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Manual Reference Pages  -  MOUNT_FUSEFS (8)


mount_fusefs - mount a Fuse file system daemon


Daemon Mounts
Strict Access Policy
Shared Mounts
See Also


mount_fusefs [-A] [-S] [-v] [-D fuse_daemon] [-O daemon_opts] [-s special] [-m node] [-h] [-V] [-o option ...] special node [fuse_daemon ...]


Basic usage is to start a fuse daemon on the given special file. In practice, the daemon is assigned a special file automatically, which can then be indentified via fstat(1). That special file can then be mounted by mount_fusefs.

However, the procedure of spawning a daemon will usually be automated so that it is performed by mount_fusefs. If the command invoking a given fuse_daemon is appended to the list of arguments, mount_fusefs will call the fuse_daemon via that command. In that way the fuse_daemon will be instructed to attach itself to special. From that on mounting goes as in the simple case. (See DAEMON MOUNTS.)

The special argument will normally be treated as the path of the special file to mount.

However, if auto is passed as special, then mount_fusefs will look for a suitable free fuse device by itself.

Finally, if special is an integer it will be interpreted as the number of the file descriptor of an already open fuse device (used when the Fuse library invokes mount_fusefs. (See DAEMON MOUNTS).

The options are as follows:
-A -, --reject-allow_other
  Prohibit the allow_other mount flag. Intended for use in scripts and the sudoers(5) file.
-S -, --safe
  Run in safe mode (i.e. reject invoking a filesystem daemon)
-v Be verbose
-D, --daemon daemon
  Call the specified daemon
-O, --daemon_opts opts
  Add opts to the daemon’s command line
-s, --special special
  Use special as special
-m, --mountpath node
  Mount on node
-h, --help
  Show help
-V, --version
  Show version information
-o Mount options are specified via -o . The following options are available (and also their negated versions, by prefixing them with "no"):
  Enable traditional (file mode based) permission checking in kernel
  Do not apply STRICT ACCESS POLICY. Only root can use this option
max_read = n
  Limit size of read requests to n
  Refuse shared mounting of the daemon. This is the default behaviour, to allow sharing, expicitly use -o noprivate
  Do not refuse unmounting if there are secondary mounts
  Prefix absolute symlinks with the mountpoint

Besides the above mount options, there is a set of pseudo-mount options which are supported by the Fuse library. One can list these by passing -h to a Fuse daemon. Most of these options have effect only on the behaviour of the daemon (that is, their scope is limited to userspace). However, there are some which do require in-kernel support. Currently the options supported by the kernel are:
  Bypass the buffer cache system
  By default cached buffers of a given file are flushed at each open(2). This option disables this behaviour


Usually users do not need to use mount_fusefs directly, as the Fuse library enables Fuse daemons to invoke mount_fusefs. That is,

    fuse_daemon device mountpoint

has the same effect as

    mount_fusefs auto mountpoint fuse_daemon

This is the recommended usage when you want basic usage (eg, run the daemon at a low privilege level but mount it as root).


The strict access policy for Fuse filesystems lets one to use the filesystem only if the filesystem daemon has the same credentials (uid, real uid, gid, real gid) as the user.

This is applied for Fuse mounts by default and only root can mount without the strict access policy (ie. the allow_other mount option).

This is to shield users from the daemon "spying" on their I/O activities.

Users might opt to willingly relax strict access policy (as far they are concerned) by doing their own secondary mount (See SHARED MOUNTS).


A Fuse daemon can be shared (ie. mounted multiple times). When doing the first (primary) mount, the spawner and the mounter of the daemon must have the same uid, or the mounter should be the superuser.

After the primary mount is in place, secondary mounts can be done by anyone unless this feature is disabled by private. The behaviour of a secondary mount is analogous to that of symbolic links: they redirect all filesystem operations to the primary mount.

Doing a secondary mount is like signing an agreement: by this action, the mounter agrees that the Fuse daemon can trace her I/O activities. From then on she is not banned from using the filesystem (either via her own mount or via the primary mount), regardless whether allow_other is used or not.

The device name of a secondary mount is the device name of the corresponding primary mount, followed by a ’#’ character and the index of the secondary mount; e.g. /dev/fuse0#3.


System administrators might want to use a custom mount policy (ie., one going beyond the vfs.usermount sysctl). The primary tool for such purposes is sudo(8). However, given that mount_fusefs is capable of invoking an arbitrary program, one must be careful when doing this. mount_fusefs is designed in a way such that it makes that easy. For this purpose, there are options which disable certain risky features (ie. -S and -A ), and command line parsing is done in a flexible way: mixing options and non-options is allowed, but processing them stops at the third non-option argument (after the first two has been utilized as device and mountpoint). The rest of the command line specifies the daemon and its arguments. (Alternatively, the daemon, the special and the mount path can be specified using the respective options.) Note that mount_fusefs ignores the environment variable POSIXLY_CORRECT and always behaves as described.

In general, to be as scripting / sudoers(5) friendly as possible, no information has a fixed position in the command line, but once a given piece of information is provided, subsequent arguments/options cannot override it (with the exception of some non-critical ones).


  This has the same effect as the -S option.
  This has the same effect as the -v option.
  If set, mount_fusefs will ignore uknown mount options.
  Adjust behaviour to the needs of the FUSE library. Currently it effects help output.

Although the following variables do not have any effect on mount_fusefs itself, they affect the behaviour of fuse daemons:
  Device to attach. If not set, the multiplexer path /dev/fuse is used.
FUSE_DEV_FD File desciptor of an opened Fuse device to use. Overrides FUSE_DEV_NAME.
  If set, the library will not attempt to mount the filesystem, even if a mountpoint argument is supplied.


  Fuse device with which the kernel and Fuse daemons can communicate.
  The multiplexer path. An open(2) performed on it automatically is passed to a free Fuse device by the kernel (which might be created just for this puprose).


Mount the example filesystem in the Fuse distribution (from its directory): either

    ./fusexmp /mnt/fuse


    mount_fusefs auto /mnt/fuse ./fusexmp

Doing the same in two steps, using /dev/fuse0:

    FUSE_DEV_NAME=/dev/fuse ./fusexmp &&

    mount_fusefs /dev/fuse /mnt/fuse

A script wrapper for fusexmp which ensures that mount_fusefs does not call any external utility and also provides a hacky (non race-free) automatic device selection:

    #!/bin/sh -e

    FUSE_DEV_NAME=/dev/fuse fusexmp

    mount_fusefs -S /dev/fuse /mnt/fuse \(lq$@\(rq


fstat(1), mount(8), umount(8), sudo(8)


mount_fusefs appears as the part of the FreeBSD implementation of the Fuse userspace filesystem framework (see This user interface is FreeBSD specific.


Secondary mounts should be unmounted via their device name. If an attempt is made to be unmount them via their filesystem root path, the unmount request will be forwarded to the primary mount path. In general, unmounting by device name is less error-prone than by mount path (although the latter will also work under normal circumstances).

If the daemon is specified via the -D and -O options, it will be invoked via system(3), and the daemon’s command line will also have an "&" control operator appended, so that we do not have to wait for its termination. You should use a simple command line when invoking the daemon via these options.


special is treated as a multiplexer if and only if it is literally the same as auto or /dev/fuse. Other paths which are equivalent with /dev/fuse (eg., /../dev/fuse) are not.
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