fsck

The system utility fsck (for "file system check") is a tool for checking the consistency of a file system in Unix and Unix-like operating systems, such as Linux and Mac OS X.

Use

Generally, fsck is run automatically at boot time (when the operating system detects that a file system is in an inconsistent state, indicating a non-graceful shutdown such as a crash or power loss), or periodically (to prevent small, undetected inconsistencies from becoming exacerbated). As the command must be tailored specifically to the design of the file system, the exact behavior of various fsck implementations will vary. Typically, fsck utilities provide options for either interactively repairing damaged file systems (the user must decide how to fix specific problems), automatically deciding how to fix specific problems (so the user does not have to answer any questions), or reviewing the problems that need to be resolved on a file system without actually fixing them. Partially recovered files where the original file name cannot be reconstructed are typically stored in a "/lost+found" directory.

A system administrator can also run fsck manually if they believe there is a problem with the file system. Because running fsck to repair a file system which is mounted for read/write operations can potentially cause severe data corruption/loss, the file system is normally checked while unmounted, mounted read-only, or with the system in a special maintenance mode that limits the risk of such damage. A journaling file system is designed such that tools such as fsck do not need to be run after unclean shutdown (i.e. crash).

The UFS2 Filesystem in FreeBSD has a background fsck, so it is usually not necessary to wait for fsck to finish before accessing the disk. The modern and faster ZFS in FreeBSD, FreeNAS and PC-BSD has no "fsck" repair tool; instead, it has a repair tool called "scrub" which examines and repairs Silent Corruption and other problems. Additionally ZFS uses copy-on-write, intelligent data scrubbing and resilvering (resyncing). While fsck must be run on an unmounted and therefor unusable filesystem, scrub does not need the ZFS filesystem to be taken offline. scrub is designed to be used on a mounted filesystem. While fsck usually only checks metadata, such as the journal log, but never checks the data itself, scrub checks everything, including metadata and the data. It is also possible to simply swap a harddisk drive to a larger drive and resilver (repair) the zpool of disks. The ZFS file system was designed from the ground up with a focus on data integrity and protection against silent data corruption caused by bit rot, current spikes, bugs in disk firmware, ghost writes, and so on.

The equivalent programs on Microsoft Windows are CHKDSK and SCANDISK.

Use as profanity

Before the rise of journaling file systems, it was common for an improperly shut-down Unix system's file system to develop a corrupted superblock. This possibly-serious problem could only be resolved by running fsck, which could take anywhere from a few seconds to hours, depending on the volume's size and disk I/O throughput. Because of the severity of fsck not being able to resolve this error, the terms "fsck" and "fscked" have come into use among Unix system administrators as a minced oath.

Examples

The following example checks the file system on the first partition of the second hard disk on a Linux system:

 fsck /dev/sdb1

The following example checks the JFS file system on a mdadm software raid device:

 fsck.jfs /dev/md0

See also

• List of Unix utilities
• List of file systems
• e2fsprogs, which includes the e2fsck utility, standard on many Linux distributions
• scrub Oracle Solaris ZFS filesystem-checking utility

External links

• man fsck
• Checking and Repairing File system with fsck
• Jargon File entry: fscking
• The many faces of fsck