How to use the command e2undo (with examples)

The command e2undo is used to replay undo logs for an ext2/ext3/ext4 filesystem. It allows for the undoing of a failed operation by an e2fsprogs program. This article will provide examples of different use cases for the e2undo command.

Use case 1: Display information about a specific undo file

Code:

e2undo -h path/to/undo_file /dev/sdXN

Motivation: This use case is helpful when you want to retrieve information about a specific undo file before performing any operations. It allows you to verify the details of the undo file beforehand.

Explanation: The -h option is used to display detailed information about the specified undo file. The path/to/undo_file argument represents the path to the undo file you want to retrieve information about. The /dev/sdXN argument represents the ext2/ext3/ext4 filesystem device.

Example output:

Undo file: path/to/undo_file
File system device: /dev/sdXN
Undo file version: 1
Creation time: Tue Jan  1 12:00:00 2022

Use case 2: Perform a dry-run and display the candidate blocks for replaying

Code:

e2undo -nv path/to/undo_file /dev/sdXN

Motivation: A dry-run allows you to simulate the undo operation without making any actual changes. This use case is useful for inspecting the candidate blocks that will be replayed during the undo operation.

Explanation: The -nv options are used to perform a dry-run and display the candidate blocks for replaying. The path/to/undo_file argument represents the path to the undo file. The /dev/sdXN argument represents the ext2/ext3/ext4 filesystem device.

Example output:

Undo file: path/to/undo_file
File system device: /dev/sdXN
Undoing operation on 10 candidate blocks:
  Block 32768: Inode 12345, ext2_new_block()
  Block 65536: Inode 67890, ext3_delete_block()
  ...

Use case 3: Perform an undo operation

Code:

e2undo path/to/undo_file /dev/sdXN

Motivation: This use case is used to perform the actual undo operation. It allows you to revert the changes made by a previous failed operation.

Explanation: The path/to/undo_file argument represents the path to the undo file. The /dev/sdXN argument represents the ext2/ext3/ext4 filesystem device.

Example output:

Undoing operation on 10 candidate blocks:
  Block 32768: Inode 12345, ext2_new_block()
  Block 65536: Inode 67890, ext3_delete_block()
  ...
Undo operation completed successfully.

Use case 4: Perform an undo operation and display verbose information

Code:

e2undo -v path/to/undo_file /dev/sdXN

Motivation: This use case is helpful when you need more detailed information about the undo operation. It provides verbose output, allowing you to better understand the changes being made.

Explanation: The -v option is used to enable verbose output during the undo operation. The path/to/undo_file argument represents the path to the undo file. The /dev/sdXN argument represents the ext2/ext3/ext4 filesystem device.

Example output:

Undoing operation on 10 candidate blocks:
  Block 32768: Inode 12345, ext2_new_block()
    Undoing change: New block 32768 freed.
  Block 65536: Inode 67890, ext3_delete_block()
    Undoing change: Block 65536 added to inode 67890.
  ...
Undo operation completed successfully.

Use case 5: Write the old contents of the block to an undo file before overwriting a filesystem block

Code:

e2undo -z path/to/file.e2undo path/to/undo_file /dev/sdXN

Motivation: This use case is useful when you want to preserve the old contents of a filesystem block before overwriting it. It allows for future undo operations, giving you the ability to restore the original data.

Explanation: The -z option is used to write the old contents of the block to an undo file before overwriting a filesystem block. The path/to/file.e2undo argument represents the path to the undo file where the old contents will be stored. The path/to/undo_file argument represents the path to the target undo file. The /dev/sdXN argument represents the ext2/ext3/ext4 filesystem device.

Example output:

Block 32768: Old contents written to path/to/file.e2undo.
Block 65536: Old contents written to path/to/file.e2undo.
...

Conclusion:

The e2undo command provides a useful way to replay undo logs for ext2/ext3/ext4 filesystems. Whether you need to retrieve information, perform a dry-run, undo operations, or write old contents to an undo file, the e2undo command offers various use cases to accommodate your needs.