filesystem(7)

filesystem -- file system organization

Synopsis

   / 
   /usr

Description

The System V file system tree is organized for administrative convenience. Distinct areas within the file system tree are provided for files that are private to one machine, files that can be shared by multiple machines of a common architecture, files that can be shared by all machines, and home directories. This organization allows sharable files to be stored on one machine but accessed by many machines using a remote file access mechanism such as NFS. Grouping together similar files makes the file system tree easier to upgrade and manage.

The file system tree consists of a root file system and a collection of mountable file systems. The mount(1M) program attaches mountable file systems to the file system tree at mount points (directory entries) in the root file system or other previously mounted file systems. If /usr is configured as a separate file system, it must be mounted in order to have a completely functional system. The root file system is mounted automatically by the kernel at boot time.

The root file system contains files that are unique to each machine. It contains the following directories:

/dev: Character and block special files. These device files provide hooks into hardware devices or operating system facilities. Typically, device files are built to match the kernel and hardware configuration of the machine.
/dev/term: Terminal devices.
/dev/pts: Pseudo-terminal devices.
/dev/sxt: Shell layers device files used by sh(1).
/etc: Machine-specific administrative configuration files and system administration databases. /etc may be viewed as the home directory of a machine, the directory that in a sense defines the machine's identity. Executable programs are no longer kept in /etc.
/home: Root of a subtree for user directories.
/mnt: Temporary mount point for file systems. This is an empty directory on which file systems may be temporarily mounted.
/opt: Root of a subtree for add-on application packages.
/proc: Root of a subtree for the process file system.
/sbin: Essential executables used in the booting process and in manual system recovery. The full complement of utilities is available only after /usr is mounted,
/tmp: Temporary files; initialized to empty during the boot operation.
/var: Root of a subtree for varying files. Varying files are files that are unique to a machine but that can grow to an arbitrary (that is, variable) size. An example is a log file.
/var/adm: System logging and accounting files.
/var/cron: cron's log file.
/var/mail: Where users' mail is kept.
/var/opt: Top-level directory used by application packages.
/var/preserve: Backup files for vi(1) and ex(1).
/var/spool: Subdirectories for files used in printer spooling, mail delivery, cron(1M), at(1), etc.
/var/tmp: Transitory files; initialized to empty during the boot operation.

Because it is desirable to keep the root file system small and not volatile, on disk-based systems larger file systems are often mounted on /home, /opt, /usr, and /var.

The file system mounted on /usr contains architecture-dependent and architecture-independent sharable files. The subtree rooted at /usr/share contains architecture-independent sharable files; the rest of the /usr tree contains architecture-dependent files. By mounting a common remote file system, a group of machines with a common architecture may share a single /usr file system. A single /usr/share file system can be shared by machines of any architecture. A machine acting as a file server may export many different /usr file systems to support several different architectures and operating system releases. Clients usually mount /usr read-only so that they don't accidentally change any shared files. The /usr file system contains the following subdirectories:

/usr/bin: Most system utilities.
/usr/sbin: Executables for system administration.
/usr/games: Game binaries and data.
/usr/include: Include header files (for C programs, etc).
/usr/lib: Program libraries, various architecture-dependent databases, and executables not invoked directly by the user (system daemons, etc).
/usr/share: Subtree for architecture-independent sharable files.
/usr/share/man: Subdirectories for on-line reference manual pages (if present).
/usr/share/lib: Architecture-independent databases.
/usr/src: Source code for utilities and libraries.
/usr/ucb: Berkeley compatibility package binaries.
/usr/ucbinclude: Berkeley compatibility package header files.
/usr/ucblib: Berkeley compatibility package libraries.

A machine with disks may export root file systems, swap files, and /usr file systems to diskless or partially-disked machines that mount them into the standard file system hierarchy. The standard directory tree for sharing these file systems from a server is:

/export: The default root of the exported file system tree.
/export/exec/architecture-name: The exported /usr file system supporting architecture-name for the current release.
/export/exec/architecture-name.release-name: The exported /usr file system supporting architecture-name for System V release-name.
/export/exec/share: The exported common /usr/share directory tree.
/export/exec/share.release-name: The exported common /usr/share directory tree for System V release-name.
/export/root/hostname: The exported root file system for hostname.
/export/swap/hostname: The exported swap file for hostname.
/export/var/hostname: The exported /var directory tree for hostname.

References

at(1), fsck(1M), init(1M), mknod(1M), mount(1M), sh(1), vi(1)