I want to make a script that monitors and compares my file sytem with a tar archive which holds all the files on my system.
What's the best way to start with this ?

Depends on which event should be monitored.
You could monitor the deletion of a file/directory and/or the creation of a new file/directory ?
and/or file manipulation.

The best way would not start with an Tar file.
because it waste a lot space and other ressources for a simple list generation.
But if you have good reasone to use a tar file you could start by sample script below:
This script shows you deletet files and directorys.

for file in `tar -tf comp.tar`
do
if [ ! -f $file -a ! -d $file ]
then
echo "$file missing"
fi
done

Frank

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