE
	# Uninstall NIS & related things
	pkg uninstall SUNWyp
	domainname ""
	rm /etc/defaultdomain
	cp /etc/nsswitch.files /etc/nsswitch.conf
	zfs set sharenfs=off rpool/export/home
	cat /etc/auto_home | grep -v "`hostname`:/export/home" > /tmp/auto_home
	mv /tmp/auto_home /etc/auto_home
	# Delete zones dir
	POOL=`zfs list | awk '{ print $1 }' | 
	grep "export" | sed 's/([a-zA-Z]*)/.*/1/g' | head -1`
	zfs destroy -Rf $POOL/export/vaes-zones
	# Undo zones
	bash /tmp/zone_setup.sh abort
	# Undo networking
	bash /tmp/network_setup.sh abort
}

By using the trap statement, you can clean up after your failed script in the majority of cases.  A word of warning though: You cannot trap the KILL (or 9) signal, if your script fails because it has received a KILL signal, it will just exit.