How to use fsck - Quick Answer
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What is fsck?
fsck, short for file system consistency check, is a utility that examines the file system for errors and attempts to repair them if possible. It uses a combination of built-in tools to check the disk and generates a report of its findings.
On some systems, fsck runs automatically after an unclean shutdown or after a certain number of reboots.
When to Use fsck
Use fsck to check your file system if your system fails to boot, if files on a specific disk become corrupt, or if an attached drive does not act as expected.
NoteTo run this utility you will want to boot into rescue mode. Please see our Troubleshooting Guide: Booting into Rescue Mode for guidance.
Verify Disks are Unmounted
Verify that the disks you wish to check are unmounted. You risk corrupting your file system and losing data if you run fsck on an active disk. To do this, enter the following command:
df -hYou will see a similar output:
root@ttyS0:~# df -h Filesystem Size Used Avail Use% Mounted on tmpfs 739M 1016K 738M 1% /media/ramdisk /dev/sdh 160M 160M 0 100% /media/sdh /dev/loop0 146M 146M 0 100% /media/compressed_root unionfs 739M 1016K 738M 1% / devtmpfs 10M 0 10M 0% /devYour primary disks should not appear in the list. As long as your device does not appear in the example output from the
df -hcommand, you can run a filesystem check on it.
How to Check for Errors on a Disk
CautionNever run fsck on a mounted disk. Do not continue unless you’re sure that the target disk is unmounted. You risk corrupting your file system and losing data if you run fsck on an active disk.
Run fsck on the target disk, using the desired options. This example checks all file systems (-A) on /dev/sdb:
fsck -A /dev/sdb
fsck Options and Arguments
| Option | Action |
|---|---|
-A | Check all disks listed in /etc/fstab. |
-M | Skip mounted file systems. |
-N | Test run. Describes what would happen without executing the check itself. |
-P | Use with the -A option to run multiple checks in parallel. |
-R | If using the -A option, do not check the root filesystem. |
-t | Check only a specific type of filesystem. |
-y | Interactive repair mode. |
Understand fsck Error Codes
The error codes that fsck returns can be understood with the following table from man7.org:
| Code | Error Code Meaning |
|---|---|
| 0 | No errors |
| 1 | Filesystem errors corrected |
| 2 | System should be rebooted |
| 4 | Filesystem errors left uncorrected |
| 8 | Operational error |
| 16 | Usage or syntax error |
| 32 | Checking canceled by user request |
| 128 | Shared-library error |
Use fsck to Repair File System Errors
Use the -r option to use the interactive repair option.
This example uses fsck to check all file systems except the root, and will attempt repair using the interactive feature:
fsck -AR -y
To check and attempt to repair any errors on /dev/sdb, use this format:
fsck -y /dev/sdb
What if fsck Gets Interrupted?
If fsck gets interrupted, it will complete any checks in process, but will not attempt to repair any errors it finds.
More Information
You may wish to consult the following resources for additional information on this topic. While these are provided in the hope that they will be useful, please note that we cannot vouch for the accuracy or timeliness of externally hosted materials.
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