fsck - file system consistency check and interactive repair
fsck -p[ # ] [ filesystem ... ]
fsck [ -y ] [ -n ] [ -sX ] [ -SX ] [ -t filename ] [ filesystem ] ...
The first form of fsck preens a standard set of file systems or the specified file sys-
tems. It is normally used in the script /etc/rc during automatic reboot. In this case
fsck reads the table /etc/fstab to determine which file systems to check. It uses the
information there to inspect groups of disks in parallel taking maximum advantage of i/o
overlap to check the file systems as quickly as possible. Normally, the root file system
will be checked on pass 1, other ``root'' (``a'' partition) file systems on pass 2, other
small file systems on separate passes (e.g. the ``d'' file systems on pass 3 and the ``e''
file systems on pass 4), and finally the large user file systems on the last pass, e.g.
pass 5. Only one file system from each disk should be checked in each pass. A pass num-
ber of 0 in fstab causes a disk to not be checked; similarly partitions which are not
shown as to be mounted ``rw'' or ``ro'' are not checked. A number or range of numbers may
be given after the -p to start or end the check at a specified pass number. A single num-
ber specifies the starting pass (e.g. -p2 or -p2-); a range specifies the starting and
endig passes (-p2-4). A missing start means to start with pass 1 (-p-2). These can be
used to stop fsck to mount a temporary file system before continuing the check on larger
file systems requiring scratch files.
The system normally takes care that only a restricted class of innocuous inconsistencies
can happen unless hardware or software failures intervene. These are limited to the fol-
Link counts in inodes too large
Missing blocks in the free list
Blocks in the free list also in files
Counts in the superblock wrong
Allocated inodes in the free inode list
These are the only inconsistencies which fsck with the -p option will correct; 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 file system on which the correction will take place, and the nature of the
correction. After successfully correcting a file system, fsck will print the number of
files on that file system and the number of used and free blocks. Warning: kernel changes
are required to limit the types of inconsistencies, and fsck -p makes assumptions about
the kernel's actions in repairing these. Vmunix, PDP-11 2.9BSD with the UCB_FSFIX option
and PDP-11 2.10BSD Unix kernels have the appropriate modifications; the -p option should
not be used on other systems.
Without the -p option, fsck audits and interactively repairs inconsistent conditions for
file systems. If the file system is inconsistent the operator is prompted for concurrence
before each correction is attempted. It should be noted that a number of the corrective
actions which are not fixable 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 fsck will default to a -n action.
Fsck has more consistency checks than its predecessors check, dcheck, fcheck, and icheck
The following flags are interpreted by fsck.
-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; do not open the file system for
-sX Ignore the actual free list and (unconditionally) reconstruct a new one by rewriting
the superblock of the file system. The file system should be unmounted while this
is done; if this is not possible, care should be taken that the system is quiescent
and that it is rebooted immediately afterwards. This precaution is necessary so
that the old, bad, in-core copy of the superblock will not continue to be used, or
written on the file system.
The -sX option allows for creating an optimal free list organization. The following
forms of X are supported for the following devices:
-s4 (RP04, RP05, RP06)
-sBlocks-per-cylinder:Blocks-to-skip (for anything else)
If X is not given, the values used when the file system was created are used. If
these values were not specified, then the value 400:9 is used.
-SX Conditionally reconstruct the free list. This option is like -sX above except that
the free list is rebuilt only if there were no discrepancies discovered in the file
system. Using -S will force a no response to all questions asked by fsck. This
option is useful for forcing free list reorganization on uncontaminated file sys-
-t If fsck cannot obtain enough memory to keep its tables, it uses a scratch file. If
the -t option is specified, the file named in the next argument is used as the
scratch file, if needed. Without the -t flag, fsck will prompt the operator for the
name of the scratch file. The file chosen should not be on the file system being
checked, and if it is not a special file or did not already exist, it is removed
when fsck completes.
If no file systems are given to fsck then a default list of file systems is read from the
Inconsistencies checked are as follows:
1. Blocks claimed by more than one inode or the free list.
2. Blocks claimed by an inode or the free list outside the range of the file system.
3. Incorrect link counts.
4. Size checks:
Directory size not 16-byte aligned.
5. Bad inode format.
6. Blocks not accounted for anywhere.
7. Directory checks:
File pointing to unallocated inode.
Inode number out of range.
8. Super Block checks:
More than 65536 inodes.
More blocks for inodes than there are in the file system.
9. Bad free block list format.
10. Total free block and/or free inode count incorrect.
11. Allocated inodes on the free inode list in the superblock.
Orphaned files and directories (allocated but unreferenced) are, with the operator's con-
currence, reconnected by placing them in the lost+found directory. The name assigned is
the inode number. The only restriction is that the directory lost+found must preexist in
the root of the file system being checked and must have empty slots in which entries can
be made. This can be accomplished manually by making lost+found, copying a number of
files to the directory, and then removing them (before fsck is executed). Mkfs(8) will
automatically create a lost+found directory.
Checking the raw device is almost always faster. The root device should not be checked
using the raw device, however, since it cannot be unmounted.
/etc/fstab default list of file systems to check
The diagnostics produced by fsck are intended to be self-explanatory. The exit codes with
the -p option are 0 (no problems that weren't fixed), 4 (root file system was modified), 8
(problems that couldn't be fixed) and 12 (fsck was interrupted).
filsys(5), fstab(5), crash(8), mkfs(8), mklost+found(8), reboot(8)
T. J. Kowalski, FSCK - The UNIX File System Check Program
Inode numbers for . and .. in each directory should be checked for validity.
3rd Berkeley Distribution FSCK(8)