File System Check (fsck)

fsck (File system check), for inconsistencies uses the known parameters and redundant information to audit and check the file systems.

The fsck Utility

The fsck utility takes its understanding of the internals of the several UNIX file systems and attempts to verify in which all the links and blocks are rightly tied together. fsck runs in five passes, every of that checks a different categories of the linkage fsck walks by the file system, beginning with the superblock. fsck then deals with the allocated disk blocks, directory connectivity, link reference counts, pathnames, and the free list of blocks and inodes.

The fsck command checks for the given inconsistencies:

1. Fragments or Blocks allocated to multiple files
2. i-nodes holding block or fragment numbers which overlap
3. i-nodes holding block or fragment numbers out of range
4. Discrepancies among the number of directory references to a file and the link count of the file
5. Illegally allocated fragments or blocks
6. i-nodes holding block or fragment numbers which are marked free in the disk map
7. i-nodes holding corrupt fragment or block numbers
8. A fragment which is not the last disk address in an i-node. The given check does not apply to compressed file systems
9. Files larger than 32KB holding a fragment. This check does not apply to compressed file systems
10. Size checks
11. Incorrect number of blocks
12. The Directory size not a multiple of 512 bytes

CAUTION

The given checks do not apply to compressed file systems.

1. Directory checks
2. Directory entry holding an i-node number marked free in the i-node map
3. i-node number out of range
4. Dot (.) link missing or not pointing to itself
5. Dot dot (..) link missing or not pointing to the parent directory
6. Files which are not referenced or directories that are not reachable
7. Inconsistent disk map
8. Inconsistent i-node map

fsck command does not create corrections to a mounted file system

fsck command can be run on a mounted file system for reasons other than repairs. Though, inaccurate error messages might be returned whenever the file system is mounted