The list of file systems to be mounted is specified in the
file-systems field of the operating system’s declaration
(see Using the Configuration System). Each file system is declared
file-system form, like this:
(file-system (mount-point "/home") (device "/dev/sda3") (type "ext4"))
As usual, some of the fields are mandatory—those shown in the example above—while others can be omitted. These are described below.
Objects of this type represent file systems to be mounted. They contain the following members:
This is a string specifying the type of the file system—e.g.,
This designates the place where the file system is to be mounted.
This names the “source” of the file system. By default it is the name
of a node under /dev, but its meaning depends on the
field described below.
This is a symbol that specifies how the
device field is to be
When it is the symbol
device, then the
device field is
interpreted as a file name; when it is
is interpreted as a partition label name; when it is
device is interpreted as a partition unique identifier (UUID).
UUIDs may be converted from their string representation (as shown by the
tune2fs -l command) using the
uuid form, like this:
(file-system (mount-point "/home") (type "ext4") (title 'uuid) (device (uuid "4dab5feb-d176-45de-b287-9b0a6e4c01cb")))
uuid options offer a way to refer to disk
partitions without having to hard-code their actual device
However, when a file system’s source is a mapped device (see Mapped Devices), its
device field must refer to the mapped
device name—e.g., /dev/mapper/root-partition—and consequently
title must be set to
'device. This is required so that
the system knows that mounting the file system depends on having the
corresponding device mapping established.
This is a list of symbols denoting mount flags. Recognized flags
access to special files),
no-suid (ignore setuid and setgid
no-exec (disallow program execution.)
This is either
#f, or a string denoting mount options.
This Boolean value indicates whether the file system is needed when booting. If that is true, then the file system is mounted when the initial RAM disk (initrd) is loaded. This is always the case, for instance, for the root file system.
This Boolean indicates whether the file system needs to be checked for errors before being mounted.
When true, the mount point is created if it does not exist yet.
This is a list of
<file-system> objects representing file systems
that must be mounted before (and unmounted after) this one.
As an example, consider a hierarchy of mounts: /sys/fs/cgroup is a dependency of /sys/fs/cgroup/cpu and /sys/fs/cgroup/memory.
(gnu system file-systems) exports the following useful
These are essential file systems that are required on normal systems, such as %pseudo-terminal-file-system and %immutable-store (see below.) Operating system declarations should always contain at least these.
This is the file system to be mounted as /dev/pts. It supports
pseudo-terminals created via
openpty and similar
functions (see Pseudo-Terminals in The GNU C Library Reference
Manual). Pseudo-terminals are used by terminal emulators such as
This file system is mounted as /dev/shm and is used to support
memory sharing across processes (see
shm_open in The GNU C Library Reference Manual).
This file system performs a read-only “bind mount” of
/gnu/store, making it read-only for all the users including
root. This prevents against accidental modification by software
root or by system administrators.
The daemon itself is still able to write to the store: it remounts it read-write in its own “name space.”
binfmt_misc file system, which allows handling of arbitrary
executable file types to be delegated to user space. This requires the
binfmt.ko kernel module to be loaded.
fusectl file system, which allows unprivileged users to mount
and unmount user-space FUSE file systems. This requires the
fuse.ko kernel module to be loaded.
Note that, while it is tempting to use /dev/disk/by-uuid and similar device names to achieve the same result, this is not recommended: These special device nodes are created by the udev daemon and may be unavailable at the time the device is mounted.