FSCK(8) BSD System Manager's Manual FSCK(8)NAMEfsck — filesystem consistency check and interactive repair
SYNOPSISfsck-p [-m mode]
fsck [-b block#] [-c level] [-l maxparallel] [-y] [-n] [-m mode]
The first form of fsck preens a standard set of filesystems or the speci‐
fied filesystems. It is normally used in the script /etc/rc during auto‐
matic reboot. Here fsck reads the table /etc/fstab to determine which
filesystems to check. Only partitions in fstab that are mounted ``rw,''
``rq'' or ``ro'' and that have non-zero pass number are checked.
Filesystems with pass number 1 (normally just the root filesystem) are
checked one at a time. When pass 1 completes, all remaining filesystems
are checked, running one process per disk drive. The disk drive contain‐
ing each filesystem is inferred from the longest prefix of the device
name that ends in a digit; the remaining characters are assumed to be the
partition designator. In preening mode, filesystems that are marked
clean are skipped. Filesystems are marked clean when they are unmounted,
when they have been mounted read-only, or when fsck runs on them success‐
The kernel takes care that only a restricted class of innocuous filesys‐
tem inconsistencies can happen unless hardware or software failures
intervene. These are limited to the following:
Link counts in inodes too large
Missing blocks in the free map
Blocks in the free map also in files
Counts in the super-block wrong
These are the only inconsistencies that fsck with the -p option will cor‐
rect; if it encounters other inconsistencies, it exits with an abnormal
return status and an automatic reboot will then fail. For each corrected
inconsistency one or more lines will be printed identifying the filesys‐
tem on which the correction will take place, and the nature of the cor‐
rection. After successfully correcting a filesystem, fsck will print 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 will finish the filesystem checks, then exit
with an abnormal return status that causes an automatic reboot to fail.
This is useful when you want to finish the filesystem checks during an
automatic reboot, but do not want the machine to come up multiuser after
the checks complete.
Without the -p option, fsck audits and interactively repairs inconsistent
conditions for filesystems. If the filesystem is inconsistent the opera‐
tor is prompted for concurrence before each correction is attempted. It
should be noted that some of the corrective actions which are not cor‐
rectable under the -p option will result in some loss of data. The
amount and severity of data lost may be determined from the diagnostic
output. The default action for each consistency correction is to wait
for the operator to respond yes or no. If the operator does not have
write permission on the filesystem fsck will default to a -n action.
Fsck has more consistency checks than its predecessors check, dcheck,
fcheck, and icheck combined.
The following flags are interpreted by fsck.
-b Use the block specified immediately after the flag as the super
block for the filesystem. Block 32 is usually an alternate super
-l Limit the number of parallel checks to the number specified in
the following argument. 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
-m Use the mode specified in octal immediately after the flag as the
permission bits to use when creating the lost+found directory
rather than the default 1777. In particular, systems that do not
wish to have lost files accessible by all users on the system
should use a more restrictive set of permissions such as 700.
-y Assume a yes response to all questions asked by fsck; this should
be used with great caution as this is a free license to continue
after essentially unlimited trouble has been encountered.
-n Assume a no response to all questions asked by fsck except for
‘CONTINUE?’, which is assumed to be affirmative; do not open the
filesystem for writing.
-c Convert the filesystem to the specified level. Note that the
level of a filesystem can only be raised.
There are currently four levels defined:
0 The filesystem is in the old (static table) format.
1 The filesystem is in the new (dynamic table) format.
2 The filesystem 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
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
If no filesystems are given to fsck then a default list of filesystems is
read from the file /etc/fstab.
Inconsistencies checked are as follows:
1. Blocks claimed by more than one inode or the free map.
2. Blocks claimed by an inode outside the range of the filesystem.
3. Incorrect link counts.
4. Size checks:
Directory size not a multiple of DIRBLKSIZ.
Partially truncated file.
5. Bad inode format.
6. Blocks not accounted for anywhere.
7. 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.
8. 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
operator's concurrence, 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.
Because of inconsistencies between the block device and the buffer cache,
the raw device should always be used.
/etc/fstab contains default list of filesystems to check.
The diagnostics produced by fsck are fully enumerated and explained in
Appendix A of Fsck - The UNIX File System Check Program.
SEE ALSOfstab(5), fs(5), fsdb(8), newfs(8), mkfs(8), reboot(8)4th Berkeley Distribution May 9, 1995 4th Berkeley Distribution