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10.3 File Systems

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 using the file-system form, like this:

  (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.

Data Type: file-system

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., "ext4".


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 name23.

File system labels are created using the file-system-label 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 e2label command:

  (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 uuid form24, like this:

  (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.

flags (default: '())

This is a list of symbols denoting mount flags. Recognized flags include read-only, bind-mount, no-dev (disallow access to special files), no-suid (ignore setuid and setgid bits), no-atime (do not update file access times), strict-atime (update file access time), lazy-time (only update time on the in-memory version of the file inode), and no-exec (disallow program execution). See Mount-Unmount-Remount in The GNU C Library Reference Manual, for more information on these flags.

options (default: #f)

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 and run man 8 mount for options for various file systems. Note that the file-system-options->alist and alist->file-system-options procedures from (gnu system file-systems) can be used to convert file system options given as an association list to the string representation, and vice-versa.

mount? (default: #t)

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.

needed-for-boot? (default: #f)

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.

check? (default: #t)

This Boolean indicates whether the file system needs to be checked for errors before being mounted.

create-mount-point? (default: #f)

When true, the mount point is created if it does not exist yet.

mount-may-fail? (default: #f)

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.

dependencies (default: '())

This is a list of <file-system> or <mapped-device> objects 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).

Scheme Procedure: file-system-label str

This procedure returns an opaque file system label from str, a string:

(file-system-label "home")
 #<file-system-label "home">

File system labels are used to refer to file systems by label rather than by device name. See above for examples.

The (gnu system file-systems) exports the following useful variables.

Scheme Variable: %base-file-systems

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.

Scheme Variable: %pseudo-terminal-file-system

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 xterm.

Scheme Variable: %shared-memory-file-system

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).

Scheme Variable: %immutable-store

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 running as 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.”

Scheme Variable: %binary-format-file-system

The 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.

Scheme Variable: %fuse-control-file-system

The 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.

The (gnu system uuid) module provides tools to deal with file system “unique identifiers” (UUIDs).

Scheme Procedure: uuid str [type]

Return an opaque UUID (unique identifier) object of the given type (a symbol) by parsing str (a string):

(uuid "4dab5feb-d176-45de-b287-9b0a6e4c01cb")
 #<<uuid> type: dce bv: …>

(uuid "1234-ABCD" 'fat)
 #<<uuid> type: fat bv: …>

type may be one of dce, iso9660, fat, 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.


The 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.

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