On Unix-like operating systems, the fsck command checks and repairs a Linux file system.
This page describes the GNU/Linux version of fsck.
Description
fsck stands for :file system consistency check:. On most systems, fsck is run at boot time if certain conditions are detected. Usually, these conditions are:
Description
Syntax
Examples
Related commands
Linux commands help
A file system is marked as “dirty” — its written state is inconsistent with data that was scheduled to be written; or,
A file system was mounted a set number of times without being checked.
The fsck command itself interacts with a matching filesystem-specific fsck command created by the filesystem’s authors. Regardless of filesystem type, 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; or,
- Check for errors, and make no attempt to repair them, but display the errors on standard output.
While normally run at boot time, fsck can run manually on unmounted filesystems by a superuser.
Technical description
fsck is used to check, and optionally repair, one or more filesystems.
filesys is a device name (e.g., /dev/hdc1, /dev/sdb2), a mount point (e.g., /, /usr, /home), or an ext2 label or UUID specifier (for example “UUID=8868abf6-88c5-4a83-98b8-bfc24057f7bd” or “LABEL=root”).
Normally, fsck tries to handle filesystems on different physical disk drives in parallel to reduce the total amount of time needed to check all of them.
If no filesystems are specified on the command line, and the -A option is not specified, fsck defaults to checking filesystems in /etc/fstab serially. This is equivalent to the combination of the -A and -s options.
The exit code returned by fsck is a unique number representing the sum of the following condition values:
0 - No errors1 - Filesystem errors corrected2 - System should be rebooted4 - Filesystem errors left uncorrected8 - Operational error16 - Usage or syntax error32 - Fsck canceled by user request128 - Shared-library error
The exit code returned when multiple filesystems are checked is the bitwise OR of the exit codes for each filesystem that is checked.
In actuality, fsck is a front-end for the various filesystem checkers (fsck.fstype) available under Linux. The filesystem-specific checker is searched for in /sbin first, then in /etc/fs and /etc, and finally in the directories listed in the PATH environment variable.
Read filesystem-specific checker manual pages for further details. For example, to learn more about ext3-specific fsck checking, run:
man fsck.ext3
Syntax
fsck [-lsAVRTMNP] [-C [fd]] [-t fstype] [filesys…] [–] [fs-specific-options]
Options
Filesystem-specific options
Options that are not understood by fsck are passed to the filesystem-specific checker. These arguments must not take arguments, as there is no way for fsck to be able to properly guess which options take arguments and which don’t.
Options and arguments which follow the – are treated as filesystem-specific options to be passed to the filesystem-specific checker.
Please note that fsck is not designed to pass arbitrarily complicated options to filesystem-specific checkers. If you’re doing something complicated, execute the filesystem-specific checker directly. If you pass fsck some horribly complicated options and arguments, and it doesn’t do what you expect, you’re almost certainly doing something you shouldn’t be doing with fsck. Options to different filesystem-specific fsck’s are not standardized. If in doubt, please consult the man pages of the filesystem-specific checker. Although not guaranteed, the following options are supported by most filesystem checkers:
Examples
Dropping into single-user mode
These instructions should help you bring your Linux system into single-user mode and unmount any filesystems you want to check with fsck.
Do not run fsck on a mounted filesystem! If you run fsck on a filesystem that is currently mounted, (at best) you receive an error message, and (at worst) corrupt your filesystem data. If you’re going to check your filesystem with fsck, drop into single-user mode and make sure the filesystem is unmounted before you proceed.
First, initiate runlevel 1 (single-user mode) with the init command:
sudo init 1
Now unmount the filesystem using umount. For instance, if /home is mounted on /dev/sda2, you could run:
umount /home
…or:
umount /dev/sda2
Make sure to run umount for any filesystem you want to check with fsck.
Checking filesystems
fsck /dev/sda2
This command attempts to check /dev/sda2, and report any errors it finds.
fsck -y /dev/sda2
Check /dev/sda2, and attempt to automatically fix any errors found.
fsck -A
Check all configured filesystems. fsck processes the file /etc/fstab and check all file systems listed there. Systems are checked in order of their
cat /etc/fstab
View all configured filesystems. Output resembles the following:
/etc/fstab: static file system information.
Use ‘blkid’ to print the universally unique identifier for a
device; this may be used with UUID= as a more robust way to name devices
that works even if disks are added and removed. See fstab man page.
/ was on /dev/sda5 during installation
UUID=c3a6839b-00f1-4cf4-8b00-e61fbcdba6c0 / ext4 errors=remount-ro 0 1
/home was on /dev/sda7 during installation
UUID=afceabb6-a3f4-41c2-9ae6-0393d85c7c70 /home ext4 defaults 0 2
swap was on /dev/sda6 during installation
UUID=c6ca8b8f-0b46-4c06-a934-a9dd3525faa7 none swap sw 0 0 #/dev/sdb1 /media/usb0 auto rw,user,noauto 0 0
ls /sbin/fsck.*
View all filesystems that can be checked with fsck. Filesystems appear as extensions to the fsck.* files; for example:
fsck.cramfs fsck.ext3 fsck.ext4dev fsck.minix fsck.nfs fsck.reiserfs fsck.xfs fsck.ext2 fsck.ext4 fsck.jfs fsck.msdos fsck.reiser4 fsck.vfat
fsck -n /dev/sda2
Check /dev/sda2 for errors, but do not attempt to repair them; instead, print any errors to standard output.
fsck -f /dev/sda2
Normally, fsck skips parts of the filesystem marked as “clean” — meaning all pending writes were successfully made. The -f (“force”) option specifies that fsck should check parts of the filesystem even if they are not “dirty”. The result is a less efficient, but a more thorough check.
fsck -t ext2 /dev/fd0
This command checks the ext2 filesystem on the floppy diskette device /dev/fd0.
Related commands
mkfs — Build a Linux file system, usually a hard disk partition.
