Automount

Automount is a mechanism in many Unix-like operating systems that automatically mounts file systems on demand when they are accessed. Its primary purpose is to simplify system administration and improve resource usage by creating mounts only as needed, especially for network shares and removable media. By delaying mounting until access, automount can reduce startup work and avoid permanently occupying mount points.

Automount works through a dedicated daemon and a set of maps that describe where and how to mount resources. A master map directs the daemon to indirect or direct maps. Indirect maps specify mount points under a common directory, mounting on first access and often unmounting after a period of inactivity. Direct maps place mounts at specific, predefined paths. When a user or process accesses a path that is managed by automount, the daemon mounts the target file system with the configured options; after use, idle mounts can be automatically unmounted to reclaim resources. Caching and timeout policies help balance responsiveness with system cleanliness.

Implementation and configuration vary across systems. Linux commonly uses the autofs subsystem and the automount daemon, with maps that can be plain files or sourced from LDAP, NIS, or other directory services. macOS and traditional BSDs include their own automount daemons, while Solaris uses a variant of automount. In many environments, NFS shares are a frequent target of automount, but it can also handle other network file systems and local devices. Mount options and security considerations, such as access controls and authentication, are specified in the maps and through the automount configuration.

Historically, automount originated in early SunOS and evolved into various implementations across Unix-like systems. It remains a tool for reducing manual mounting work, improving boot times, and keeping mount points clean, at the cost of potential first-access latency and added system complexity.