automounters

An automounter, or automount daemon, is a service in UNIX-like systems that mounts filesystems automatically on demand when they are accessed. It keeps filesystems unmounted until needed, reducing startup time, network traffic, and the number of active mounts. Common targets include network shares such as NFS and various local or remote resources that are not always required.

The core design uses mount maps and a daemon that watches access to predefined directories. A master map (typically /etc/auto.master) defines top-level mount points and references to indirect or direct maps. Indirect maps describe a directory under a mount point whose subdirectories map to specific filesystems; direct maps provide non-hierarchical mount points. Maps can reside locally or be retrieved from a server.

Operation proceeds when a user or process accesses a trigger directory. The automount daemon consults the maps, performs the necessary mounts, and provides access to the resource. The mounted filesystem remains available while it is in use, and after a configurable period of inactivity it is automatically unmounted. This on-demand behavior can be combined with options that control mount behavior, caching, and protocol details (for example, NFS or CIFS).

Use cases include network home directories, shared project directories, and remote data repositories, as well as managing access to removable media. Benefits include reduced system boot time, lower resource consumption, transparent access for users, and automatic cleanup of unused mounts. Potential drawbacks involve configuration complexity and the need to ensure network reliability and proper authorization in maps.

Implementations vary by platform but commonly include autofs in Linux and BSDs, with systemd-automount offerings on systemd-based systems. Historical and alternative automounters have existed on other UNIX systems, each with its own map formats and integration points.