fsck: File System Check .

filesystem consistency check and repair

fsck [-p][-f] [-m mode]

fsck [-b block# ] [-c level] [-l maxparallel] [-q] [-y ] [-n ] [-m mode] [filesystem] ...

During boot, /etc/rc ( launchd on Mac OSx 10.5) specifies filesystems to be checked.
In addition the filesystems in /etc/fstab which are not marked clean are checked.
(Filesystems are marked clean when they are unmounted (as in shutdown), have been mounted read-only, or when fsck runs on them successfully).

-p

Unreferenced inodes
Link counts in inodes too large
Missing blocks in the free map
Blocks in the free map and allocated to files
incorrect super-block counts

Other inconsistencies, cause an exit with abnormal return status and an automatic boot fails.

During processing, the filesystem, the corrective action, and the nature of the correction are displayed.
After successfully correcting a filesystem, fsck displays the number of files on that filesystem, the number of used and free blocks, and the percentage of fragmentation.

If sent a QUIT signal, fsck completes the checks but then exits with an abnormal return status which causes boot to fail. Used to prevent starting multiuser mode.

Without -p, fsck audits and interactively repairs inconsistent conditions. The user is prompted for permission before each correction is attempted (unless -y or -n is specified).
Some of the corrective actions , not correctable under -p, will result in some loss of data.
The amount and severity of data lost is displayed .
If the user does not have write permission on the filesystem no action is taken.

-p preen
-f Force ie include clean filesystems when preening.
-m rwxrwxrwx If a lost+found directory is created, use the mode specified in octal as the permission bits rather than 1777 which is a security risk, 700 is suggested.

‑b alt‑super‑block Block 32 is usually an alternate super block.
-l p Limit the number of parallel checks. By default, the limit is the number of disks, running one process per disk. If a smaller limit is given, the disks are checked round-robin, one filesystem at a time.
-q quickly determine if filesystem was unmounted cleanly.
-y Supply a yes response to all questions ; this should be used with great caution
-n Supply a no response to all questions except for `CONTINUE?', which is assumed to be affirmative; do not open the filesystem for writing.
-c level Convert the filesystem to a higher level.

0 old (static table) format.
1 new (dynamic table) format.
2 supports 32-bit uid's and gid's, short symbolic links are stored in the inode, and directories have an added field showing the file type.
3 If maxcontig is greater than one, build the free segment maps to aid in finding contiguous sets of blocks.
If maxcontig is equal to one, delete any existing segment maps.
In interactive mode, fsck will list the conversion to be made and ask whether the conversion should be done.
If a negative answer is given, no further operations are done on the filesystem.
In preen mode, the conversion is listed and done if possible without user interaction. Conversion in preen mode is best used when all the filesystems are being converted at once. The format of a filesystem can be determined from the first line of output from dumpfs(8).
If no filesystems are specified filesystems as per /etc/fstab.

Blocks claimed by more than one inode or the free map.
Blocks claimed by an inode outside the range of the filesystem.
Incorrect link counts.
Size checks:
- Directory size not a multiple of DIRBLKSIZ.
- Partially truncated file.
- Bad inode format.
- Blocks not accounted for anywhere.
Directory checks:
- File pointing to unallocated inode.
- Inode number out of range.
- Dot or dot-dot not the first two entries of a directory or having the wrong inode number.
Super Block checks:
- More blocks for inodes than there are in the filesystem.
- Bad free block map format.
- Total free block and/or free inode count incorrect.
Orphaned files and directories (allocated but unreferenced) are, with the user's permission, reconnected by placing them in the lost+found directory. The name assigned is the inode number. If the lost+found directory does not exist, it is created. If there is insufficient space its size is increased.

The raw device should always be used.

FILES /etc/fstab contains default list of filesystems to check.

Appendix A of Fsck - The UNIX File System Check Program.

Fsck has more consistency checks than its predecessors check, dcheck, fcheck, and icheck combined.

see fs(5), fstab(5), newfs(8), reboot(8)

FSCK(8) BSD System Manager's Manual
4th Berkeley Distribution May 9, 1995

-p	preen
-f	Force ie include `clean` filesystems when preening.
-m `rwxrwxrwx`	If a `lost+found` directory is created, use the mode specified in octal as the permission bits rather than 1777 which is a security risk, 700 is suggested.

‑b `alt‑super‑block`	Block 32 is usually an alternate super block.
-l `p`	Limit the number of parallel checks. By default, the limit is the number of disks, running one process per disk. If a smaller limit is given, the disks are checked round-robin, one filesystem at a time.
-q	quickly determine if `filesystem` was unmounted cleanly.
-y	Supply a yes response to all questions ; this should be used with great caution
-n	Supply a no response to all questions except for `CONTINUE?', which is assumed to be affirmative; do not open the filesystem for writing.
-c `level`	Convert the filesystem to a higher level. 0 old (static table) format. 1 new (dynamic table) format. 2 supports 32-bit uid's and gid's, short symbolic links are stored in the inode, and directories have an added field showing the file type. 3 If maxcontig is greater than one, build the free segment maps to aid in finding contiguous sets of blocks. If maxcontig is equal to one, delete any existing segment maps. In interactive mode, fsck will list the conversion to be made and ask whether the conversion should be done. If a negative answer is given, no further operations are done on the filesystem. In preen mode, the conversion is listed and done if possible without user interaction. Conversion in preen mode is best used when all the filesystems are being converted at once. The format of a filesystem can be determined from the first line of output from dumpfs(8).