The fsck utility invokes file system-specific programs to check the special devices listed in the fstab(5) file or in the command line for consistency.
It is normally used in the script /etc/rc during automatic reboot. Traditionally, fsck is invoked before the file systems are mounted and all checks are done to completion at that time. If background checking is available, fsck is invoked twice. It is first invoked at the traditional time, before the file systems are mounted, with the -F flag to do checking on all the file systems that cannot do background checking. It is then invoked a second time, after the system has completed going multiuser, with the -B flag to do checking on all the file systems that can do background checking. Unlike the foreground checking, the background checking is started asynchronously so that other system activity can proceed even on the file systems that are being checked.
If no file systems are specified, fsck reads the table /etc/fstab to determine which file systems to check. Only partitions in /etc/fstab that are mounted "rw", "rq" or "ro" and that have non-zero pass number are checked. File systems with pass number 1 (normally just the root file system) are always checked one at a time.
If not in preen mode, the remaining entries are checked in order of increasing pass number one at a time. This is needed when interaction with fsck is required.
In preen mode, after pass 1 completes, all remaining file systems are checked, in pass number order running one process per disk drive in parallel for each pass number in increasing order.
In other words: In preen mode all pass 1 partitions are checked sequentially. Next all pass 2 partitions are checked in parallel, one process per disk drive. Next all pass 3 partitions are checked in parallel, one process per disk drive. etc.
The disk drive containing each file system is inferred from the shortest prefix of the device name that ends in a digit; the remaining characters are assumed to be the partition and slice designators.
The options are as follows: