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.

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 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.

A system administrator can also run fsck manually if there is believed to be 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.