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.
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. 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 doesn't 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.
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 background fsck, so it is usually not necessary to wait for fsck to finish before accessing the disk.
The Microsoft equivalent programs 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.
It is unclear whether this usage was cause or effect, as it has been anecdotally reported that Dennis Ritchie has claimed "The second letter was originally different."
The following example checks the file system on the first partition of the second hard disk on a Linux system:
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