File Permissions

Linux file permissions are a core part of the operating system's security model. They determine who can read, write, or execute files and directories.

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Understanding the Permission Format

Run ls -l in a terminal to see file permissions:

ls -l

Example:

-rwxr-xr-- 1 user group 1234 Jul 20 10:00 example.sh

Breakdown:

[File Type][User][Group][Others]
-rwx        r-x    r--

File Type

• - : Regular file
• d : Directory
• l : Symbolic link

Permission Characters

• r : Read
• w : Write
• x : Execute
• - : No permission

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Permission Categories

1. User (Owner) – Permissions for the file's owner
2. Group – Permissions for the group associated with the file
3. Others – Permissions for everyone else

Each permission category contains 3 characters: r, w, and x.

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Numeric (Octal) Representation

Each permission set maps to a number:

Permission	Binary	Octal
r	100	4
w	010	2
x	001	1

Examples:

Symbolic	Octal	Meaning
rwxr-xr-x	755	Owner: all, Group/Others: read + exec
rw-r--r--	644	Owner: read/write, others: read only
rwx------	700	Only owner can read/write/execute

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Changing Permissions

chmod – Change file mode (permissions)

Symbolic Mode:

chmod u+x file     # Add execute to user
chmod g-w file     # Remove write from group
chmod o=r file     # Set read-only for others

Numeric Mode:

chmod 755 file     # Set rwxr-xr-x

chown – Change file owner

chown user:group file

chgrp – Change group ownership

chgrp groupname file

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Directory Permissions

• r : List contents (e.g., ls)
• w : Create/delete files in the directory
• x : Enter the directory (e.g., cd)

To access a directory, execute (x) permission is required.

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Special Permissions

1. SetUID (s) – Runs the file as the file owner (user)
2. SetGID (s) – Runs the file as the group owner or preserves group in directories
3. Sticky Bit (t) – In directories, only file owners can delete their own files

chmod u+s filename   # SetUID
chmod g+s filename   # SetGID
chmod +t directory   # Sticky bit

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Quick Practice

touch testfile
chmod 755 testfile
ls -l testfile

Expected output:

-rwxr-xr-x 1 user group 0 Jul 20 10:00 testfile

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Summary Table

Symbol	Meaning	Who
r	Read	View file
w	Write	Edit file
x	Execute	Run file
u	User (owner)	File owner
g	Group	Group users
o	Others	Everyone
a	All	u + g + o

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Calculate Permissions Easily

Use this online tool to calculate symbolic and numeric permissions:

👉 https://chmod-calculator.com/

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