The system utilityfsck(file system consistency check) is a tool for checking the consistency of afile systeminUnixandUnix-likeoperating systems, such asLinux,macOS,andFreeBSD.[1]The equivalent programs onMS-DOSandMicrosoft WindowsareCHKDSK,SFC,andSCANDISK.
 fsck in action on aLinuxsystem. | |
| Operating system | UnixandUnix-like |
|---|---|
| Type | Command |
Use
editGenerally, fsck is run either automatically at boot time, or manually by the system administrator. The command works directly on data structures stored on disk, which are internal and specific to the particular file system in use - so an fsck command tailored to the file system is generally required. The exact behaviors of various fsck implementations vary, but they typically follow a common order of internal operations and provide a common command-line interface to the user. On modern systems, fsck simply detects the type of filesystem and calls the specializedfsck.type(Linux) orfsck_type(BSD, macOS) program for each type.[1][2]
Most 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 fi xing them. Partially recovered files where the original file name cannot be reconstructed are typically recovered to a"lost+found"directory that is stored at the root of the file system.
Asystem administratorcan also run fsck manually if they believe there is a problem with the file system. The file system is normally checked while unmounted, mounted read-only, or with the system in a special maintenance mode.
Boot time
editAs boot time fsck is expected to run without user intervention, it generally defaults to not perform any destructive operations. This may be in the form of a read-only check (failing whenever issues are found), or more commonly, a "preen"-pmode that only fixes innocuous issues commonly found after an unclean shutdown (i.e. crash, power fail).[2]
ext2/3/4 offers an option to force a boot-time check after a specified number of mounts, so that periodic checking can be done.[3]
Some modern file systems do not require fsck to be at boot after an unclean shutdown. Some examples are:
- XFS,ajournaling file system.It has a dummy fsck which does nothing[4]and an actual
xfs_repairtool to be run when problems are suspected. - UFS2file system inFreeBSD,which can delay the check to background ifsoft updatesare enabled.[5]As a result, it is usually not necessary to wait for fsck to finish before accessing the disk. This design is reflected by the
-Fflag used at boot.[2] - ZFSandBtrfs,two fullcopy-on-writefile systems. They avoid in-place changes to assure levels of consistency similar to a journal. They also provide a dummy fsck.[6]
btrfs-checkis still available to check for suspected problems in filesystem structure (e.g. when a software bug or hardware issue is suspected).
Independent of checking the file system structure, modern file systems may offer adata scrubbingtool to check for silent corruption in stored data against a mirror or achecksum.Scrubs tend to be slow as they cover all data on a disk, but periodic runs can defend againstdata rotand help identify failing drives.[7]
History
editfsck first appeared in 4.0BSD of 1980. It turned into its modern wrapper form in NetBSD 1.3 (1998). fsck is not defined by any extant standard,[2]but the primitive non-wrapper form is present in the 1995 draftSystems Management: File System and Scheduling Utilities (FSSU)fromX/Open.[8]
As an expletive
editThe severity of file system corruption led to the terms "fsck" and "fscked" becoming used among Unix system administrators as aminced oathfor "fuck"and" fucked ".[9]It is unclear whether this usage was cause or effect, as a report from a question and answer session atUSENIX1998 claims that "fsck" originally had a different name:
- Dennis Ritchie:"So fsck was originally called something else"
- Question:"What was it called?"
- Dennis Ritchie:"Well, the second letter was different"[10][11]
That story has been confirmed byRob Pikeon the Mastodon social network on December 17, 2023:
Ted Kowalski, username frodo, may he rest in peace, was the original author, just down the hall from my office in Murray Hill, and his name for the program had a 'u' where there is now an 's'. Management made him change it for distribution, but they couldn't make him change his pronunciation.
"Go fsck yourself", is occasionally used online as an injunction to a person to go and correct their issue (attitude, ignorance of the subject matter, etc.) - in the same way that running fsck involves fi xing fundamental errors.
Examples
editThe following example checks the file system configured to be mounted on /usr partition; the file system needs to be unmounted first:
fsck/usr
The following example checks the LinuxJFSfile system on a mdadm softwareRAIDdevice:
fsck-tjfs/dev/md0
See also
edit- List of Unix commands
- List of file systems
- e2fsprogs,which includes the
e2fsckutility, standard on many Linux distributions - scrub,Oracle Solaris ZFS file system checking utility
References
edit- ^ab–LinuxProgrammer'sManual– Administration and Privileged Commands "In actuality, fsck is simply a front-end for the various filesystem checkers (fsck.fstype) available under Linux."
- ^abcd–FreeBSDSystem Manager'sManual"It was reimplemented as a filesystem independent wrapper in NetBSD 1.3 and first appeared in FreeBSD 5.0."
- ^–LinuxProgrammer'sManual– Administration and Privileged Commands
- ^–LinuxProgrammer'sManual– Administration and Privileged Commands: "XFS is a journaling filesystem and performs recovery at mount(8) time if necessary"
- ^–FreeBSDSystem Manager'sManual
- ^–LinuxProgrammer'sManual– Administration and Privileged Commands,: "do nothing, successfully" "Traditional filesystems need to run their respective fsck utility in case the filesystem was not unmounted cleanly and the log needs to be replayed before mount. This is not needed for BTRFS."
- ^Swearingen, Cindy."Scheduled Pool Scrubs in Oracle Solaris ZFS".
- ^"Systems Management: File System and Scheduling Utilities (FSSU)"(PDF).opengroup.org.1995. p. 18.Retrieved8 April2023.
- ^Raymond, Eric S(September 24, 1999)."fscking".The Jargon File.Archived fromthe originalon 2009-01-03.
fcking: /fus'-king/ or /eff'-seek-ing/ adj. [Usenet; common]Fucking,in the expletive sense (it refers to the Unix filesystem-repair command fsck(1), of which it can be said that if you have to use it at all you are having a bad day). Originated on {scary devil monastery} and thebofh.netnewsgroups, but became much more widespread following the passage of {CDA}. Also occasionally seen in the variant 'What the fsck?'
- ^Crosby, Matthew."Report from Usenix".alt.sysadmin.recovery.Usenet.Retrieved2016-02-20.
- ^"Mind Fsck".WikiWikiWeb.C2.
- ^Rob Pike,Rob Pike on Mastodon: the truth on fsck pronunciation and original naming,retrieved2023-12-18