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:
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 name28.
File system labels are created using the
procedure, UUIDs are created using
uuid, and /dev nodes are
plain strings. Here’s an example of a file system referred to by its
label, as shown by the
UUIDs are converted from their string representation (as shown by the
tune2fs -l command) using the
(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-atime (do not update file access times),
no-diratime (likewise for directories only),
strict-atime (update file access time),
update time on the in-memory version of the file inode),
no-exec (disallow program execution), and
shared (make the
See Mount-Unmount-Remount in The GNU C Library Reference
Manual, for more information on these flags.
This is either
#f, or a string denoting mount options passed to
the file system driver. See Mount-Unmount-Remount in The GNU C
Library Reference Manual, for details.
man 8 mount for options for various file systems, but
beware that what it lists as file-system-independent “mount options” are
in fact flags, and belong in the
flags field described above.
(gnu system file-systems) can be used to convert
file system options given as an association list to the string
representation, and vice-versa.
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 should be checked for errors before being mounted. How and when this happens can be further adjusted with the following options.
When true, this Boolean indicates that a file system check triggered
check? may exit early if the file system is marked as
“clean”, meaning that it was previously correctly unmounted and
should not contain errors.
Setting this to false will always force a full consistency check when
check? is true. This may take a very long time and is not
recommended on healthy systems—in fact, it may reduce reliability!
Conversely, some primitive file systems like
fat do not keep
track of clean shutdowns and will perform a full scan regardless of the
value of this option.
check? finds errors, it can (try to) repair them and
continue booting. This option controls when and how to do so.
If false, try not to modify the file system at all. Checking certain
file systems like
jfs may still write to the device to replay
the journal. No repairs will be attempted.
#t, try to repair any errors found and assume “yes” to
all questions. This will fix the most errors, but may be risky.
'preen, repair only errors that are safe to fix without
human interaction. What that means is left up to the developers of
each file system and may be equivalent to “none” or “all”.
When true, the mount point is created if it does not exist yet.
When true, this indicates that mounting this file system can fail but
that should not be considered an error. This is useful in unusual
cases; an example of this is
efivarfs, a file system that can
only be mounted on EFI/UEFI systems.
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).
This procedure returns an opaque file system label from str, a string:
File system labels are used to refer to file systems by label rather than by device name. See above for examples.
(gnu system file-systems) exports the following useful
These are essential file systems that are required on normal systems,
below). Operating system declarations should always contain at least
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.
(gnu system uuid) module provides tools to deal with file
system “unique identifiers” (UUIDs).
Return an opaque UUID (unique identifier) object of the given type (a symbol) by parsing str (a string):
type may be one of
ntfs, or one of the commonly found synonyms for these.
UUIDs are another way to unambiguously refer to file systems in operating system configuration. See the examples above.
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.