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I'm aware that a similar question has been brought up already about Debian, though I'll be using Arch instead, and I'm looking for the inverse of the same question. (I haven't installed just yet) The reason that I'm looking for folders not to back up is because I'll be using Btrfs snapshots, which more easily exclude folders than include them.

Just to clarify, I'm interested in knowing which folders I should explicitly not back up, where restoring a backup of such folders might:

  • damage the system, or cause certain programs to not work correctly
  • be erased, inaccessible, or otherwise become useless when backed up

If there are other folders which don't fit in the above categories but are worth mentioning, I'd like to know about those as well.

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While it won't break the system, you probably want to exclude /var/lib/pacman/sync, that's where pacman stores the locally cached copies of the repository indexes, which are trivial to regenerate. If you're planning on just backing up user data instead of the whole system, then you absolutely want to exclude /var/lib/pacman/local, as that stores the local database of what is installed and what isn't (messing that up will essentially brick pacman). Those are the only two I know that are specific to Arch, but there may be others, and you probably will have others depending on what software is installed. On a slightly more generic note, many people I know advocate not including /var/log so that you don't screw up your system logs when you restore data, and /var/spool so that you aren't accidentally re-queueing long-expired print jobs or e-mails.

Additionally, while not Arch specific, you can safely exclude /tmp, /var/tmp, /var/cache/*/*, and /home/*/.cache from backups on pretty much any Linux system, as all of them are only supposed to contain data that applications can work without or regenerate themselves.