Unix File Systems

With a working Linux environment in place, this week is about the file system: how a Unix-like system organizes everything on disk, how you move around it from the shell, and how ownership and permissions control who can do what.

Everything is a file

Unix takes a famous stance: almost everything is a file — not just documents, but directories, devices, and communication channels. ls -l shows the type in the first character of each line:

The directory tree

Everything hangs off a single root, /. A Unix system follows a fairly standard layout, so you can find your way around any distribution:

/
├── bin/    core user programs (ls, cp, sh, …)
├── sbin/   system/administrative programs
├── etc/    system-wide configuration files
├── home/   user home directories
├── lib/    essential shared libraries
├── usr/    the bulk of user programs & libraries
├── var/    variable data — logs, spools, databases
├── tmp/    temporary files (may be auto-cleaned)
├── mnt/    mount points for other filesystems
├── dev/    device files
└── proc/   virtual files: live process & kernel info

Navigating

Paths are either absolute (from the root, e.g. /home/your-username/notes) or relative to where you are now. Three shortcuts do most of the work:

• ~ — your home directory
• . — the current directory
• .. — the parent directory

Hard links and symbolic links

A filename is just a pointer to an inode — the on-disk record that actually describes a file. That indirection gives two kinds of links:

• A hard link is another name for the same inode. The data survives as long as any hard link to it exists. Create one with ln target name.
• A symbolic (soft) link is a small file that just stores a path to another name. If the target is removed, the link dangles. Create one with ln -s target name.

ls -i shows inode numbers, so you can see two hard links share one inode while a symlink has its own.

Users, groups, and permissions

Every file has an owner (a user) and a group. Permissions are granted separately to three classes — user (owner), group, and others — each with read (r), write (w), and execute (x) bits:

 -   rwx   r-x   r--
 │    │     │     └── others: read
 │    │     └──────── group: read + execute
 │    └────────────── owner: read + write + execute
 └─────────────────── type (- = regular file)

The same bits are often written in octal, where r=4, w=2, x=1:

Manage them from the shell:

• chmod 644 file or chmod u+x file — change permissions
• chown user:group file — change ownership
• ls -l — inspect type, permissions, owner, and group

Practice / lab

• Walk the tree: inspect the top-level folders under /, then peek into /proc and /dev.
• Discover commands with man, which, and type; navigate with cd, pwd, ls -l.
• Create hard and symbolic links with ln / ln -s and compare inodes with ls -i.
• Change modes and ownership with chmod / chown; experiment with umask.
• Look at the account model in /etc/passwd and /etc/group; try id, who, groups.

References & resources

Required

• Filesystem Hierarchy Standard (FHS)
• Linux file types (TLDP)
• Linux file permissions explained (Red Hat)
• ECE 252 — The File System (video)

Optional / enrichment

• Linux Journey: The Filesystem
• An introduction to Linux permissions (DigitalOcean)
• Linux file system explained (video)
• Права доступа и владения файлами (RUS, video)

Self-check

1. What is the difference between a hard link and a symbolic link?
2. Which directory holds configuration files for most system services?
3. What do the three groups in rwxr-xr-- represent, and what is it in octal?
4. How do you find the manual page for a command?
5. Which virtual directory exposes information about running processes?