Section (2) mount

Linux manual pages Section 2  


mount — mount filesystem


#include <sys/mount.h>
int mount( const char *source,
  const char *target,
  const char *filesystemtype,
  unsigned long mountflags,
  const void *data);


mount() attaches the filesystem specified by source (which is often a pathname referring to a device, but can also be the pathname of a directory or file, or a dummy string) to the location (a directory or file) specified by the pathname in target.

Appropriate privilege (Linux: the CAP_SYS_ADMIN capability) is required to mount filesystems.

Values for the filesystemtype argument supported by the kernel are listed in /proc/filesystems (e.g., btrfs, ext4, jfs, xfs, vfat, fuse, tmpfs, cgroup, proc, mqueue, nfs, cifs, iso9660). Further types may become available when the appropriate modules are loaded.

The data argument is interpreted by the different filesystems. Typically it is a string of comma-separated options understood by this filesystem. See mount(8) for details of the options available for each filesystem type.

A call to mount() performs one of a number of general types of operation, depending on the bits specified in mountflags. The choice of which operation to perform is determined by testing the bits set in mountflags, with the tests being conducted in the order listed here:

  • Remount an existing mount: mountflags includes MS_REMOUNT.

  • Create a bind mount: mountflags includes MS_BIND.

  • Change the propagation type of an existing mount: mountflags includes one of MS_SHARED, MS_PRIVATE, MS_SLAVE, or MS_UNBINDABLE.

  • Move an existing mount to a new location: mountflags includes MS_MOVE.

  • Create a new mount: mountflags includes none of the above flags.

Each of these operations is detailed later in this page. Further flags may be specified in mountflags to modify the behavior of mount(), as described below.

Additional mount flags

The list below describes the additional flags that can be specified in mountflags. Note that some operation types ignore some or all of these flags, as described later in this page.

MS_DIRSYNC (since Linux 2.5.19)

Make directory changes on this filesystem synchronous. (This property can be obtained for individual directories or subtrees using chattr(1).)

MS_LAZYTIME (since Linux 4.0)

Reduce on-disk updates of inode timestamps (atime, mtime, ctime) by maintaining these changes only in memory. The on-disk timestamps are updated only when:


the inode needs to be updated for some change unrelated to file timestamps;


the application employs fsync(2), syncfs(2), or sync(2);


an undeleted inode is evicted from memory; or


more than 24 hours have passed since the inode was written to disk.

This mount option significantly reduces writes needed to update the inode_zsingle_quotesz_s timestamps, especially mtime and atime. However, in the event of a system crash, the atime and mtime fields on disk might be out of date by up to 24 hours.

Examples of workloads where this option could be of significant benefit include frequent random writes to preallocated files, as well as cases where the MS_STRICTATIME mount option is also enabled. (The advantage of combining MS_STRICTATIME and MS_LAZYTIME is that stat(2) will return the correctly updated atime, but the atime updates will be flushed to disk only in the cases listed above.)


Permit mandatory locking on files in this filesystem. (Mandatory locking must still be enabled on a per-file basis, as described in fcntl(2).) Since Linux 4.5, this mount option requires the CAP_SYS_ADMIN capability and a kernel configured with the CONFIG_MANDATORY_FILE_LOCKING option.


Do not update access times for (all types of) files on this filesystem.


Do not allow access to devices (special files) on this filesystem.


Do not update access times for directories on this filesystem. This flag provides a subset of the functionality provided by MS_NOATIME; that is, MS_NOATIME implies MS_NODIRATIME.


Do not allow programs to be executed from this filesystem.


Do not honor set-user-ID and set-group-ID bits or file capabilities when executing programs from this filesystem.


Mount filesystem read-only.

MS_REC (since Linux 2.4.11)

Used in conjunction with MS_BIND to create a recursive bind mount, and in conjunction with the propagation type flags to recursively change the propagation type of all of the mounts in a subtree. See below for further details.

MS_RELATIME (since Linux 2.6.20)

When a file on this filesystem is accessed, update the file_zsingle_quotesz_s last access time (atime) only if the current value of atime is less than or equal to the file_zsingle_quotesz_s last modification time (mtime) or last status change time (ctime). This option is useful for programs, such as mutt(1), that need to know when a file has been read since it was last modified. Since Linux 2.6.30, the kernel defaults to the behavior provided by this flag (unless MS_NOATIME was specified), and the MS_STRICTATIME flag is required to obtain traditional semantics. In addition, since Linux 2.6.30, the file_zsingle_quotesz_s last access time is always updated if it is more than 1 day old.

MS_SILENT (since Linux 2.6.17)

Suppress the display of certain (printk()) warning messages in the kernel log. This flag supersedes the misnamed and obsolete MS_VERBOSE flag (available since Linux 2.4.12), which has the same meaning.

MS_STRICTATIME (since Linux 2.6.30)

Always update the last access time (atime) when files on this filesystem are accessed. (This was the default behavior before Linux 2.6.30.) Specifying this flag overrides the effect of setting the MS_NOATIME and MS_RELATIME flags.


Make writes on this filesystem synchronous (as though the O_SYNC flag to open(2) was specified for all file opens to this filesystem).

From Linux 2.4 onward, some of the above flags are settable on a per-mount basis, while others apply to the superblock of the mounted filesystem, meaning that all mounts of the same filesystem share those flags. (Previously, all of the flags were per-superblock.)

The per-mount-point flags are as follows:

  • Since Linux 2.4: MS_NODEV, MS_NOEXEC, and MS_NOSUID flags are settable on a per-mount-point basis.

  • Since Linux 2.6.16: MS_NOATIME and MS_NODIRATIME.

  • Since Linux 2.6.20: MS_RELATIME.

The following flags are per-superblock: MS_DIRSYNC, MS_LAZYTIME, MS_MANDLOCK, MS_SILENT, and MS_SYNCHRONOUS. The initial settings of these flags are determined on the first mount of the filesystem, and will be shared by all subsequent mounts of the same filesystem. Subsequently, the settings of the flags can be changed via a remount operation (see below). Such changes will be visible via all mount points associated with the filesystem.

Since Linux 2.6.16, MS_RDONLY can be set or cleared on a per-mount-point basis as well as on the underlying filesystem superblock. The mounted filesystem will be writable only if neither the filesystem nor the mountpoint are flagged as read-only.

Remounting an existing mount

An existing mount may be remounted by specifying MS_REMOUNT in mountflags. This allows you to change the mountflags and data of an existing mount without having to unmount and remount the filesystem. target should be the same value specified in the initial mount() call.

The source and filesystemtype arguments are ignored.

The mountflags and data arguments should match the values used in the original mount() call, except for those parameters that are being deliberately changed.

The following mountflags can be changed: MS_LAZYTIME, MS_MANDLOCK, MS_NOATIME, MS_NODEV, MS_NODIRATIME, MS_NOEXEC, MS_NOSUID, MS_RELATIME, MS_RDONLY, MS_STRICTATIME (whose effect is to clear the MS_NOATIME and MS_RELATIME flags), and MS_SYNCHRONOUS. Attempts to change the setting of the MS_DIRSYNC and MS_SILENT flags during a remount are silently ignored. Note that changes to per-superblock flags are visible via all mount points of the associated filesystem (because the per-superblock flags are shared by all mount points).

Since Linux 3.17, if none of MS_NOATIME, MS_NODIRATIME, MS_RELATIME, or MS_STRICTATIME is specified in mountflags, then the remount operation preserves the existing values of these flags (rather than defaulting to MS_RELATIME).

Since Linux 2.6.26, the MS_REMOUNT flag can be used with MS_BIND to modify only the per-mount-point flags. This is particularly useful for setting or clearing the read-only flag on a mount point without changing the underlying filesystem. Specifying mountflags as:


will make access through this mountpoint read-only, without affecting other mount points.

Creating a bind mount

If mountflags includes MS_BIND (available since Linux 2.4), then perform a bind mount. A bind mount makes a file or a directory subtree visible at another point within the single directory hierarchy. Bind mounts may cross filesystem boundaries and span chroot(2) jails.

The filesystemtype and data arguments are ignored.

The remaining bits (other than MS_REC, described below) in the mountflags argument are also ignored. (The bind mount has the same mount options as the underlying mount point.) However, see the discussion of remounting above, for a method of making an existing bind mount read-only.

By default, when a directory is bind mounted, only that directory is mounted; if there are any submounts under the directory tree, they are not bind mounted. If the MS_REC flag is also specified, then a recursive bind mount operation is performed: all submounts under the source subtree (other than unbindable mounts) are also bind mounted at the corresponding location in the target subtree.

Changing the propagation type of an existing mount

If mountflags includes one of MS_SHARED, MS_PRIVATE, MS_SLAVE, or MS_UNBINDABLE (all available since Linux 2.6.15), then the propagation type of an existing mount is changed. If more than one of these flags is specified, an error results.

The only other flags that can be specified while changing the propagation type are MS_REC (described below) and MS_SILENT (which is ignored).

The source, filesystemtype, and data arguments are ignored.

The meanings of the propagation type flags are as follows:


Make this mount point shared. Mount and unmount events immediately under this mount point will propagate to the other mount points that are members of this mount_zsingle_quotesz_s peer group. Propagation here means that the same mount or unmount will automatically occur under all of the other mount points in the peer group. Conversely, mount and unmount events that take place under peer mount points will propagate to this mount point.


Make this mount point private. Mount and unmount events do not propagate into or out of this mount point.


If this is a shared mount point that is a member of a peer group that contains other members, convert it to a slave mount. If this is a shared mount point that is a member of a peer group that contains no other members, convert it to a private mount. Otherwise, the propagation type of the mount point is left unchanged.

When a mount point is a slave, mount and unmount events propagate into this mount point from the (master) shared peer group of which it was formerly a member. Mount and unmount events under this mount point do not propagate to any peer.

A mount point can be the slave of another peer group while at the same time sharing mount and unmount events with a peer group of which it is a member.


Make this mount unbindable. This is like a private mount, and in addition this mount can_zsingle_quotesz_t be bind mounted. When a recursive bind mount (mount() with the MS_BIND and MS_REC flags) is performed on a directory subtree, any unbindable mounts within the subtree are automatically pruned (i.e., not replicated) when replicating that subtree to produce the target subtree.

By default, changing the propagation type affects only the target mount point. If the MS_REC flag is also specified in mountflags, then the propagation type of all mount points under target is also changed.

For further details regarding mount propagation types (including the default propagation type assigned to new mounts), see mount_namespaces(7).

Moving a mount

If mountflags contains the flag MS_MOVE (available since Linux 2.4.18), then move a subtree: source specifies an existing mount point and target specifies the new location to which that mount point is to be relocated. The move is atomic: at no point is the subtree unmounted.

The remaining bits in the mountflags argument are ignored, as are the filesystemtype and data arguments.

Creating a new mount point

If none of MS_REMOUNT, MS_BIND, MS_MOVE, MS_SHARED, MS_PRIVATE, MS_SLAVE, or MS_UNBINDABLE is specified in mountflags, then mount() performs its default action: creating a new mount point. source specifies the source for the new mount point, and target specifies the directory at which to create the mount point.

The filesystemtype and data arguments are employed, and further bits may be specified in mountflags to modify the behavior of the call.


On success, zero is returned. On error, −1 is returned, and errno is set appropriately.


The error values given below result from filesystem type independent errors. Each filesystem type may have its own special errors and its own special behavior. See the Linux kernel source code for details.


A component of a path was not searchable. (See also path_resolution(7).)


Mounting a read-only filesystem was attempted without giving the MS_RDONLY flag.

The filesystem may be read-only for various reasons, including: it resides on a read-only optical disk; it is resides on a device with a physical switch that has been set to mark the device read-only; the filesystem implementation was compiled with read-only support; or errors were detected when initially mounting the filesystem, so that it was marked read-only and can_zsingle_quotesz_t be remounted as read-write (until the errors are fixed).

Some filesystems instead return the error EROFS on an attempt to mount a read-only filesystem.


The block device source is located on a filesystem mounted with the MS_NODEV option.


An attempt was made to stack a new mount directly on top of an existing mount point that was created in this mount namespace with the same source and target.


source cannot be remounted read-only, because it still holds files open for writing.


One of the pointer arguments points outside the user address space.


source had an invalid superblock.


A remount operation (MS_REMOUNT) was attempted, but source was not already mounted on target.


A move operation (MS_MOVE) was attempted, but the mount tree under source includes unbindable mounts and target is a mount point that has propagation type MS_SHARED.


A move operation (MS_MOVE) was attempted, but the parent mount of source mount has propagation type MS_SHARED.


A move operation (MS_MOVE) was attempted, but source was not a mount point, or was _zsingle_quotesz_/_zsingle_quotesz_.


mountflags includes more than one of MS_SHARED, MS_PRIVATE, MS_SLAVE, or MS_UNBINDABLE.


mountflags includes MS_SHARED, MS_PRIVATE, MS_SLAVE, or MS_UNBINDABLE and also includes a flag other than MS_REC or MS_SILENT.


An attempt was made to bind mount an unbindable mount.


In an unprivileged mount namespace (i.e., a mount namespace owned by a user namespace that was created by an unprivileged user), a bind mount operation (MS_BIND) was attempted without specifying (MS_REC), which would have revealed the filesystem tree underneath one of the submounts of the directory being bound.


Too many links encountered during pathname resolution.


A move operation was attempted, and target is a descendant of source.


(In case no block device is required:) Table of dummy devices is full.


A pathname was longer than MAXPATHLEN.


filesystemtype not configured in the kernel.


A pathname was empty or had a nonexistent component.


The kernel could not allocate a free page to copy filenames or data into.


source is not a block device (and a device was required).


target, or a prefix of source, is not a directory.


The major number of the block device source is out of range.


The caller does not have the required privileges.


Mounting a read-only filesystem was attempted without giving the MS_RDONLY flag. See EACCES, above.


The definitions of MS_DIRSYNC, MS_MOVE, MS_PRIVATE, MS_REC, MS_RELATIME, MS_SHARED, MS_SLAVE, MS_STRICTATIME, and MS_UNBINDABLE were added to glibc headers in version 2.12.


This function is Linux-specific and should not be used in programs intended to be portable.


Since Linux 2.4 a single filesystem can be mounted at multiple mount points, and multiple mounts can be stacked on the same mount point.

The mountflags argument may have the magic number 0xC0ED (MS_MGC_VAL) in the top 16 bits. (All of the other flags discussed in DESCRIPTION occupy the low order 16 bits of mountflags.) Specifying MS_MGC_VAL was required in kernel versions prior to 2.4, but since Linux 2.4 is no longer required and is ignored if specified.

The original MS_SYNC flag was renamed MS_SYNCHRONOUS in 1.1.69 when a different MS_SYNC was added to <mman.h>

Before Linux 2.4 an attempt to execute a set-user-ID or set-group-ID program on a filesystem mounted with MS_NOSUID would fail with EPERM. Since Linux 2.4 the set-user-ID and set-group-ID bits are just silently ignored in this case.

Mount namespaces

Starting with kernel 2.4.19, Linux provides mount namespaces. A mount namespace is the set of filesystem mounts that are visible to a process. Mount namespaces can be (and usually are) shared between multiple processes, and changes to the namespace (i.e., mounts and unmounts) by one process are visible to all other processes sharing the same namespace. (The pre-2.4.19 Linux situation can be considered as one in which a single namespace was shared by every process on the system.)

A child process created by fork(2) shares its parent_zsingle_quotesz_s mount namespace; the mount namespace is preserved across an execve(2).

A process can obtain a private mount namespace if: it was created using the clone(2) CLONE_NEWNS flag, in which case its new namespace is initialized to be a copy of the namespace of the process that called clone(2); or it calls unshare(2) with the CLONE_NEWNS flag, which causes the caller_zsingle_quotesz_s mount namespace to obtain a private copy of the namespace that it was previously sharing with other processes, so that future mounts and unmounts by the caller are invisible to other processes (except child processes that the caller subsequently creates) and vice versa.

For further details on mount namespaces, see mount_namespaces(7).

Parental relationship between mount points

Each mount point has a parent mount point. The overall parental relationship of all mount points defines the single directory hierarchy seen by the processes within a mount namespace.

The parent of a new mount point is defined when the mount point is created. In the usual case, the parent of a new mount is the mount point of the filesystem containing the directory or file at which the new mount is attached. In the case where a new mount is stacked on top of an existing mount, the parent of the new mount is the previous mount that was stacked at that location.

The parental relationship between mount points can be discovered via the /proc/[pid]/mountinfo file (see below).

/proc/[pid]/mounts and /proc/[pid]/mountinfo

The Linux-specific /proc/[pid]/mounts file exposes the list of mount points in the mount namespace of the process with the specified ID. The /proc/[pid]/mountinfo file exposes even more information about mount points, including the propagation type and mount ID information that makes it possible to discover the parental relationship between mount points. See proc(5) and mount_namespaces(7) for details of this file.


mountpoint(1), chroot(2), ioctl_iflags(2), pivot_root(2), umount(2), mount_namespaces(7), path_resolution(7), findmnt(8), lsblk(8), mount(8), umount(8)


This page is part of release 5.04 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at−pages/.

  Copyright (C) 1993 Rickard E. Faith <>
and Copyright (C) 1994 Andries E. Brouwer <>
and Copyright (C) 2002, 2005, 2016 Michael Kerrisk <>

Permission is granted to make and distribute verbatim copies of this
manual provided the copyright notice and this permission notice are
preserved on all copies.

Permission is granted to copy and distribute modified versions of this
manual under the conditions for verbatim copying, provided that the
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permission notice identical to this one.

Since the Linux kernel and libraries are constantly changing, this
manual page may be incorrect or out-of-date.  The author(s) assume no
responsibility for errors or omissions, or for damages resulting from
the use of the information contained herein.  The author(s) may not
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Formatted or processed versions of this manual, if unaccompanied by
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Modified 1996-11-04 by Eric S. Raymond <>
Modified 2001-10-13 by Michael Kerrisk <>
Added note on historical behavior of MS_NOSUID
Modified 2002-05-16 by Michael Kerrisk <>
Extensive changes and additions
Modified 2002-05-27 by aeb
Modified 2002-06-11 by Michael Kerrisk <>
Enhanced descriptions of MS_MOVE, MS_BIND, and MS_REMOUNT
Modified 2004-06-17 by Michael Kerrisk <>
2005-05-18, mtk, Added MNT_EXPIRE, plus a few other tidy-ups.
2008-10-06, mtk: move umount*() material into separate umount.2 page.
2008-10-06, mtk: Add discussion of namespaces.

Section (8) mount

Linux manual pages Section 8  


mount — mount a filesystem


mount [ −l | −h | −V ]

mount -a [−fFnrsvw] [ −t fstype ] [ −O optlist ]

mount [−fnrsvw] [ −o options ] device | dir

mount [−fnrsvw] [ −t fstype ] [ −o options ] device dir


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 filesystem found on some device to the big file tree. Conversely, the umount(8) command will detach it again. The filesystem is used to control how data is stored on the device or provided in a virtual way by network or another services.

The standard form of the mount command is:

  mount −t type device dir

This tells the kernel to attach the filesystem found on device (which is of type type) at the directory dir. The option −t type is optional. The mount command is usually able to detect a filesystem. The root permissions are necessary to mount a filesystem by default. See section Non-superuser mounts below for more details. The previous contents (if any) and owner and mode of dir become invisible, and as long as this filesystem remains mounted, the pathname dir refers to the root of the filesystem on device.

If only the directory or the device is given, for example:

mount /dir

then mount looks for a mountpoint (and if not found then for a device) in the /etc/fstab file. It_zsingle_quotesz_s possible to use the −−target or −−source options to avoid ambivalent interpretation of the given argument. For example:

mount −−target /mountpoint

The same filesystem may be mounted more than once, and in some cases (e.g. network filesystems) the same filesystem maybe be mounted on the same mountpoint more times. The mount command does not implement any policy to control this behavior. All behavior is controlled by kernel and it is usually specific to filesystem driver. The exception is −−all, in this case already mounted filesystems are ignored (see −−all below for more details).

Listing the mounts

The listing mode is maintained for backward compatibility only.

For more robust and customizable output use findmnt(8), especially in your scripts. Note that control characters in the mountpoint name are replaced with _zsingle_quotesz_?_zsingle_quotesz_.

The following command lists all mounted filesystems (of type type):

mount [ −l ] [ −t type/ ]

The option −l adds labels to this listing. See below.

Indicating the device and filesystem

Most devices are indicated by a filename (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 It is also possible to indicate a block special device using its filesystem label or UUID (see the −L and −U options below), or its partition label or UUID. Partition identifiers are supported for example for GUID Partition Tables (GPT).

The device name of disk partitions are unstable; hardware reconfiguration, adding or removing a device can cause change in names. This is reason why it_zsingle_quotesz_s strongly recommended to use filesystem or partition identificators like UUID or LABEL.

The command lsblk --fs provides overview of filesystems, LABELs and UUIDs on available block devices. The command blkid -p <device> provides details about a filesystem on the specified device.

Don_zsingle_quotesz_t forget that there is no guarantee that UUIDs and labels are really unique, especially if you move, share or copy the device. Use lsblk −o +UUID,PARTUUID to verify that the UUIDs are really unique in your system.

The recommended setup is to use tags (e.g. UUID=uuid) rather than /dev/disk/by-{label,uuid,partuuid,partlabel} udev symlinks in the /etc/fstab file. Tags are more readable, robust and portable. The mount(8) command internally uses udev symlinks, so the use of symlinks in /etc/fstab has no advantage over tags. For more details see libblkid(3).

Note that mount(8) uses UUIDs as strings. The UUIDs from the command line or from fstab(5) are not converted to internal binary representation. The string representation of the UUID should be based on lower case characters.

The proc filesystem 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 already mounted_zsingle_quotesz_ from mount can be confusing.)

The files /etc/fstab, /etc/mtab and /proc/mounts

The file /etc/fstab (see fstab(5)), may contain lines describing what devices are usually mounted where, using which options. The default location of the fstab(5) file can be overridden with the −−fstab path command-line option (see below for more details).

The command

mount −a [−t type] [−O optlist]

(usually given in a bootscript) causes all filesystems 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.

When mounting a filesystem mentioned in fstab or mtab, it suffices to specify on the command line only the device, or only the mount point.

The programs mount and umount traditionally maintained a list of currently mounted filesystems in the file /etc/mtab. The support for regular classic /etc/mtab is completely disabled in compile time by default, because on current Linux systems it is better to make it a symlink to /proc/mounts instead. The regular mtab file maintained in userspace cannot reliably work with namespaces, containers and other advanced Linux features. If the regular mtab support is enabled than it_zsingle_quotesz_s possible to use the file as well as the symlink.

If no arguments are given to mount, the list of mounted filesystems is printed.

If you want to override mount options from /etc/fstab you have to use the −o option:

mount device | dir −o options

and then the mount options from the command line will be appended to the list of options from /etc/fstab. This default behaviour is possible to change by command line option −−options−mode. The usual behavior is that the last option wins if there are conflicting ones.

The mount program does not read the /etc/fstab file if both device (or LABEL, UUID, PARTUUID or PARTLABEL) and dir are specified. For example, to mount device foo at /dir:

mount /dev/foo /dir

This default behaviour is possible to change by command line option −−options−source−force to always read configuration from fstab. For non-root users mount always read fstab configuration.

Non-superuser mounts

Normally, only the superuser can mount filesystems. However, when fstab contains the user option on a line, anybody can mount the corresponding filesystem.

Thus, given a line

/dev/cdrom  /cd  iso9660  ro,user,noauto,unhide

any user can mount the iso9660 filesystem found on an inserted CDROM using the command:

mount /cd

Note that mount is very strict about non-root users and all paths specified on command line are verified before fstab is parsed or a helper program is executed. It_zsingle_quotesz_s strongly recommended to use a valid mountpoint to specify filesystem, otherwise mount may fail. For example it_zsingle_quotesz_s bad idea to use NFS or CIFS source on command line.

For more details, see fstab(5). Only the user that mounted a filesystem can unmount it again. If any user should be able to unmount it, 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 group option is similar, with the restriction that the user must be member of the group of the special file.

Bind mount operation

Remount part of the file hierarchy somewhere else. The call is:

mount −−bind olddir newdir

or by using this fstab entry:

/olddir /newdir none bind

After this call the same contents are accessible in two places.

It is important to understand that bind does not to create any second-class or special node in the kernel VFS. The bind is just another operation to attach a filesystem. There is nowhere stored information that the filesystem has been attached by bind operation. The olddir and newdir are independent and the olddir maybe be umounted.

One can also remount a single file (on a single file). It_zsingle_quotesz_s also possible to use the bind mount to create a mountpoint from a regular directory, for example:

mount −−bind foo foo

The bind mount call attaches only (part of) a single filesystem, not possible submounts. The entire file hierarchy including submounts is attached a second place by using:

mount −−rbind olddir newdir

Note that the filesystem mount options maintained by kernel will remain the same as those on the original mount point. The userspace mount options (e.g. _netdev) will not be copied by mount(8) and it_zsingle_quotesz_s necessary explicitly specify the options on mount command line.

mount(8) since v2.27 allows to change the mount options by passing the relevant options along with −−bind. For example:

mount -o bind,ro foo foo

This feature is not supported by the Linux kernel; it is implemented in userspace by an additional mount(2) remounting system call. This solution is not atomic.

The alternative (classic) way to create a read-only bind mount is to use the remount operation, for example:

mount −−bind olddir newdir

mount −o remount,bind,ro olddir newdir

Note that a read-only bind will create a read-only mountpoint (VFS entry), but the original filesystem superblock will still be writable, meaning that the olddir will be writable, but the newdir will be read-only.

It_zsingle_quotesz_s also possible to change nosuid, nodev, noexec, noatime, nodiratime and relatime VFS entry flags by remount,bind operation. The another (for example filesystem specific flags) are silently ignored. It_zsingle_quotesz_s impossible to change mount options recursively (for example with −o rbind,ro).

mount(8) since v2.31 ignores the bind flag from /etc/fstab on remount operation (if -o remount specified on command line). This is necessary to fully control mount options on remount by command line. In the previous versions the bind flag has been always applied and it was impossible to re-define mount options without interaction with the bind semantic. This mount(8) behavior does not affect situations when remount,bind is specified in the /etc/fstab file.

The move operation

Move a mounted tree to another place (atomically). The call is:

mount −−move olddir newdir

This will cause the contents which previously appeared under olddir to now be accessible under newdir. The physical location of the files is not changed. Note that olddir has to be a mountpoint.

Note also that moving a mount residing under a shared mount is invalid and unsupported. Use findmnt −o TARGET,PROPAGATION to see the current propagation flags.

Shared subtree operations

Since Linux 2.6.15 it is possible to mark a mount and its submounts as shared, private, slave or unbindable. A shared mount provides the ability to create mirrors of that mount such that mounts and unmounts within any of the mirrors propagate to the other mirror. A slave mount receives propagation from its master, but not vice versa. A private mount carries no propagation abilities. An unbindable mount is a private mount which cannot be cloned through a bind operation. The detailed semantics are documented in Documentation/filesystems/sharedsubtree.txt file in the kernel source tree.

Supported operations are:

  mount −−make−shared mountpoint
  mount −−make−slave mountpoint
  mount −−make−private mountpoint
  mount −−make−unbindable mountpoint

The following commands allow one to recursively change the type of all the mounts under a given mountpoint.

  mount −−make−rshared mountpoint
  mount −−make−rslave mountpoint
  mount −−make−rprivate mountpoint
  mount −−make−runbindable mountpoint

mount(8) does not read fstab(5) when a −−make−* operation is requested. All necessary information has to be specified on the command line.

Note that the Linux kernel does not allow to change multiple propagation flags with a single mount(2) system call, and the flags cannot be mixed with other mount options and operations.

Since util-linux 2.23 the mount command allows to do more propagation (topology) changes by one mount(8) call and do it also together with other mount operations. This feature is EXPERIMENTAL. The propagation flags are applied by additional mount(2) system calls when the preceding mount operations were successful. Note that this use case is not atomic. It is possible to specify the propagation flags in fstab(5) as mount options (private, slave, shared, unbindable, rprivate, rslave, rshared, runbindable).

For example:

mount −−make−private −−make−unbindable /dev/sda1 /foo

is the same as:

mount /dev/sda1 /foox
mount −−make−private /foo
mount −−make−unbindable /foo


The full set of mount options used by an invocation of mount is determined by first extracting the mount options for the filesystem from the fstab table, then applying any options specified by the −o argument, and finally applying a −r or −w option, when present.

The command mount does not pass all command-line options to the /sbin/mount.suffix mount helpers. The interface between mount and the mount helpers is described below in the section EXTERNAL HELPERS.

Command-line options available for the mount command are:

−a, −−all

Mount all filesystems (of the given types) mentioned in fstab (except for those whose line contains the noauto keyword). The filesystems are mounted following their order in fstab. The mount command compares filesystem source, target (and fs root for bind mount or btrfs) to detect already mounted filesystems. The kernel table with already mounted filesystems is cached during mount −−all. It means that all duplicated fstab entries will be mounted.

Note that it is a bad practice to use mount −a for fstab checking. The recommended solution is findmnt −−verify.

−B, −−bind

Remount a subtree somewhere else (so that its contents are available in both places). See above, under Bind mounts.

−c, −−no−canonicalize

Don_zsingle_quotesz_t canonicalize paths. The mount command canonicalizes all paths (from command line or fstab) by default. This option can be used together with the −f flag for already canonicalized absolute paths. The option is designed for mount helpers which call mount -i. It is strongly recommended to not use this command-line option for normal mount operations.

Note that mount(8) does not pass this option to the /sbin/mount.type helpers.

−F, −−fork

(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.

−f, −−fake

Causes everything to be done except for the actual system call; if it_zsingle_quotesz_s not obvious, this ``fakes_zsingle_quotesz__zsingle_quotesz_ mounting the filesystem. 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. The −f option checks for an existing record in /etc/mtab and fails when the record already exists (with a regular non-fake mount, this check is done by the kernel).

−i, −−internal−only

Don_zsingle_quotesz_t call the /sbin/mount.filesystem helper even if it exists.

−L, −−label label

Mount the partition that has the specified label.

−l, −−show−labels

Add the labels in the mount output. mount must have permission to read the disk device (e.g. be set-user-ID root) for this to work. One can set such a label for ext2, ext3 or ext4 using the e2label(8) utility, or for XFS using xfs_admin(8), or for reiserfs using reiserfstune(8).

−M, −−move

Move a subtree to some other place. See above, the subsection The move operation.

−n, −−no−mtab

Mount without writing in /etc/mtab. This is necessary for example when /etc is on a read-only filesystem.

−N, −−namespace ns

Perform mount in namespace specified by ns. ns is either PID of process running in that namespace or special file representing that namespace.

mount(8) switches to the namespace when it reads /etc/fstab, writes /etc/mtab (or writes to /run/mount) and calls mount(2) system call, otherwise it runs in the original namespace. It means that the target namespace does not have to contain any libraries or another requirements necessary to execute mount(2) command.

See namespaces(7) for more information.

−O, −−test−opts opts

Limit the set of filesystems to which the −a option applies. In this regard it is like the −t option except that −O is useless without −a. For example, the command:

mount −a −O no_netdev

mounts all filesystems 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.

−o, −−options opts

Use the specified mount options. The opts argument is a comma-separated list. For example:

mount LABEL=mydisk −o noatime,nodev,nosuid


−−options−mode mode

Controls how to combine options from fstab/mtab with options from command line. mode can be one of ignore, append, prepend or replace. For example append means that options from fstab are appended to options from command line. Default value is prepend -- it means command line options are evaluated after fstab options. Note that the last option wins if there are conflicting ones.

−−options−source source

Source of default options. source is comma separated list of fstab, mtab and disable. disable disables fstab and mtab and disables −−options−source−force. Default value is fstab,mtab.


Use options from fstab/mtab even if both device and dir are specified.

−R, −−rbind

Remount a subtree and all possible submounts somewhere else (so that its contents are available in both places). See above, the subsection Bind mounts.

−r, −−read−only

Mount the filesystem read-only. A synonym is −o ro.

Note that, depending on the filesystem type, state and kernel behavior, the system may still write to the device. For example, ext3 and ext4 will replay the journal if the filesystem is dirty. To prevent this kind of write access, you may want to mount an ext3 or ext4 filesystem with the ro,noload mount options or set the block device itself to read-only mode, see the blockdev(8) command.


Tolerate sloppy mount options rather than failing. This will ignore mount options not supported by a filesystem type. Not all filesystems support this option. Currently it_zsingle_quotesz_s supported by the mount.nfs mount helper only.

−−source device

If only one argument for the mount command is given then the argument might be interpreted as target (mountpoint) or source (device). This option allows to explicitly define that the argument is the mount source.

−−target directory

If only one argument for the mount command is given then the argument might be interpreted as target (mountpoint) or source (device). This option allows to explicitly define that the argument is the mount target.

−T, −−fstab path

Specifies an alternative fstab file. If path is a directory then the files in the directory are sorted by strverscmp(3); files that start with . or without an .fstab extension are ignored. The option can be specified more than once. This option is mostly designed for initramfs or chroot scripts where additional configuration is specified beyond standard system configuration.

Note that mount(8) does not pass the option −−fstab to the /sbin/mount.type helpers, meaning that the alternative fstab files will be invisible for the helpers. This is no problem for normal mounts, but user (non-root) mounts always require fstab to verify the user_zsingle_quotesz_s rights.

−t, −−types fstype

The argument following the −t is used to indicate the filesystem type. The filesystem types which are currently supported depend on the running kernel. See /proc/filesystems and /lib/modules/$(uname -r)/kernel/fs for a complete list of the filesystems. The most common are ext2, ext3, ext4, xfs, btrfs, vfat, sysfs, proc, nfs and cifs.

The programs mount and umount support filesystem subtypes. The subtype is defined by a _zsingle_quotesz_.subtype_zsingle_quotesz_ suffix. For example _zsingle_quotesz_fuse.sshfs_zsingle_quotesz_. It_zsingle_quotesz_s recommended to use subtype notation rather than add any prefix to the mount source (for example _zsingle_quotesz_sshfs#example.com_zsingle_quotesz_ is deprecated).

If no −t option is given, or if the auto type is specified, mount will try to guess the desired type. Mount uses the blkid library for guessing the filesystem type; if that does not turn up anything that looks familiar, 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 *, mount will read /proc/filesystems afterwards. While trying, all filesystem types will be mounted with the mount option silent.

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 ext3 before ext2) or if you use a kernel module autoloader.

More than one type may be specified in a comma-separated list, for option −t as well as in an /etc/fstab entry. The list of filesystem types for option −t can be prefixed with no to specify the filesystem types on which no action should be taken. The prefix no has no effect when specified in an /etc/fstab entry.

The prefix no can be meaningful with the −a option. For example, the command

mount −a −t nomsdos,smbfs

mounts all filesystems except those of type msdos and smbfs.

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, nfs4, cifs, smbfs, ncpfs) an ad hoc code is necessary. The nfs, nfs4, cifs, smbfs, and ncpfs filesystems 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 different versions of the smbmount program have different calling conventions, /sbin/mount.smbfs may have to be a shell script that sets up the desired call.

−U, −−uuid uuid

Mount the partition that has the specified uuid.

−v, −−verbose

Verbose mode.

−w, −−rw, −−read−write

Mount the filesystem read/write. The read-write is kernel default. A synonym is −o rw.

Note that specify −w on command line forces mount command to never try read-only mount on write-protected devices. The default is try read-only if the previous mount syscall with read-write flags failed.

−V, −−version

Display version information and exit.

−h, −−help

Display help text and exit.


Some of these options are only useful when they appear in the /etc/fstab file.

Some of these options could be enabled or disabled by default in the system kernel. To check the current setting see the options in /proc/mounts. Note that filesystems also have per-filesystem specific default mount options (see for example tune2fs −l output for extN filesystems).

The following options apply to any filesystem that is being mounted (but not every filesystem actually honors them – e.g., the sync option today has an effect only for ext2, ext3, ext4, fat, vfat and ufs):


All I/O to the filesystem should be done asynchronously. (See also the sync option.)


Do not use the noatime feature, so the inode access time is controlled by kernel defaults. See also the descriptions of the relatime and strictatime mount options.


Do not update inode access times on this filesystem (e.g. for faster access on the news spool to speed up news servers). This works for all inode types (directories too), so it implies nodiratime.


Can be mounted with the −a option.


Can only be mounted explicitly (i.e., the −a option will not cause the filesystem to be mounted).

context=context, fscontext=context, defcontext=context, and rootcontext=context

The context= option is useful when mounting filesystems that do not support extended attributes, such as a floppy or hard disk formatted with VFAT, or systems that are not normally running under SELinux, such as an ext3 or ext4 formatted

disk from a non-SELinux workstation. You can also use context= on filesystems you do not trust, such as a floppy. It also helps in compatibility with xattr-supporting filesystems on earlier 2.4.<x> kernel versions. Even where xattrs are supported, you can save time not having to label every file by assigning the entire disk one security context.

A commonly used option for removable media is context=system_u:object_r:removable_t.

Two other options are fscontext= and defcontext=, both of which are mutually exclusive of the context option. This means you can use fscontext and defcontext with each other, but neither can be used with context.

The fscontext= option works for all filesystems, regardless of their xattr support. The fscontext option sets the overarching filesystem label to a specific security context. This filesystem label is separate from the individual labels on the files. It represents the entire filesystem for certain kinds of permission checks, such as during mount or file creation. Individual file labels are still obtained from the xattrs on the files themselves. The context option actually sets the aggregate context that fscontext provides, in addition to supplying the same label for individual files.

You can set the default security context for unlabeled files using defcontext= option. This overrides the value set for unlabeled files in the policy and requires a filesystem that supports xattr labeling.

The rootcontext= option allows you to explicitly label the root inode of a FS being mounted before that FS or inode becomes visible to userspace. This was found to be useful for things like stateless linux.

Note that the kernel rejects any remount request that includes the context option, even when unchanged from the current context.

Warning: the context value might contain commas, in which case the value has to be properly quoted, otherwise mount(8) will interpret the comma as a separator between mount options. Don_zsingle_quotesz_t forget that the shell strips off quotes and thus double quoting is required. For example:

mount −t tmpfs none /mnt −o 

For more details, see selinux(8).


Use the default options: rw, suid, dev, exec, auto, nouser, and async.

Note that the real set of all default mount options depends on kernel and filesystem type. See the beginning of this section for more details.


Interpret character or block special devices on the filesystem.


Do not interpret character or block special devices on the file system.


Update directory inode access times on this filesystem. This is the default. (This option is ignored when noatime is set.)


Do not update directory inode access times on this filesystem. (This option is implied when noatime is set.)


All directory updates within the filesystem should be done synchronously. This affects the following system calls: creat, link, unlink, symlink, mkdir, rmdir, mknod and rename.


Permit execution of binaries.


Do not permit direct execution of any binaries on the mounted filesystem.


Allow an ordinary user to mount the filesystem if one of that user_zsingle_quotesz_s groups matches the group of the device. This option implies the options nosuid and nodev (unless overridden by subsequent options, as in the option line group,dev,suid).


Every time the inode is modified, the i_version field will be incremented.


Do not increment the i_version inode field.


Allow mandatory locks on this filesystem. See fcntl(2).


Do not allow mandatory locks on this filesystem.


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).


Do not report errors for this device if it does not exist.


Update inode access times relative to modify or change time. Access time is only updated if the previous access time was earlier than the current modify or change time. (Similar to noatime, but it doesn_zsingle_quotesz_t break mutt or other applications that need to know if a file has been read since the last time it was modified.)

Since Linux 2.6.30, the kernel defaults to the behavior provided by this option (unless noatime was specified), and the strictatime option is required to obtain traditional semantics. In addition, since Linux 2.6.30, the file_zsingle_quotesz_s last access time is always updated if it is more than 1 day old.


Do not use the relatime feature. See also the strictatime mount option.


Allows to explicitly request full atime updates. This makes it possible for the kernel to default to relatime or noatime but still allow userspace to override it. For more details about the default system mount options see /proc/mounts.


Use the kernel_zsingle_quotesz_s default behavior for inode access time updates.


Only update times (atime, mtime, ctime) on the in-memory version of the file inode.

This mount option significantly reduces writes to the inode table for workloads that perform frequent random writes to preallocated files.

The on-disk timestamps are updated only when:

- the inode needs to be updated for some change unrelated to file timestamps

- the application employs fsync(2), syncfs(2), or sync(2)

- an undeleted inode is evicted from memory

- more than 24 hours have passed since the i-node was written to disk.


Do not use the lazytime feature.


Allow set-user-ID or set-group-ID bits to take effect.


Do not allow set-user-ID or set-group-ID bits to take effect.


Turn on the silent flag.


Turn off the silent flag.


Allow an ordinary user to mount the filesystem if that user is the owner of the device. This option implies the options nosuid and nodev (unless overridden by subsequent options, as in the option line owner,dev,suid).


Attempt to remount an already-mounted filesystem. This is commonly used to change the mount flags for a filesystem, especially to make a readonly filesystem writable. It does not change device or mount point.

The remount operation together with the bind flag has special semantic. See above, the subsection Bind mounts.

The remount functionality follows the standard way the mount command works with options from fstab. This means that mount does not read fstab (or mtab) only when both device and dir are specified.

mount −o remount,rw /dev/foo /dir

After this call all old mount options are replaced and arbitrary stuff from fstab (or mtab) is ignored, except the loop= option which is internally generated and maintained by the mount command.

mount −o remount,rw /dir

After this call, mount reads fstab and merges these options with the options from the command line (−o). If no mountpoint is found in fstab, then a remount with unspecified source is allowed.


Mount the filesystem read-only.


Mount the filesystem read-write.


All I/O to the filesystem should be done synchronously. In the case of media with a limited number of write cycles (e.g. some flash drives), sync may cause life-cycle shortening.


Allow an ordinary user to mount the filesystem. The name of the mounting user is written to the mtab file (or to the private libmount file in /run/mount on systems without a regular mtab) so that this same user can unmount the filesystem again. This option implies the options noexec, nosuid, and nodev (unless overridden by subsequent options, as in the option line user,exec,dev,suid).


Forbid an ordinary user to mount the filesystem. This is the default; it does not imply any other options.


Allow any user to mount and to unmount the filesystem, even when some other ordinary user mounted it. This option implies the options noexec, nosuid, and nodev (unless overridden by subsequent options, as in the option line users,exec,dev,suid).


All options prefixed with X- are interpreted as comments or as userspace application-specific options. These options are not stored in the user space (e.g. mtab file), nor sent to the mount.type helpers nor to the mount(2) system call. The suggested format is X-appname.option.


The same as X-* options, but stored permanently in the user space. It means the options are also available for umount or another operations. Note that maintain mount options in user space is tricky, because it_zsingle_quotesz_s necessary use libmount based tools and there is no guarantee that the options will be always available (for example after a move mount operation or in unshared namespace).

Note that before util-linux v2.30 the x-* options have not been maintained by libmount and stored in user space (functionality was the same as have X-* now), but due to growing number of use-cases (in initrd, systemd etc.) the functionality have been extended to keep existing fstab configurations usable without a change.


Allow to make a target directory (mountpoint). The optional argument mode specifies the filesystem access mode used for mkdir(2) in octal notation. The default mode is 0755. This functionality is supported only for root users. The option is also supported as x-mount.mkdir, this notation is deprecated for mount.mkdir since v2.30.


You should consult the respective man page for the filesystem first. If you want to know what options the ext4 filesystem supports, then check the ext4(5) man page. If that doesn_zsingle_quotesz_t exist, you can also check the corresponding mount page like mount.cifs(8). Note that you might have to install the respective userland tools.

The following options apply only to certain filesystems. We sort them by filesystem. They all follow the −o flag.

What options are supported depends a bit on the running kernel. More info may be found in the kernel source subdirectory Documentation/filesystems.

Mount options for adfs

uid=value and gid=value

Set the owner and group of the files in the filesystem (default: uid=gid=0).

ownmask=value and othmask=value

Set the permission mask for ADFS _zsingle_quotesz_owner_zsingle_quotesz_ permissions and _zsingle_quotesz_other_zsingle_quotesz_ permissions, respectively (default: 0700 and 0077, respectively). See also /usr/src/linux/Documentation/filesystems/adfs.txt.

Mount options for affs

uid=value and gid=value

Set the owner and group of the root of the filesystem (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.


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.


Do not allow any changes to the protection bits on the filesystem.


Set UID and GID of the root of the filesystem to the UID and GID of the mount point upon the first sync or umount, and then clear this option. Strange...


Print an informational message for each successful mount.


Prefix used before volume name, when following a link.


Prefix (of length at most 30) used before _zsingle_quotesz_/_zsingle_quotesz_ when following a symbolic link.


(Default: 2.) Number of unused blocks at the start of the device.


Give explicitly the location of the root block.


Give blocksize. Allowed values are 512, 1024, 2048, 4096.


These options are accepted but ignored. (However, quota utilities may react to such strings in /etc/fstab.)

Mount options for debugfs

The debugfs filesystem is a pseudo filesystem, traditionally mounted on /sys/kernel/debug. As of kernel version 3.4, debugfs has the following options:

uid=n, gid=n

Set the owner and group of the mountpoint.


Sets the mode of the mountpoint.

Mount options for devpts

The devpts filesystem is a pseudo filesystem, 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/<number>.

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.


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.


Create a private instance of devpts filesystem, such that indices of ptys allocated in this new instance are independent of indices created in other instances of devpts.

All mounts of devpts without this newinstance option share the same set of pty indices (i.e. legacy mode). Each mount of devpts with the newinstance option has a private set of pty indices.

This option is mainly used to support containers in the linux kernel. It is implemented in linux kernel versions starting with 2.6.29. Further, this mount option is valid only if CONFIG_DEVPTS_MULTIPLE_INSTANCES is enabled in the kernel configuration.

To use this option effectively, /dev/ptmx must be a symbolic link to pts/ptmx. See Documentation/filesystems/devpts.txt in the linux kernel source tree for details.


Set the mode for the new ptmx device node in the devpts filesystem.

With the support for multiple instances of devpts (see newinstance option above), each instance has a private ptmx node in the root of the devpts filesystem (typically /dev/pts/ptmx).

For compatibility with older versions of the kernel, the default mode of the new ptmx node is 0000. ptmxmode=value specifies a more useful mode for the ptmx node and is highly recommended when the newinstance option is specified.

This option is only implemented in linux kernel versions starting with 2.6.29. Further, this option is valid only if CONFIG_DEVPTS_MULTIPLE_INSTANCES is enabled in the kernel configuration.

Mount options for fat

[Note] Note

fat is not a separate filesystem, but a common part of the msdos, umsdos and vfat filesystems.


Set blocksize (default 512). This option is obsolete.

uid=value and gid=value

Set the owner and group of all files. (Default: the UID and GID of the current process.)


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.


Set the umask applied to directories only. The default is the umask of the current process. The value is given in octal.


Set the umask applied to regular files only. The default is the umask of the current process. The value is given in octal.


This option controls the permission check of mtime/atime.


If current process is in group of file_zsingle_quotesz_s group ID, you can change timestamp.


Other users can change timestamp.

The default is set from `dmask_zsingle_quotesz_ option. (If the directory is writable, utime(2) is also allowed. I.e. ~dmask & 022)

Normally utime(2) checks current process is owner of the file, or it has CAP_FOWNER capability. But FAT filesystem doesn_zsingle_quotesz_t have UID/GID on disk, so normal check is too inflexible. With this option you can relax it.


Three different levels of pickiness can be chosen:


Upper and lower case are accepted and equivalent, long name parts are truncated (e.g. verylongname.foobar becomes, leading and embedded spaces are accepted in each name part (name and extension).


Like relaxed, but many special characters (*, ?, <, spaces, etc.) are rejected. This is the default.


Like normal, but names that contain long parts or special characters that are sometimes used on Linux but are not accepted by MS-DOS (+, =, etc.) are rejected.


Sets the codepage for converting to shortname characters on FAT and VFAT filesystems. By default, codepage 437 is used.


This option is obsolete and may fail or being ignored.


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. This option is obsolete.


Option passed to the CVF module. This option is obsolete.


Turn on the debug flag. A version string and a list of filesystem parameters will be printed (these data are also printed if the parameters appear to be inconsistent).


If set, causes discard/TRIM commands to be issued to the block device when blocks are freed. This is useful for SSD devices and sparse/thinly-provisioned LUNs.


If set, use a fallback default BIOS Parameter Block configuration, determined by backing device size. These static parameters match defaults assumed by DOS 1.x for 160 kiB, 180 kiB, 320 kiB, and 360 kiB floppies and floppy images.


Specify FAT behavior on critical errors: panic, continue without doing anything, or remount the partition in read-only mode (default behavior).


Specify a 12, 16 or 32 bit fat. This overrides the automatic FAT type detection routine. Use with caution!


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.


Enable this only if you want to export the FAT filesystem over NFS.

stale_rw: This option maintains an index (cache) of directory inodes which is used by the nfs-related code to improve look-ups. Full file operations (read/write) over NFS are supported but with cache eviction at NFS server, this could result in spurious ESTALE errors.

nostale_ro: This option bases the inode number and file handle on the on-disk location of a file in the FAT directory entry. This ensures that ESTALE will not be returned after a file is evicted from the inode cache. However, it means that operations such as rename, create and unlink could cause file handles that previously pointed at one file to point at a different file, potentially causing data corruption. For this reason, this option also mounts the filesystem readonly.

To maintain backward compatibility, _zsingle_quotesz_-o nfs_zsingle_quotesz_ is also accepted, defaulting to stale_rw.


This option disables the conversion of timestamps between local time (as used by Windows on FAT) and UTC (which Linux uses internally). This is particularly useful when mounting devices (like digital cameras) that are set to UTC in order to avoid the pitfalls of local time.


Set offset for conversion of timestamps from local time used by FAT to UTC. I.e., minutes will be subtracted from each timestamp to convert it to UTC used internally by Linux. This is useful when the time zone set in the kernel via settimeofday(2) is not the time zone used by the filesystem. Note that this option still does not provide correct time stamps in all cases in presence of DST - time stamps in a different DST setting will be off by one hour.


Turn on the quiet flag. Attempts to chown or chmod files do not return errors, although they fail. Use with caution!


FAT has the ATTR_RO (read-only) attribute. On Windows, the ATTR_RO of the directory will just be ignored, and is used only by applications as a flag (e.g. it_zsingle_quotesz_s set for the customized folder).

If you want to use ATTR_RO as read-only flag even for the directory, set this option.


If set, the execute permission bits of the file will be allowed only if the extension part of the name is .EXE, .COM, or .BAT. Not set by default.


If set, ATTR_SYS attribute on FAT is handled as IMMUTABLE flag on Linux. Not set by default.


If set, the filesystem will try to flush to disk more early than normal. Not set by default.


Use the free clusters value stored on FSINFO. It_zsingle_quotesz_ll be used to determine number of free clusters without scanning disk. But it_zsingle_quotesz_s not used by default, because recent Windows don_zsingle_quotesz_t update it correctly in some case. If you are sure the free clusters on FSINFO is correct, by this option you can avoid scanning disk.

dots, nodots, dotsOK=[yes|no]

Various misguided attempts to force Unix or DOS conventions onto a FAT filesystem.

Mount options for hfs

creator=cccc, type=cccc

Set the creator/type values as shown by the MacOS finder used for creating new files. Default values: _zsingle_quotesz_????_zsingle_quotesz_.

uid=n, gid=n

Set the owner and group of all files. (Default: the UID and GID of the current process.)

dir_umask=n, file_umask=n, umask=n

Set the umask used for all directories, all regular files, or all files and directories. Defaults to the umask of the current process.


Select the CDROM session to mount. Defaults to leaving that decision to the CDROM driver. This option will fail with anything but a CDROM as underlying device.


Select partition number n from the device. Only makes sense for CDROMs. Defaults to not parsing the partition table at all.


Don_zsingle_quotesz_t complain about invalid mount options.

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.)


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.


Convert all files names to lower case, or leave them. (Default: case=lower.)


This option is obsolete and may fail or being ignored.


Do not abort mounting when certain consistency checks fail.

Mount options for iso9660

ISO 9660 is a standard describing a filesystem structure to be used on CD-ROMs. (This filesystem type is also seen on some DVDs. See also the udf filesystem.)

Normal iso9660 filenames appear in an 8.3 format (i.e., DOS-like restrictions on filename length), and in addition all characters are in upper case. Also there is no field for file ownership, protection, number of links, provision for block/character devices, etc.

Rock Ridge is an extension to iso9660 that provides all of these UNIX-like features. Basically there are extensions to each directory record that supply all of the additional information, and when Rock Ridge is in use, the filesystem is indistinguishable from a normal UNIX filesystem (except that it is read-only, of course).


Disable the use of Rock Ridge extensions, even if available. Cf. map.


Disable the use of Microsoft Joliet extensions, even if available. Cf. map.


With check=relaxed, a filename is first converted to lower case before doing the lookup. This is probably only meaningful together with norock and map=normal. (Default: check=strict.)

uid=value and gid=value

Give all files in the filesystem the indicated user or group id, possibly overriding the information found in the Rock Ridge extensions. (Default: uid=0,gid=0.)


For non-Rock Ridge volumes, normal name translation maps upper to lower case ASCII, drops a trailing `;1_zsingle_quotesz_, and converts `;_zsingle_quotesz_ to `._zsingle_quotesz_. With map=off no name translation is done. See norock. (Default: map=normal.) map=acorn is like map=normal but also apply Acorn extensions if present.


For non-Rock Ridge volumes, give all files the indicated mode. (Default: read and execute permission for everybody.) Octal mode values require a leading 0.


Also show hidden and associated files. (If the ordinary files and the associated or hidden files have the same filenames, this may make the ordinary files inaccessible.)


Set the block size to the indicated value. (Default: block=1024.)


This option is obsolete and may fail or being ignored.


If the high byte of the file length contains other garbage, set this mount option to ignore the high order bits of the file length. This implies that a file cannot be larger than 16 MB.


Select number of session on multisession CD.


Session begins from sector xxx.

The following options are the same as for vfat and specifying them only makes sense when using discs encoded using Microsoft_zsingle_quotesz_s Joliet extensions.


Character set to use for converting 16 bit Unicode characters on CD to 8 bit characters. The default is iso8859-1.


Convert 16 bit Unicode characters on CD to UTF-8.

Mount options for jfs


Character set to use for converting from Unicode to ASCII. The default is to do no conversion. Use iocharset=utf8 for UTF8 translations. This requires CONFIG_NLS_UTF8 to be set in the kernel .config file.


Resize the volume to value blocks. JFS only supports growing a volume, not shrinking it. This option is only valid during a remount, when the volume is mounted read-write. The resize keyword with no value will grow the volume to the full size of the partition.


Do not write to the journal. The primary use of this option is to allow for higher performance when restoring a volume from backup media. The integrity of the volume is not guaranteed if the system abnormally ends.


Default. Commit metadata changes to the journal. Use this option to remount a volume where the nointegrity option was previously specified in order to restore normal behavior.


Define the behavior when an error is encountered. (Either ignore errors and just mark the filesystem erroneous and continue, or remount the filesystem read-only, or panic and halt the system.)


These options are accepted but ignored.

Mount options for msdos

See mount options for fat. If the msdos filesystem detects an inconsistency, it reports an error and sets the file system read-only. The filesystem can be made writable again by remounting it.

Mount options for ncpfs

Just like nfs, the ncpfs implementation expects a binary argument (a struct ncp_mount_data) to the mount system call. This argument is constructed by ncpmount(8) and the current version of mount (2.12) does not know anything about ncpfs.

Mount options for ntfs


Character set to use when returning file names. Unlike VFAT, NTFS suppresses names that contain nonconvertible characters. Deprecated.


New name for the option earlier called iocharset.


Use UTF-8 for converting file names.


For 0 (or `no_zsingle_quotesz_ or `false_zsingle_quotesz_), do not use escape sequences for unknown Unicode characters. For 1 (or `yes_zsingle_quotesz_ or `true_zsingle_quotesz_) or 2, use vfat-style 4-byte escape sequences starting with :. Here 2 give a little-endian encoding and 1 a byteswapped bigendian encoding.


If enabled (posix=1), the filesystem distinguishes between upper and lower case. The 8.3 alias names are presented as hard links instead of being suppressed. This option is obsolete.

uid=value, gid=value and umask=value

Set the file permission on the filesystem. The umask value is given in octal. By default, the files are owned by root and not readable by somebody else.

Mount options for overlay

Since Linux 3.18 the overlay pseudo filesystem implements a union mount for other filesystems.

An overlay filesystem combines two filesystems - an upper filesystem and a lower filesystem. When a name exists in both filesystems, the object in the upper filesystem is visible while the object in the lower filesystem is either hidden or, in the case of directories, merged with the upper object.

The lower filesystem can be any filesystem supported by Linux and does not need to be writable. The lower filesystem can even be another overlayfs. The upper filesystem will normally be writable and if it is it must support the creation of trusted.* extended attributes, and must provide a valid d_type in readdir responses, so NFS is not suitable.

A read-only overlay of two read-only filesystems may use any filesystem type. The options lowerdir and upperdir are combined into a merged directory by using:

mount −t overlay  overlay  
  −olowerdir=/lower,upperdir=/upper,workdir=/work  /merged

Any filesystem, does not need to be on a writable filesystem.


The upperdir is normally on a writable filesystem.


The workdir needs to be an empty directory on the same filesystem as upperdir.

Mount options for reiserfs

Reiserfs is a journaling filesystem.


Instructs version 3.6 reiserfs software to mount a version 3.5 filesystem, using the 3.6 format for newly created objects. This filesystem will no longer be compatible with reiserfs 3.5 tools.


Choose which hash function reiserfs will use to find files within directories.


A hash invented by Yury Yu. Rupasov. It is fast and preserves locality, mapping lexicographically close file names to close hash values. This option should not be used, as it causes a high probability of hash collisions.


A Davis-Meyer function implemented by Jeremy Fitzhardinge. It uses hash permuting bits in the name. It gets high randomness and, therefore, low probability of hash collisions at some CPU cost. This may be used if EHASHCOLLISION errors are experienced with the r5 hash.


A modified version of the rupasov hash. It is used by default and is the best choice unless the filesystem has huge directories and unusual file-name patterns.


Instructs mount to detect which hash function is in use by examining the filesystem being mounted, and to write this information into the reiserfs superblock. This is only useful on the first mount of an old format filesystem.


Tunes the block allocator. This may provide performance improvements in some situations.


Tunes the block allocator. This may provide performance improvements in some situations.


Disable the border allocator algorithm invented by Yury Yu. Rupasov. This may provide performance improvements in some situations.


Disable journaling. This will provide slight performance improvements in some situations at the cost of losing reiserfs_zsingle_quotesz_s fast recovery from crashes. Even with this option turned on, reiserfs still performs all journaling operations, save for actual writes into its journaling area. Implementation of nolog is a work in progress.


By default, reiserfs stores small files and `file tails_zsingle_quotesz_ directly into its tree. This confuses some utilities such as LILO(8). This option is used to disable packing of files into the tree.


Replay the transactions which are in the journal, but do not actually mount the filesystem. Mainly used by reiserfsck.


A remount option which permits online expansion of reiserfs partitions. Instructs reiserfs to assume that the device has number blocks. This option is designed for use with devices which are under logical volume management (LVM). There is a special resizer utility which can be obtained from


Enable Extended User Attributes. See the attr(5) manual page.


Enable POSIX Access Control Lists. See the acl(5) manual page.

barrier=none / barrier=flush

This disables / enables the use of write barriers in the journaling code. barrier=none disables, barrier=flush enables (default). This also requires an IO stack which can support barriers, and if reiserfs gets an error on a barrier write, it will disable barriers again with a warning. Write barriers enforce proper on-disk ordering of journal commits, making volatile disk write caches safe to use, at some performance penalty. If your disks are battery-backed in one way or another, disabling barriers may safely improve performance.

Mount options for ubifs

UBIFS is a flash filesystem which works on top of UBI volumes. Note that atime is not supported and is always turned off.

The device name may be specified as

ubiX_Y UBI device number X, volume number Y


UBI device number 0, volume number Y


UBI device number X, volume with name NAME


UBI device number 0, volume with name NAME

Alternative ! separator may be used instead of :.

The following mount options are available:

Enable bulk-read. VFS read-ahead is disabled because it slows down the file system. Bulk-Read is an internal optimization. Some flashes may read faster if the data are read at one go, rather than at several read requests. For example, OneNAND can do read-while-load if it reads more than one NAND page.


Do not bulk-read. This is the default.


Check data CRC-32 checksums. This is the default.


Do not check data CRC-32 checksums. With this option, the filesystem does not check CRC-32 checksum for data, but it does check it for the internal indexing information. This option only affects reading, not writing. CRC-32 is always calculated when writing the data.


Select the default compressor which is used when new files are written. It is still possible to read compressed files if mounted with the none option.

Mount options for udf

UDF is the Universal Disk Format filesystem defined by OSTA, the Optical Storage Technology Association, and is often used for DVD-ROM, frequently in the form of a hybrid UDF/ISO-9660 filesystem. It is, however, perfectly usable by itself on disk drives, flash drives and other block devices. See also iso9660.


Make all files in the filesystem belong to the given user. uid=forget can be specified independently of (or usually in addition to) uid=<user> and results in UDF not storing uids to the media. In fact the recorded uid is the 32-bit overflow uid -1 as defined by the UDF standard. The value is given as either <user> which is a valid user name or the corresponding decimal user id, or the special string forget.


Make all files in the filesystem belong to the given group. gid=forget can be specified independently of (or usually in addition to) gid=<group> and results in UDF not storing gids to the media. In fact the recorded gid is the 32-bit overflow gid -1 as defined by the UDF standard. The value is given as either <group> which is a valid group name or the corresponding decimal group id, or the special string forget.


Mask out the given permissions from all inodes read from the filesystem. The value is given in octal.


If mode= is set the permissions of all non-directory inodes read from the filesystem will be set to the given mode. The value is given in octal.


If dmode= is set the permissions of all directory inodes read from the filesystem will be set to the given dmode. The value is given in octal.


Set the block size. Default value prior to kernel version 2.6.30 was 2048. Since 2.6.30 and prior to 4.11 it was logical device block size with fallback to 2048. Since 4.11 it is logical block size with fallback to any valid block size between logical device block size and 4096.

For other details see the mkudffs(8) 2.0+ manpage, sections COMPATIBILITY and BLOCK SIZE.


Show otherwise hidden files.


Show deleted files in lists.


Embed data in the inode. (default)


Don_zsingle_quotesz_t embed data in the inode.


Use short UDF address descriptors.


Use long UDF address descriptors. (default)


Unset strict conformance.


Set the NLS character set. This requires kernel compiled with CONFIG_UDF_NLS option.


Set the UTF-8 character set.

Mount options for debugging and disaster recovery


Ignore the Volume Recognition Sequence and attempt to mount anyway.


Select the session number for multi-session recorded optical media. (default= last session)


Override standard anchor location. (default= 256)


Set the last block of the filesystem.

Unused historical mount options that may be encountered and should be removed


Ignored, use uid=<user> instead.


Ignored, use gid=<group> instead.


Unimplemented and ignored.


Unimplemented and ignored.


Unimplemented and ignored.


Unimplemented and ignored.

Mount options for ufs


UFS is a filesystem widely used in different operating systems. The problem are differences among implementations. Features of some implementations are undocumented, so its hard to recognize the type of ufs automatically. That_zsingle_quotesz_s why the user must specify the type of ufs by mount option. Possible values are:


Old format of ufs, this is the default, read only. (Don_zsingle_quotesz_t forget to give the −r option.)


For filesystems created by a BSD-like system (NetBSD, FreeBSD, OpenBSD).


Used in FreeBSD 5.x supported as read-write.


Synonym for ufs2.


For filesystems created by SunOS or Solaris on Sparc.


For filesystems created by Solaris on x86.


For filesystems created by HP-UX, read-only.


For filesystems created by NeXTStep (on NeXT station) (currently read only).


For NextStep CDROMs (block_size == 2048), read-only.


For filesystems created by OpenStep (currently read only). The same filesystem type is also used by Mac OS X.


Set behavior on error:


If an error is encountered, cause a kernel panic.


These mount options don_zsingle_quotesz_t do anything at present; when an error is encountered only a console message is printed.

Mount options for umsdos

See mount options for msdos. The dotsOK option is explicitly killed by umsdos.

Mount options for vfat

First of all, the mount options for fat are recognized. The dotsOK option is explicitly killed by vfat. Furthermore, there are


Translate unhandled Unicode characters to special escaped sequences. This lets you backup and restore filenames that are created with any Unicode characters. Without this option, a _zsingle_quotesz_?_zsingle_quotesz_ is used when no translation is possible. The escape character is _zsingle_quotesz_:_zsingle_quotesz_ because it is otherwise invalid on the vfat filesystem. The escape sequence that gets used, where u is the Unicode character, is: _zsingle_quotesz_:_zsingle_quotesz_, (u & 0x3f), ((u>>6) & 0x3f), (u>>12).


Allow two files with names that only differ in case. This option is obsolete.


First try to make a short name without sequence number, before trying name~num.ext.


UTF8 is the filesystem safe 8-bit encoding of Unicode that is used by the console. It can be enabled for the filesystem with this option or disabled with utf8=0, utf8=no or utf8=false. If `uni_xlate_zsingle_quotesz_ gets set, UTF8 gets disabled.


Defines the behavior for creation and display of filenames which fit into 8.3 characters. If a long name for a file exists, it will always be the preferred one for display. There are four modes:


Force the short name to lower case upon display; store a long name when the short name is not all upper case.


Force the short name to upper case upon display; store a long name when the short name is not all upper case.


Display the short name as is; store a long name when the short name is not all lower case or all upper case.


Display the short name as is; store a long name when the short name is not all upper case. This mode is the default since Linux 2.6.32.

Mount options for usbfs

devuid=uid and devgid=gid and devmode=mode

Set the owner and group and mode of the device files in the usbfs filesystem (default: uid=gid=0, mode=0644). The mode is given in octal.

busuid=uid and busgid=gid and busmode=mode

Set the owner and group and mode of the bus directories in the usbfs filesystem (default: uid=gid=0, mode=0555). The mode is given in octal.

listuid=uid and listgid=gid and listmode=mode

Set the owner and group and mode of the file devices (default: uid=gid=0, mode=0444). The mode is given in octal.


One further possible type is a mount via the loop device. For example, the command

mount /tmp/disk.img /mnt −t vfat −o loop=/dev/loop3

will set up the loop device /dev/loop3 to correspond to the file /tmp/disk.img, and then mount this device on /mnt.

If no explicit loop device is mentioned (but just an option `−o loop_zsingle_quotesz_ is given), then mount will try to find some unused loop device and use that, for example

mount /tmp/disk.img /mnt −o loop

The mount command automatically creates a loop device from a regular file if a filesystem type is not specified or the filesystem is known for libblkid, for example:

mount /tmp/disk.img /mnt

mount −t ext4 /tmp/disk.img /mnt

This type of mount knows about three options, namely loop, offset and sizelimit, that are really options to losetup(8). (These options can be used in addition to those specific to the filesystem type.)

Since Linux 2.6.25 auto-destruction of loop devices is supported, meaning that any loop device allocated by mount will be freed by umount independently of /etc/mtab.

You can also free a loop device by hand, using losetup −dor umount −d.

Since util-linux v2.29 mount command re-uses the loop device rather than initialize a new device if the same backing file is already used for some loop device with the same offset and sizelimit. This is necessary to avoid a filesystem corruption.


mount has the following return codes (the bits can be ORed):




incorrect invocation or permissions


system error (out of memory, cannot fork, no more loop devices)


internal mount bug


user interrupt


problems writing or locking /etc/mtab


mount failure


some mount succeeded

The command mount −a returns 0 (all succeeded), 32 (all failed), or 64 (some failed, some succeeded).


The syntax of external mount helpers is:

/sbin/mount.suffix spec dir [−sfnv] [−N namespace] [−o options] [−t type.subtype]

where the suffix is the filesystem type and the −sfnvoN options have the same meaning as the normal mount options. The −t option is used for filesystems with subtypes support (for example /sbin/mount.fuse −t fuse.sshfs).

The command mount does not pass the mount options unbindable, runbindable, private, rprivate, slave, rslave, shared, rshared, auto, noauto, comment, x-*, loop, offset and sizelimit to the mount.<suffix> helpers. All other options are used in a comma-separated list as argument to the −o option.


See also The files /etc/fstab, /etc/mtab and /proc/mounts section above.


filesystem table


libmount private runtime directory


table of mounted filesystems or symlink to /proc/mounts


lock file (unused on systems with mtab symlink)


temporary file (unused on systems with mtab symlink)


a list of filesystem types to try



overrides the default location of the fstab file (ignored for suid)


overrides the default location of the mtab file (ignored for suid)


enables libmount debug output


enables libblkid debug output


enables loop device setup debug output


mount(2), umount(2), umount(8), fstab(5), nfs(5), xfs(5), e2label(8), findmnt(8), losetup(8), mke2fs(8), mountd(8), nfsd(8), swapon(8), tune2fs(8), xfs_admin(8)


It is possible for a corrupted filesystem to cause a crash.

Some Linux filesystems don_zsingle_quotesz_t support −o syncnor −o dirsync (the ext2, ext3, ext4, fat and vfat filesystems do support synchronous updates (a la BSD) when mounted with the sync option).

The −o remount may not be able to change mount parameters (all ext2fs-specific parameters, except sb, are changeable with a remount, for example, but you can_zsingle_quotesz_t change gid or umask for the fatfs).

It is possible that the files /etc/mtab and /proc/mounts don_zsingle_quotesz_t match on systems with a regular mtab file. The first file is based only on the mount command options, but the content of the second file also depends on the kernel and others settings (e.g. on a remote NFS server -- in certain cases the mount command may report unreliable information about an NFS mount point and the /proc/mounts file usually contains more reliable information.) This is another reason to replace the mtab file with a symlink to the /proc/mounts file.

Checking files on NFS filesystems referenced by file descriptors (i.e. the fcntl and ioctl families of functions) may lead to inconsistent results due to the lack of a consistency check in the kernel even if noac is used.

The loop option with the offset or sizelimit options used may fail when using older kernels if the mount command can_zsingle_quotesz_t confirm that the size of the block device has been configured as requested. This situation can be worked around by using the losetup command manually before calling mount with the configured loop device.


A mount command existed in Version 5 AT&T UNIX.


Karel Zak <[email protected]>


The mount command is part of the util-linux package and is available from

  Copyright (c) 1996-2004 Andries Brouwer
Copyright (C) 2006-2012 Karel Zak <>

This page is somewhat derived from a page that was
(c) 1980, 1989, 1991 The Regents of the University of California
and had been heavily modified by Rik Faith and myself.
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