The file system is best visualized as a tree, rooted, as it were, at /. /dev, /usr, and the other directories in the root directory are branches, which may have their own branches, such as /usr/local, and so on.
There are various reasons to house some of these directories on separate file systems. /var contains the directories log/, spool/, and various types of temporary files, and as such, may get filled up. Filling up the root file system is not a good idea, so splitting /var from / is often favorable.
Another common reason to contain certain directory trees on other file systems is if they are to be housed on separate physical disks, or are separate virtual disks, such as Network File System mounts, or CDROM drives.
During the boot process, file systems listed in /etc/fstab are automatically mounted (unless they are listed
with the noauto option).
The /etc/fstab file contains a list of lines of the following format:
device /mount-point fstype options dumpfreq passno
A device name (which should exist), as explained in Section 19.2.
A directory (which should exist), on which to mount the file system.
The file system type to pass to mount(8). The default FreeBSD file system is ufs.
Either rw for read-write file systems, or ro for read-only file systems, followed by any other options
that may be needed. A common option is noauto for
file systems not normally mounted during the boot sequence. Other options are
listed in the mount(8) manual
page.
This is used by dump(8) to determine which file systems require dumping. If the field is missing, a value of zero is assumed.
This determines the order in which file systems should be checked. File systems that should be skipped should have their passno set to zero. The root file system (which needs to be checked before everything else) should have its passno set to one, and other file systems' passno should be set to values greater than one. If more than one file systems have the same passno then fsck(8) will attempt to check file systems in parallel if possible.
Consult the fstab(5) manual page for more information on the format of the /etc/fstab file and the options it contains.
The mount(8) command is what is ultimately used to mount file systems.
In its most basic form, you use:
There are plenty of options, as mentioned in the mount(8) manual page, but the most common are:
Mount Options
-aMount all the file systems listed in /etc/fstab.
Except those marked as “noauto”, excluded by the -t flag, or those that are already mounted.
-dDo everything except for the actual mount system call. This option is useful in
conjunction with the -v flag to determine what mount(8) is
actually trying to do.
-fForce the mount of an unclean file system (dangerous), or forces the revocation of write access when downgrading a file system's mount status from read-write to read-only.
-rMount the file system read-only. This is identical to using the ro argument to the -o
option.
-t fstypeMount the given file system as the given file system type, or mount only file
systems of the given type, if given the -a option.
“ufs” is the default file system type.
-uUpdate mount options on the file system.
-vBe verbose.
-wMount the file system read-write.
The -o option takes a comma-separated list of the
options, including the following:
Do not allow execution of binaries on this file system. This is also a useful security option.
Do not interpret setuid or setgid flags on the file system. This is also a useful security option.
The umount(8) command
takes, as a parameter, one of a mountpoint, a device name, or the -a or -A option.
All forms take -f to force unmounting, and -v for verbosity. Be warned that -f is not generally a good idea. Forcibly unmounting file
systems might crash the computer or damage data on the file system.
-a and -A are used to
unmount all mounted file systems, possibly modified by the file system types
listed after -t. -A,
however, does not attempt to unmount the root file system.