All files accessible in a Unix system are arranged in one big tree, the file hierarchy, rooted at /. These files can be spread out over several devices. The mount command serves to attach the file system found on some device to the big file tree. Conversely, the umount (8) command will detach it again.
The standard form of the mount command, is
mount -t type device dir
This tells the kernel to attach the file system found on device (which is of type type) at the directory dir. The previous contents (if any) and owner and mode of dir become invisible, and as long as this file system remains mounted, the pathname dir refers to the root of the file system on device.
Three forms of invocation do not actually mount anything:
mount -h
prints a help message;
mount -V
prints a version string; and just
mount [-l] [-t type]
lists all mounted file systems (of type type). The option -l adds the (ext2, ext3 and XFS) labels in this listing.
Since Linux 2.4.0 it is possible to remount part of the file hierarchy somewhere else. The call is
mount –bind olddir newdir
After this call the same contents is accessible in two places. This call attaches only (part of) a single filesystem, not possible submounts. The entire file hierarchy including submounts is attached a second place using
mount –rbind olddir newdir
Since Linux 2.5.1 it is possible to atomically move a subtree to another place. The call is
mount –move olddir newdir
The proc file system is not associated with a special device, and when mounting it, an arbitrary keyword, such as proc can be used instead of a device specification. (The customary choice none is less fortunate: the error message `none busy’ from umount can be confusing.)
Most devices are indicated by a file name (of a block special device), like /dev/sda1, but there are other possibilities. For example, in the case of an NFS mount, device may look like knuth.cwi.nl:/dir. It is possible to indicate a block special device using its volume label or UUID (see the -L and -U options below).
The file /etc/fstab, may contain lines describing what devices are usually mounted where, using which options. This file is used in three ways:
(i) The command
mount -a [-t type] [-O optlist]
(usually given in a bootscript) causes all file systems mentioned in fstab (of the proper type and/or having or not having the proper options) to be mounted as indicated, except for those whose line contains the noauto keyword. Adding the -F option will make mount fork, so that the filesystems are mounted simultaneously.
(ii) When mounting a file system mentioned in fstab, it suffices to give only the device, or only the mount point.
(iii) Normally, only the superuser can mount file systems. However, when fstab contains the user option on a line, then anybody can mount the corresponding system.
Thus, given a line
/dev/cdrom /cd iso9660 ro,user,noauto,unhide
any user can mount the iso9660 file system found on his CDROM using the command
mount /dev/cdrom
or
mount /cd
Only the user that mounted a filesystem can unmount it again. If any user should be able to unmount, then use users instead of user in the fstab line. The owner option is similar to the user option, with the restriction that the user must be the owner of the special file. This may be useful e.g. for /dev/fd if a login script makes the console user owner of this device.
The programs mount and umount maintain a list of currently mounted file systems in the file /etc/mtab. If no arguments are given to mount, this list is printed. When the proc filesystem is mounted (say at /proc), the files /etc/mtab and /proc/mounts have very similar contents. The former has somewhat more information, such as the mount options used, but is not necessarily up-to-date (cf. the -n option below). It is possible to replace /etc/mtab by a symbolic link to /proc/mounts, but some information is lost that way, and in particular working with the loop device will be less convenient.
OPTIONS
The full set of options used by an invocation of mount is determined by first extracting the options for the file system from the fstab table, then applying any options specified by the -o argument, and finally applying a -r or -w option, when present.
Options available for the mount command:
-V
Output version.
-h
Print a help message.
-v
Verbose mode.
Mount all filesystems (of the given types) mentioned in fstab.
(Used in conjunction with -a.) Fork off a new incarnation of mount for each device. This will do the mounts on different devices or different NFS servers in parallel. This has the advantage that it is faster; also NFS timeouts go in parallel. A disadvantage is that the mounts are done in undefined order. Thus, you cannot use this option if you want to mount both /usr and /usr/spool.
Causes everything to be done except for the actual system call; if it’s not obvious, this “fakes” mounting the file system. This option is useful in conjunction with the -v flag to determine what the mount command is trying to do. It can also be used to add entries for devices that were mounted earlier with the -n option.
Add the ext2, ext3 and XFS labels in the mount output. Mount must have permission to read the disk device (e.g. be suid root) for this to work. One can set such a label for ext2 or ext3 using the e2label(8) utility, or for XFS using xfs_admin(8).
Mount without writing in /etc/mtab. This is necessary for example when /etc is on a read-only file system. Tolerate sloppy mount options rather than failing. This will ignore mount options not supported by a filesystem type. Not all filesystems support this option. This option exists for support of the Linux autofs-based automounter.
Mount the file system read-only. A synonym is -o ro.
Mount the file system read/write. This is the default. A synonym is -o rw.
Mount the partition that has the specified label.
Mount the partition that has the specified uuid. These two options require the file /proc/partitions (present since Linux 2.1.116) to exist.
-t vfstype
The argument following the -t is used to indicate the file system type. The file system types which are currently supported are: adfs, affs, autofs, coda, coherent, cramfs, devpts, efs, ext, ext2, ext3, hfs, hpfs, iso9660, jfs, minix, msdos, ncpfs, nfs, ntfs, proc, qnx4, ramfs, reiserfs, romfs, smbfs, sysv, tmpfs, udf, ufs, umsdos, vfat, xenix, xfs, xiafs. Note that coherent, sysv and xenix are equivalent and that xenix and coherent will be removed at some point in the future — use sysv instead. Since kernel version 2.1.21 the types ext and xiafs do not exist anymore.
For most types all the mount program has to do is issue a simple mount (2) system call, and no detailed knowledge of the filesystem type is required. For a few types however (like nfs, smbfs, ncpfs) ad hoc code is necessary. The nfs ad hoc code is built in, but smbfs and ncpfs have a separate mount program. In order to make it possible to treat all types in a uniform way, mount will execute the program /sbin/mount.TYPE (if that exists) when called with type TYPE. Since various versions of the smbmount program have different calling conventions, /sbin/mount.smb may have to be a shell script that sets up the desired call.
The type iso9660 is the default. If no -t option is given, or if the auto type is specified, the superblock is probed for the filesystem type (adfs, bfs, cramfs, ext, ext2, ext3, hfs, hpfs, iso9660, jfs, minix, ntfs, qnx4, reiserfs, romfs, udf, ufs, vxfs, xfs, xiafs are supported). If this probe fails, mount will try to read the file /etc/filesystems, or, if that does not exist, /proc/filesystems. All of the filesystem types listed there will be tried, except for those that are labeled “nodev” (e.g., devpts, proc and nfs). If /etc/filesystems ends in a line with a single * only, mount will read /proc/filesystems afterwards.
The auto type may be useful for user-mounted floppies. Creating a file /etc/filesystems can be useful to change the probe order (e.g., to try vfat before msdos) or if you use a kernel module autoloader. Warning: the probing uses a heuristic (the presence of appropriate `magic’), and could recognize the wrong filesystem type, possibly with catastrophic consequences. If your data is valuable, don’t ask mount to guess.
More than one type may be specified in a comma separated list. The list of file system types can be prefixed with no to specify the file system types on which no action should be taken. (This can be meaningful with the -a option.)
For example, the command:
mount -a -t nomsdos,ext
mounts all file systems except those of type msdos and ext.
Used in conjunction with -a, to limit the set of filesystems to which the -a is applied. Like -t in this regard except that it is useless except in the context of -a. For example, the command:
mount -a -O no_netdev
mounts all file systems except those which have the option _netdev specified in the options field in the /etc/fstab file.
It is different from -t in that each option is matched exactly; a leading no at the beginning of one option does not negate the rest.
The -t and -O options are cumulative in effect; that is, the command
mount -a -t ext2 -O _netdev
mounts all ext2 filesystems with the _netdev option, not all filesystems that are either ext2 or have the _netdev option specified. Some of the options are only useful when they appear in the /etc/fstab file. The following options apply to any file system that is being mounted (but not every file system actually honors them - e.g., the sync option today has effect only for ext2, ext3 and ufs):
async
All I/O to the file system should be done asynchronously.
atime
Update inode access time for each access. This is the default.
auto
Can be mounted with the -a option.
defaults
Use default options: rw, suid, dev, exec, auto, nouser, and async.
dev
Interpret character or block special devices on the file system.
exec
Permit execution of binaries.
_netdev
The filesystem resides on a device that requires network access (used to prevent the system from attempting to mount these filesystems until the network has been enabled on the system).
noatime
Do not update inode access times on this file system (e.g, for faster access on the news spool to speed up news servers).
noauto
Can only be mounted explicitly (i.e., the -a option will not cause the file system to be mounted).
nodev
Do not interpret character or block special devices on the file system.
noexec
Do not allow execution of any binaries on the mounted file system. This option might be useful for a server that has file systems containing binaries for architectures other than its own.
nosuid
Do not allow set-user-identifier or set-group-identifier bits to take effect. (This seems safe, but is in fact rather unsafe if you have suidperl (1) installed.)
nouser
Forbid an ordinary (i.e., non-root) user to mount the file system. This is the default.
remount
Attempt to remount an already-mounted file system. This is commonly used to change the mount flags for a file system, especially to make a readonly file system writeable. It does not change device or mount point.
Mount the file system read-only.
Mount the file system read-write.
Allow set-user-identifier or set-group-identifier bits to take effect.
All I/O to the file system should be done synchronously.
All directory updates within the file system should be done synchronously. This affects the following system calls: creat, link, unlink, symlink, mkdir, rmdir, mknod and rename.
user
Allow an ordinary user to mount the file system. The name of the mounting user is written to mtab so that he can unmount the file system again. This option implies the options noexec, nosuid, and nodev (unless overridden by subsequent options, as in the option line user,exec,dev,suid).
Allow every user to mount and unmount the file system. This option implies the options noexec, nosuid, and nodev (unless overridden by subsequent options, as in the option line users,exec,dev,suid).
Remount a subtree somewhere else (so that its contents are available in both places). See above.
Move a subtree to some other place. See above.
FILESYSTEM SPECIFIC MOUNT OPTIONS. The following options apply only to certain file systems. We sort them by file system. They all follow the -o flag.
Mount options for adfs uid=value and gid=value
Set the owner and group of the files in the file system (default: uid=gid=0). ownmask=value and othmask=value
Set the permission mask for ADFS ‘owner’ permissions and ‘other’ permissions, respectively (default: 0700 and 0077, respectively).
Mount options for affs
uid=value and gid=value
Set the owner and group of the root of the file system (default: uid=gid=0, but with option uid or gid without specified value, the uid and gid of the current process are taken).
setuid=value and setgid=value
Set the owner and group of all files.
mode=value
Set the mode of all files to value & 0777 disregarding the original permissions. Add search permission to directories that have read permission. The value is given in octal. protect
Do not allow any changes to the protection bits on the file system.
usemp
Set uid and gid of the root of the file system to the uid and gid of the mount point upon the first sync or umount, and then clear this option.
verbose
Print an informational message for each successful mount.
prefix=string
Prefix used before volume name, when following a link.
volume=string
Prefix (of length at most 30) used before ‘/’ when following a symbolic link.
reserved=value
(Default: 2.) Number of unused blocks at the start of the device.
root=value
Give explicitly the location of the root block.
bs=value
Give blocksize. Allowed values are 512, 1024, 2048, 4096.
grpquota / noquota / quota / usrquota
These options are accepted but ignored. (However, quota utilities may react to such strings in /etc/fstab.)
Mount options for coherent
None.
Mount options for devpts
The devpts file system is a pseudo file system, traditionally mounted on /dev/pts. In order to acquire a pseudo terminal, a process opens /dev/ptmx; the number of the pseudo terminal is then made available to the process and the pseudo terminal slave can be accessed as /dev/pts/.
uid=value and gid=value
This sets the owner or the group of newly created PTYs to the specified values. When nothing is specified, they will be set to the UID and GID of the creating process. For example, if there is a tty group with GID 5, then gid=5 will cause newly created PTYs to belong to the tty group.
mode=value
Set the mode of newly created PTYs to the specified value. The default is 0600. A value of mode=620 and gid=5 makes “mesg y” the default on newly created PTYs.
Mount options for ext
None. Note that the `ext’ file system is obsolete. Don’t use it. Since Linux version 2.1.21 extfs is no longer part of the kernel source.
Mount options for ext2
The `ext2′ file system is the standard Linux file system. Due to a kernel bug, it may be mounted with random mount options (fixed in Linux 2.0.4).
bsddf / minixdf
Set the behaviour for the statfs system call. The minixdf behaviour is to return in the f_blocks field the total number of blocks of the file system, while the bsddf behaviour (which is the default) is to subtract the overhead blocks used by the ext2 file system and not available for file storage. Thus
% mount /k -o minixdf; df /k; umount /k
Filesystem 1024-blocks Used Available Capacity Mounted on
/dev/sda6 2630655 86954 2412169 3% /k
% mount /k -o bsddf; df /k; umount /k
Filesystem 1024-blocks Used Available Capacity Mounted on
/dev/sda6 2543714 13 2412169 0% /k
(Note that this example shows that one can add command line options to the options given in /etc/fstab.)
check / check=normal / check=strict
Set checking level. When at least one of these options is set (and check=normal is set by default) the inodes and blocks bitmaps are checked upon mount (which can take half a minute or so on a big disk, and is rather useless). With strict checking, block deallocation checks that the block to free is in the data zone.
No checking is done. This is fast. Recent kernels do not have a check option anymore - checking with e2fsck (8) is more meaningful.
debug
Print debugging info upon each (re)mount.
errors=continue / errors=remount-ro / errors=panic
Define the behaviour when an error is encountered. (Either ignore errors and just mark the file system erroneous and continue, or remount the file system read-only, or panic and halt the system.) The default is set in the filesystem superblock, and can be changed using tune2fs(8).
grpid or bsdgroups / nogrpid or sysvgroups
These options define what group id a newly created file gets. When grpid is set, it takes the group id of the directory in which it is created; otherwise (the default) it takes the fsgid of the current process, unless the directory has the setgid bit set, in which case it takes the gid from the parent directory, and also gets the setgid bit set if it is a directory itself.
resgid=n and resuid=n
The ext2 file system reserves a certain percentage of the available space (by default 5%, see mke2fs (8) and tune2fs (8)). These options determine who can use the reserved blocks. (Roughly: whoever has the specified uid, or belongs to the specified group.) sb=n
Instead of block 1, use block n as superblock. This could be useful when the filesystem has been damaged. (Earlier, copies of the superblock would be made every 8192 blocks: in block 1, 8193, 16385, … (and one got hundreds or even thousands of copies on a big filesystem). Since version 1.08, mke2fs has a -s (sparse superblock) option to reduce the number of backup superblocks, and since version 1.15 this is the default. Note that this may mean that ext2 filesystems created by a recent mke2fs cannot be mounted r/w under Linux 2.0.*.) The block number here uses 1k units. Thus, if you want to use logical block 32768 on a filesystem with 4k blocks, use “sb=131072″.
grpquota / noquota / quota / usrquota
These options are accepted but ignored.
nouid32
Disables 32-bit UIDs and GIDs. This is for interoperability with older kernels which only store and expect 16-bit values.
Mount options for ext3
The `ext3′ file system is version of the ext2 file system which has been enhanced with journalling. It supports the same options as ext2 as well as the following additions: journal=update
Update the ext3 file system’s journal to the current format.
journal=inum
When a journal already exists, this option is ignored. Otherwise, it specifies the number of the inode which will represent the ext3 file system’s journal file; ext3 will create a new journal, overwriting the old contents of the file whose inode number is inum.
noload
Do not load the ext3 file system’s journal on mounting.
data=journal / data=ordered / data=writeback
Specifies the journalling mode for file data. Metadata is always journaled.
journal
All data is committed into the journal prior to being written into the main file system.
ordered
This is the default mode. All data is forced directly out to the main file system prior to its metadata being committed to the journal.
writeback
Data ordering is not preserved - data may be written into the main file system after its metadata has been committed to the journal. This is rumoured to be the highest-throughput option. It guarantees internal file system integrity, however it can allow old data to appear in files after a crash and journal recovery.
Mount options for fat (Note: fat is not a separate filesystem, but a common part of the msdos, umsdos and vfat filesystems.)
blocksize=512 / blocksize=1024 / blocksize=2048
Set blocksize (default 512).
uid=value and gid=value
Set the owner and group of all files. (Default: the uid and gid of the current process.) umask=value
Set the umask (the bitmask of the permissions that are not present). The default is the umask of the current process. The value is given in octal.
dmask=value
Set the umask applied to directories only. The default is the umask of the current process. The value is given in octal. Present since 2.5.43.
fmask=value
Set the umask applied to regular files only. The default is the umask of the current process. The value is given in octal. Present since 2.5.43.
check=value
Three different levels of pickyness can be chosen:
r[elaxed]
Upper and lower case are accepted and equivalent, long name parts are truncated (e.g. verylongname.foobar becomes verylong.foo), leading and embedded spaces are accepted in each name part (name and extension).
n[ormal]
Like “relaxed”, but many special characters (*, ?, <, spaces, etc.) are rejected. This is the default.
s[trict]
Like “normal”, but names may not contain long parts and special characters that are sometimes used on Linux, but are not accepted by MS-DOS are rejected. (+, =, spaces, etc.)
codepage=value
Sets the codepage for converting to shortname characters on FAT and VFAT filesystems. By default, codepage 437 is used.
conv=b[inary] / conv=t[ext] / conv=a[uto]
The fat file system can perform CRLFNL (MS-DOS text format to UNIX text format) conversion in the kernel. The following conversion modes are available:
binary
no translation is performed. This is the default.
text
CRLFNL translation is performed on all files.
auto
CRLFNL translation is performed on all files that don’t have a “well-known binary” extension. The list of known extensions can be found at the beginning of fs/fat/misc.c (as of 2.0, the list is: exe, com, bin, app, sys, drv, ovl, ovr, obj, lib, dll, pif, arc, zip, lha, lzh, zoo, tar, z, arj, tz, taz, tzp, tpz, gz, tgz, deb, gif, bmp, tif, gl, jpg, pcx, tfm, vf, gf, pk, pxl, dvi).
Programs that do computed lseeks won’t like in-kernel text conversion. Several people have had their data ruined by this translation. Beware!
For file systems mounted in binary mode, a conversion tool (fromdos/todos) is available. cvf_format=module
Forces the driver to use the CVF (Compressed Volume File) module cvf_module instead of auto-detection. If the kernel supports kmod, the cvf_format=xxx option also controls on-demand CVF module loading.
cvf_option=option
Option passed to the CVF module.
debug
Turn on the debug flag. A version string and a list of file system parameters will be printed (these data are also printed if the parameters appear to be inconsistent).
fat=12 / fat=16 / fat=32
Specify a 12, 16 or 32 bit fat. This overrides the automatic FAT type detection routine. Use with caution!
iocharset=value
Character set to use for converting between 8 bit characters and 16 bit Unicode characters. The default is iso8859-1. Long filenames are stored on disk in Unicode format.
quiet
Turn on the quiet flag. Attempts to chown or chmod files do not return errors, although they fail. Use with caution!
sys_immutable, showexec, dots, nodots, dotsOK=[yes|no]
Various misguided attempts to force Unix or DOS conventions onto a FAT file system.
Mount options for hpfs
uid=value and gid=value
Set the owner and group of all files. (Default: the uid and gid of the current process.)
umask=value
Set the umask (the bitmask of the permissions that are not present). The default is the umask of the current process. The value is given in octal.
case=lower / case=asis
Convert all files names to lower case, or leave them. (Default: case=lower.)
conv=binary / conv=text / conv=auto
For conv=text, delete some random CRs (in particular, all followed by NL) when reading a file. For conv=auto, choose more or less at random between conv=binary and conv=text. For conv=binary, just read what is in the file. This is the default.
Posted in How To's by Shafkat Shahzad, M.Sc on March 28th, 2009 
