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# The expire_backups command

## Synopsis

    expire_backups [options] <age> [<age> ...]

## Description

The expire_backups command intelligently remove old backups that are no longer needed.

To define what backups you want to keep for how long, you define a number of age ranges. expire_backups ensures that you will have at least one backup in each age range at all times. It will keep exactly as many backups as are required for that and delete any backups that become redundant.

Age ranges are specified by giving a list of range boundaries in terms of backup cycles. Every time you create a new backup, the existing backups age by one cycle.

Example: when expire_backups is called with the age range definition `1 3 7 14 31`, it will guarantee that you always have the following backups available:

1. A backup that is 0 to 1 cycles old (i.e, the most recent backup)
2. A backup that is 1 to 3 cycles old
3. A backup that is 3 to 7 cycles old
4. A backup that is 7 to 14 cycles old
5. A backup that is 14 to 31 cycles old

Note

If you do backups in fixed intervals, then one cycle will be equivalent to the backup interval. The advantage of specifying the age ranges in terms of backup cycles rather than days or weeks is that it allows you to gracefully handle irregular backup intervals. Imagine that for some reason you do not turn on your computer for one month. Now all your backups are at least a month old, and if you had specified the above backup strategy in terms of absolute ages, they would all be deleted! Specifying age ranges in terms of backup cycles avoids these sort of problems.

expire_backups usage is simple. It requires backups to have names of the forms `year-month-day_hour:minute:seconds` (`YYYY-MM-DD_HH:mm:ss`) and works on all backups in the current directory. So for the above backup strategy, the correct invocation would be:

    expire_backups.py 1 3 7 14 31

When storing your backups on an S3QL file system, you probably want to specify the `--use-s3qlrm` option as well. This tells expire_backups to use the s3qlrm command to delete directories.

expire_backups uses a “state file” to keep track which backups are how many cycles old (since this cannot be inferred from the dates contained in the directory names). The standard name for this state file is `.expire_backups.dat`. If this file gets damaged or deleted, expire_backups no longer knows the ages of the backups and refuses to work. In this case you can use the `--reconstruct-state` option to try to reconstruct the state from the backup dates. However, the accuracy of this reconstruction depends strongly on how rigorous you have been with making backups (it is only completely correct if the time between subsequent backups has always been exactly the same), so it’s generally a good idea not to tamper with the state file.

## Options

The expire_backups command accepts the following options:

> `--quiet` | be really quiet
> `--debug` | activate debugging output
> `--version` | just print program version and exit
> `--state <file>` | File to save state information in (default: ”.expire_backups.dat”)
> `-n` | Dry run. Just show which backups would be deleted.
> `--reconstruct-state`
> | Try to reconstruct a missing state file from backup dates.
> `--use-s3qlrm` | Use `s3qlrm` command to delete backups.

## Exit Status

expire_backups returns exit code 0 if the operation succeeded and 1 if some error occured.

## See Also

expire_backups is shipped as part of S3QL, http://code.google.com/p/s3ql/.

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