Short note: UNIX file system

The UNIX file system is a hierarchical, tree-structured file system with a single root /. Everything is a file: ordinary files, directories and special (device) files, so the same system calls (open, read, write, close) are used for files and I/O devices.

Main features:

• Hierarchical directories: a directory is a file containing (name, i-node number) entries; path names can be absolute (/usr/bin/ls) or relative; . and .. refer to the current and parent directory.
• I-nodes: each file has an i-node with attributes and block addresses (10 direct, single, double and triple indirect pointers), so large files are supported and access is efficient.
• Disk layout: boot block, superblock, i-node list, data blocks.
• Mounting: other file systems are attached to a directory with mount, giving one unified tree.
• Links: hard links (several names for one i-node) and symbolic links.
• Protection: 9 permission bits (read, write, execute for owner, group, others), plus the set-UID, set-GID and sticky bits.
• File types: regular, directory, character special, block special, pipe/FIFO, socket.
• A buffer cache is used for performance, and free blocks are managed by a free list or bit map.

          /
   +------+------+
  bin    usr    dev
   |      |      |
  ls   +--+--+  tty0
      bin  lib