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
- device
-
A device name (which should exist), as explained in Section 18.2.
- mount-point
-
A directory (which should exist), on which to mount the file system.
- fstype
-
The file system type to pass to mount(8). The default
FreeBSD file system is ufs.
- options
-
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.
- dumpfreq
-
This is used by dump(8) to determine
which file systems require dumping. If the field is missing, a value of zero is
assumed.
- passno
-
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
-a-
Mount all the file systems listed in /etc/fstab. Except
those marked as “noauto”, excluded by the -t
flag, or those that are already mounted.
-d-
Do 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.
-f-
Force 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.
-r-
Mount the file system read-only. This is identical to using the ro (rdonly for FreeBSD versions older
than 5.2) argument to the -o option.
-t fstype-
Mount 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.
-u-
Update mount options on the file system.
-v-
Be verbose.
-w-
Mount the file system read-write.
The -o option takes a comma-separated list of the options,
including the following:
- noexec
-
Do not allow execution of binaries on this file system. This is also a useful security
option.
- nosuid
-
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.
