14.4. Reversing Changes in Between Snapshots

To reverse changes made between two existing Snapper snapshots, use the undochange command in the following format, where 1 is the first snapshot and 2 is the second snapshot:

The following diagram demonstrates how the undochange command works:

The diagram shows the point in time in which snapshot_1 is created, file_a is created, then file_b deleted. Snapshot_2 is then created, after which file_a is edited and file_c is created. This is now the current state of the system. The current system has an edited version of file_a, no file_b, and a newly created file_c.

When the undochange command is called, Snapper generates a list of modified files between the first listed snapshot and the second. In the diagram, if you use the snapper -c SnapperExample undochange 1..2 command, Snapper creates a list of modified files (that is, file_a is created; file_b is deleted) and applies them to the current system. Therefore: Be aware that if file_b and file_c conflict, the system can become corrupted.

You can also use the snapper -c SnapperExample undochange 2..1 command. In this case, the current system replaces the edited version of file_a with one copied from snapshot_1, which undoes edits of that file made after snapshot_2 was created.

The undochange command is not always the best way to revert modifications. With the status and diff command, you can make a qualified decision, and use the mount and unmount commands instead of Snapper. The mount and unmount commands are only useful if you want to mount snapshots and browse their content independently of Snapper workflow.

If needed, the mount command activates respective LVM Snapper snapshot before mounting. Use the mount and unmount commands if you are, for example, interested in mounting snapshots and extracting older version of several files manually. To revert files manually, copy them from a mounted snapshot to the current file system. The current file system, snapshot 0, is the live file system created in Procedure 14.1, “Creating a Snapper Configuration File”. Copy the files to the subtree of the original /mount-point.

Use the mount and unmount commands for explicit client-side requests. The /etc/snapper/configs/config_name file contains the ALLOW_USERS= and ALLOW_GROUPS= variables where you can add users and groups. Then, snapperd allows you to perform mount operations for the added users and groups.