Permissions#

Linux permissions control access to files and directories via a user–group–others model. Each filesystem object is associated with:

• Owner (user)
• Group
• Permissions defining allowed operations

Users can belong to multiple groups, inheriting access rights accordingly.

Permission Types#

Permission Scope#

Permissions are defined for three classes:

Directory vs File Semantics#

Permissions behave differently depending on object type.

Files#

• r: Read file contents
• w: Modify file contents
• x: Execute file (if binary/script)

Directories#

• r: List directory contents (ls)
• w: Create, delete, rename entries
• x: Traverse directory (cd, access entries)

Important constraints:

• Directory traversal requires x, regardless of r
• x on a directory does not grant execution of contained files
• Modifying directory contents requires w on the directory, not the files

Permission Representation#

Permissions are displayed via ls -l:

ls -l /etc/passwd

Example output:

-rwxrw-r-- 1 root root 1641 May 4 23:42 /etc/passwd

Breakdown#

-rwxrw-r--
││  │  │
││  │  └── Others:  r--
││  └───── Group:   rw-
│└──────── Owner:   rwx
└───────── Type:    -

Fields#

Octal (Numeric) Representation#

Permissions map to octal values:

Each scope is summed:

Example#

rwxrw-r-- → 764

• Owner: rwx = 7
• Group: rw- = 6
• Others: r-- = 4

Mental Model#

• Ownership defines baseline control
• Groups extend access across users
• Permissions gate operations
• Directories control access to structure; files control access to content