What to backup?
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Many people advise not to do upgrades but to do fresh installs, reformatting / (the partition with the root directory) and keeping /home.
However, this point of view is misleading as there are many user generated files that live out of /home. Such files and settings would be lost after a fresh install.
The purpose of with wiki page is to make a list of all such files that a user ought to include in the backups. OS or installer generated settings files do not need to be backup up, but as a matter of rule, do backup all files that have receive a user input, may it be directly (from the console) or via a graphical front end.
Please, help us make the list below as comprehensive as possible.
/etc is where all the system configuration files lay. Have a good look around and see which files have been modified by you.
/tmp is supposed to include only temporary files, but you'd be surprised how much desktop environments rely on them for data that a user might want to be more persistent.
KDE keeps its favicon cache there, and losing /tmp might mean losing all the pretty favicons in konqueror's bookmarks.
User crontab or the data for user front end like Task Scheduler (KDE) are stored in
/var/games : all your games, system-wide highscores.