Repairing a Linux file system using fsck utility

This note was created on 2020-06-17
This note was last edited on 2023-01-31

fsck ("file system consistency check") is a tool for checking the consistency of a file system in Unix and Unix-like operating systems.
"fsck" generally has three modes of operation:
- Check for errors, and prompt the user interactively to decide how to resolve individual problems;
- Check for errors, and attempt to fix any errors automatically;
- Check for errors, and make no attempt to repair them, but display the errors on standard output.

=== Usage ===
Warning: Do not run fsck on a mounted file system! 
"fsck" is a front-end for the various filesystem checkers (fsck._fstype_) available under Linux. In examples below, we use "fsck.ext4".

Check and repair file system:
# fsck.ext4 /dev/sdb1 

Check file system for errors, but do not attempt to repair them and print any errors to standard output:
# sudo fsck.ext4 -n /dev/sdb1 

Check and repair file system; before overwriting a file system block, write the old contents of the block to an undo file:
# fsck.ext4 -v /dev/sdb1 -z /mnt/wd2tb/backup/sdb1undofile.bak

Use undo file to undo changes in file system:
# e2undo /mnt/wd2tb/backup/sdb1undofile.bak /dev/sdb1