Bash disaster prevention

Like any other versatile tool, consequences of misusing of UNIX scripting can be disasterous. It often helps to save the day

A horror story

You listen to music and have built up a quite extensive library of digital music in your computer under the directory /home/alex/music. Everyday, you copy a folder from under music folder (e.g. beatles/yellowsubmarine) to your MP3 player, which appears as a usb drive (say /mnt/usbstick). You write a small script called loadmusic.bash and save it in /home/alex/,

#!/bin/bash
cd /mnt/usbstick/todaysmusic # Go to the appropriate folder
rm -rf #Delete all the old files 
cp -r /home/alex/music/$1 . 
echo 'Done!'

and call it as

$ loadmusic.bash yellowsubmarine

On one day, you simply forget to insert the usb key, before running the command:

loadmusic.bash : line 2: cd: /mnt/usbstick/todaysmusic: No such file or directory
Done!

What happens here. Bash tries to change directory to /mnt/usbstick/todaysmusic , but can't. Does it stop there? Not unless we ask it to. It simply forgets what happened and executes the next command, namely rm -rf and innocently deletes ALL your digital music albums! (And we don't waste time on useless stuff like backups!!)

Error handling

Never, ever use dangerous commands like rm -rf (or rm by itself, for that matter,) or mv in a script that doesn't have a proper error handler.

Whenever there is the potential for pitfalls like the above, use some form of general error handler to at least tell the script to exit without trying the rest of the commands.

#!/bin/bash
## End of error handling
function handle {
   echo "Error"
   exit 1
}
trap handle ERR
## End of error handling
cd /mnt/usbstick/todaysmusic # Go to the appropriate folder
rm -rf #Delete all the old files 
cp -r /home/alex/music/$1 . 
echo 'Done!'

which will execute the function handle() as soon as it encounters and error. And inside handle() , we have exit , so there's no danger of bash trying to be smart.

: line 7: cd: /mnt/usbstick/todaysmusic: No such file or directory
Error

More useful error handling

trap type of error handling is useful for preventing disasters. For more graceful error handling, the return value of each individual command can be checked.