The file fstab contains descriptive information about the various file systems. fstab is only read by programs, and not written; it is the duty of the system administrator to properly create and maintain this file. Each filesystem is described on a separate line; fields on each line are separated by tabs or spaces. The order of records in fstab is important because fsck(8), mount(8), and umount(8) sequentially iterate through fstab doing their thing.
The first field, (fs_spec), describes the block special device or remote filesystem to be mounted.
For ordinary mounts it will hold (a link to) a block special device node (as created by mknod(8)?) for the device to be mounted, like `/dev/cdrom' or `/dev/sdb7'. For NFS mounts one will have <host>:<dir>, e.g., `knuth.aeb.nl:/'. For procfs, use `proc'.
Instead of giving the device explicitly, one may indicate the (ext2 or xfs) filesystem that is to be mounted by its UUID or volume label (cf. e2label(8) or xfs_admin(8)?), writing LABEL=<label> or UUID=<uuid>, e.g., `LABEL=Boot' or `UUID=3e6be9de-8139-11d1-9106-a43f08d823a6'. This will make the system more robust: adding or removing a SCSI disk changes the disk device name but not the filesystem volume label.
The second field, (fs_file), describes the mount point for the filesystem. For swap partitions, this field should be specified as `none'. If the name of the mount point contains spaces these can be escaped as `040'.
The third field, (fs_vfstype), describes the type of the filesystem. The system currently supports these types of filesystems (and possibly others - consult /proc/filesystems):
If fs_vfstype is specified as ``ignore'' the entry is ignored. This is useful to show disk partitions which are currently unused.
The fourth field, (fs_mntops), describes the mount options associated with the filesystem.
It is formatted as a comma separated list of options. It contains at least the type of mount plus any additional options appropriate to the filesystem type. For documentation on the available options for non-nfs file systems, see mount(8). For documentation on all nfs-specific options have a look at nfs(5). Common for all types of file system are the options ``noauto'' (do not mount when mount(8).
The fifth field, (fs_freq), is used for these filesystems by the dump(8)?1? command to determine which filesystems need to be dumped. If the fifth field is not present, a value of zero is returned and dump will assume that the filesystem does not need to be dumped.
The sixth field, (fs_passno), is used by the fsck(8) program to determine the order in which filesystem checks are done at reboot time. The root filesystem should be specified with a fs_passno of 1, and other filesystems should have a fs_passno of 2. Filesystems within a drive will be checked sequentially, but filesystems on different drives will be checked at the same time to utilize parallelism available in the hardware. If the sixth field is not present or zero, a value of zero is returned and fsck will assume that the filesystem does not need to be checked.
The proper way to read records from fstab is to use the routines getmntent(3).
The documentation in mount(8) is often more up-to-date.
The fstab file format appeared in 4.0BSD.