The list of file systems to be mounted is specified in the
file-systems field of the operating system 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. It can be one of three things: a file system label, a file system UUID, or the name of a /dev node. Labels and UUIDs offer a way to refer to file systems without having to hard-code their actual device name22.
File system labels are created using the
procedure, UUIDs are created using
uuid, and /dev node are
plain strings. Here’s an example of a file system referred to by its
label, as shown by the
(file-system (mount-point "/home") (type "ext4") (device (file-system-label "my-home")))
UUIDs are converted from their string representation (as shown by the
tune2fs -l command) using the
(file-system (mount-point "/home") (type "ext4") (device (uuid "4dab5feb-d176-45de-b287-9b0a6e4c01cb")))
When the source of a file system is a mapped device (see Mapped Devices), its
device field must refer to the mapped
device name—e.g., "/dev/mapper/root-partition".
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 value indicates whether to automatically mount the file system when
the system is brought up. When set to
#f, the file system gets
an entry in /etc/fstab (read by the
mount command) but
is not automatically mounted.
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
representing file systems that must be mounted or mapped devices that
must be opened before (and unmounted or closed 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.
Another example is a file system that depends on a mapped device, for example for an encrypted partition (see Mapped Devices).
(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.
uuid form expects 16-byte UUIDs as defined in
RFC 4122. This is the
form of UUID used by the ext2 family of file systems and others, but it
is different from “UUIDs” found in FAT file systems, for instance.