Tar incremental option does not add new files

Question

In an incremental backup, tar does not seem to be able to add new files :

$ tar -cvzf test.tar.gz -g test.snar a.txt
a.txt
$ tar -cvzf test2.tar.gz -g test.snar a.txt b.txt

The second command returns nothing and the archive is empty. Yet it works if I specify the directory instead of the files. Anything I'm missing here ?

Answer 1

The snapshot file contains a.txt on the first tar command. On the second tar the snapshot does not know about b.txt. a.txt has not changed so the incremental archive is empty.

If the incremental archive is at directory level, it will detect the change (b.txt) because b.txt exists within the directory's heirarchy.

If the snapshot file exists, it determines which files are modified.

See GNU Tar reference incremental backups

The option ‘--listed-incremental’ instructs tar to operate on an incremental archive with additional metadata stored in a standalone file, called a snapshot file. The purpose of this file is to help determine which files have been changed, added or deleted since the last backup, so that the next incremental backup will contain only modified files. The name of the snapshot file is given as an argument to the option:

Answer 2

Have you looked at using the "–listed-incremental" option to tar ? It will allow you to incrementally add specific folders and helps to ensure you only compress the files changed since the last time they were added to tar with the --list-incremental option.

A more generic option is simply to append new files using -a or the append option, which puts the file onto the end of the archive.

If this is a backup situation, I'd suggest you look at rsync as a more feature rich option but again all depends on what you're trying to do with your backup procedure.