Incremental Backups
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## Incremental Backups

Cedar Backup supports three different kinds of backups for individual collect directories. These are daily, weekly and incremental backups. Directories using the daily mode are backed up every day. Directories using the weekly mode are only backed up on the first day of the week, or when the `--full` option is used. Directories using the incremental mode are always backed up on the first day of the week (like a weekly backup), but after that only the files which have changed are actually backed up on a daily basis.

In Cedar Backup, incremental backups are not based on date, but are instead based on saved checksums, one for each backed-up file. When a full backup is run, Cedar Backup gathers a checksum value [12] for each backed-up file. The next time an incremental backup is run, Cedar Backup checks its list of file/checksum pairs for each file that might be backed up. If the file's checksum value does not match the saved value, or if the file does not appear in the list of file/checksum pairs, then it will be backed up and a new checksum value will be placed into the list. Otherwise, the file will be ignored and the checksum value will be left unchanged.

Cedar Backup stores the file/checksum pairs in `.sha` files in its working directory, one file per configured collect directory. The mappings in these files are reset at the start of the week or when the `--full` option is used. Because these files are used for an entire week, you should never purge the working directory more frequently than once per week.

[12] The checksum is actually an SHA cryptographic hash. See Wikipedia for more information: http://en.wikipedia.org/wiki/SHA-1.

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