Linux File Permissions Explained: A Beginner's Complete Guide

Understanding Linux File Permissions

One of the first concepts every Linux user needs to master is file permissions. Linux is a multi-user operating system, and its permission system controls exactly who can read, write, or execute any file or directory. Once you understand this system, you'll feel far more confident navigating and managing your Linux environment.

The Three Permission Types

Every file and directory in Linux has three types of permissions:

• Read (r) — Allows viewing the contents of a file, or listing a directory's contents.
• Write (w) — Allows modifying a file, or creating/deleting files within a directory.
• Execute (x) — Allows running a file as a program, or entering a directory with cd.

The Three Permission Groups

These permissions are assigned to three distinct groups:

• Owner (u) — The user who owns the file.
• Group (g) — Users who belong to the file's assigned group.
• Others (o) — Everyone else on the system.

Reading Permission Output

When you run ls -l in a terminal, you'll see output like this:

-rwxr-xr-- 1 alice devs 4096 Jan 10 09:00 script.sh

Breaking down the first field -rwxr-xr--:

1. The first character (-) indicates file type: - for file, d for directory, l for symlink.
2. Next three characters (rwx) — Owner permissions: read, write, execute.
3. Next three characters (r-x) — Group permissions: read, no write, execute.
4. Last three characters (r--) — Others permissions: read only.

Changing Permissions with chmod

The chmod command changes file permissions. You can use symbolic or numeric (octal) mode.

Symbolic Mode

Symbolic mode uses letters to add (+), remove (-), or set (=) permissions:

• chmod u+x file.sh — Add execute permission for the owner.
• chmod g-w file.txt — Remove write permission from the group.
• chmod o=r file.txt — Set others to read-only.
• chmod a+x script.sh — Add execute for all (owner, group, others).

Numeric (Octal) Mode

Each permission type has a numeric value: read = 4, write = 2, execute = 1. Add them together for each group:

Permission	Value
rwx	7
rw-	6
r-x	5
r--	4
---	0

Example: chmod 755 script.sh gives the owner full permissions and everyone else read+execute.

Changing Ownership with chown

The chown command changes the owner and/or group of a file:

• chown alice file.txt — Change owner to alice.
• chown alice:devs file.txt — Change owner to alice and group to devs.
• chown -R alice:devs /project/ — Recursively change ownership of a directory.

Special Permissions: SUID, SGID, and Sticky Bit

Beyond the basic permissions, Linux has three special bits worth knowing:

• SUID (Set User ID) — When set on an executable, it runs as the file's owner. Example: chmod u+s program.
• SGID (Set Group ID) — Files created in an SGID directory inherit the directory's group. Example: chmod g+s /shared/.
• Sticky Bit — On directories, only the file's owner can delete it. Commonly used on /tmp. Example: chmod +t /tmp/.

Key Takeaways

Understanding file permissions is essential for Linux security and system management. Start by practicing ls -l to read permissions, then experiment with chmod in a safe environment. Always be cautious when using chmod 777 as it grants full access to everyone — a common security mistake beginners make.