borgmatic/docs/how-to/backup-to-a-removable-drive-or-an-intermittent-server.md

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How to backup to a removable drive or an intermittent server	key parent order Backup to a removable drive or server How-to guides 9

Occasional backups

A common situation is backing up to a repository that's only sometimes online. For instance, you might send most of your backups to the cloud, but occasionally you want to plug in an external hard drive or backup to your buddy's sometimes-online server for that extra level of redundancy.

But if you run borgmatic and your hard drive isn't plugged in, or your buddy's server is offline, then you'll get an annoying error message and the overall borgmatic run will fail (even if individual repositories still complete).

Another variant is when the source machine is only sometimes available for backups, e.g. a laptop where you want to skip backups when the battery falls below a certain level.

So what if you want borgmatic to swallow the error of a missing drive or an offline server or a low battery—and exit gracefully? That's where the concept of "soft failure" come in.

Soft failure command hooks

This feature leverages borgmatic command hooks, so first familiarize yourself with them. The idea is that you write a simple test in the form of a borgmatic hook to see if backups should proceed or not.

The way the test works is that if any of your hook commands return a special exit status of 75, that indicates to borgmatic that it's a temporary failure, and borgmatic should skip all subsequent actions for that configuration file. If you return any other status, then it's a standard success or error. (Zero is success; anything else other than 75 is an error).

So for instance, if you have an external drive that's only sometimes mounted, declare its repository in its own separate configuration file, say at /etc/borgmatic.d/removable.yaml:

location:
    source_directories:
        - /home

    repositories:
        - /mnt/removable/backup.borg

Then, write a before_backup hook in that same configuration file that uses the external findmnt utility to see whether the drive is mounted before proceeding.

hooks:
    before_backup:
      - findmnt /mnt/removable > /dev/null || exit 75

What this does is check if the findmnt command errors when probing for a particular mount point. If it does error, then it returns exit code 75 to borgmatic. borgmatic logs the soft failure, skips all further actions in that configurable file, and proceeds onward to any other borgmatic configuration files you may have.

You can imagine a similar check for the sometimes-online server case:

location:
    source_directories:
        - /home

    repositories:
        - me@buddys-server.org:backup.borg

hooks:
    before_backup:
      - ping -q -c 1 buddys-server.org > /dev/null || exit 75

Or to only run backups if the battery level is high enough:

hooks:
    before_backup:
      - is_battery_percent_at_least.sh 25

(Writing the battery script is left as an exercise to the reader.)

Caveats and details

There are some caveats you should be aware of with this feature.

• You'll generally want to put a soft failure command in the before_backup hook, so as to gate whether the backup action occurs. While a soft failure is also supported in the after_backup hook, returning a soft failure there won't prevent any actions from occuring, because they've already occurred! Similiarly, you can return a soft failure from an on_error hook, but at that point it's too late to prevent the error.
• Returning a soft failure does prevent further commands in the same hook from executing. So, like a standard error, it is an "early out". Unlike a standard error, borgmatic does not display it in angry red text or consider it a failure.
• The soft failure only applies to the scope of a single borgmatic configuration file. So put anything that you don't want soft-failed, like always-online cloud backups, in separate configuration files from your soft-failing repositories.
• The soft failure doesn't have to apply to a repository. You can even perform a test to make sure that individual source directories are mounted and available. Use your imagination!
• The soft failure feature also works for before_prune, after_prune, before_check, and after_check hooks. But it is not implemented for before_everything or after_everything.