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fsck Command Cheat Sheet

fsck (FileSystem Consistency ChecK) acts as a frontend for various filesystem checkers (fsck.ext4, fsck.xfs, fsck.vfat, etc.). It allows you to check and repair Linux filesystems.

⚠️ WARNING: Never run fsck on a mounted partition (except inside a Read-Only mount). Doing so can severely corrupt data.

Synopsis

fsck [OPTION]... [FILESYSTEM]...

Basic Usage

Check a Partition

Unmount first!

sudo umount /dev/sdb1
sudo fsck /dev/sdb1

Auto-Repair (-y)

Answer "yes" to all repair questions automatically (non-interactive).

sudo fsck -y /dev/sdb1

Dry Run (-n)

Don't make changes. Just check and report what would be done.

sudo fsck -n /dev/sdb1

Root Filesystem Check

Since you cannot unmount the root partition (/) while the OS is running, you have two options:

1. Force Check on Next Boot

Create a file named forcefsck in the root directory.

sudo touch /forcefsck
sudo reboot
The system will detect this file during boot, run fsck, and remove the file.

2. Boot into Recovery Mode

Boot from a Live USB or select "Recovery Mode" in GRUB to run fsck on the unmounted root.

Common Options

Force Check (-f)

By default, fsck skips filesystems marked as "clean". Use -f to force a scan anyway.

sudo fsck -f /dev/sdb1

Scanning All Filesystems (-A)

Usually used in system boot scripts. Checks all FS listed in /etc/fstab.

sudo fsck -A
  • -R: Skip the root filesystem (useful if running script on live system).
  • -M: Do not check mounted filesystems.

Filesystem Specifics

fsck detects the type and calls the appropriate backend.

Ext4 / Ext3 / Ext2

Uses e2fsck. - Supports bad block check (-c). - Supports optimizing directories (-D).

XFS

XFS is designed to essentially rarely need fsck. fsck.xfs does nothing! If meaningful repair is needed, use xfs_repair (requires unmount).

sudo xfs_repair /dev/sdb1

FAT / VFAT

Uses fsck.vfat (or dosfsck).

Superblock Recovery

If a filesystem's superblock is corrupted, it won't mount.

  1. Find backup superblocks: 

    sudo mke2fs -n /dev/sdb1
    (Note: -n acts as dry-run make, listing where superblocks WOULD be).

  2. Restore from backup: 

    sudo e2fsck -b 32768 /dev/sdb1
    (Replace 32768 with a block number found in step 1).

Exit Codes

The exit code of fsck is a bitmask of the following values:

Bit Value Meaning
0 1 Filesystem errors corrected
1 2 System should be rebooted
2 4 Filesystem errors left uncorrected
3 8 Operational error
4 16 Usage or syntax error
5 32 Checkout canceled by user
7 128 Shared library error

Example: Exit code 3 (1 + 2) means errors were corrected AND you should reboot.

Notes

  • Lost+Found: Recovered file fragments (orphaned inodes) are placed in the lost+found directory of the partition. They are named by their inode number (e.g., #12345). Use file command to identify them.
  • Dirty Bit: If a system crashes, the "dirty bit" is set on the FS. fsck runs automatically on boot to clean it.