Permissions

Linux is a multi-user operating system with a robust file permission system to control access to files and directories. Every file and directory has an owner, a group, and access rights defined for the owner, the group, and all other users (others). These permissions determine who can read, write, or execute a file or directory.

Types of Permissions

There are three basic permission types:

• Read (r): Ability to view the contents of a file or list a directory.
• Write (w): Ability to modify a file or add/delete files in a directory.
• Execute (x): Ability to run a file as a program or enter a directory.

Permission Values Table

These values are combined using addition. For example, read + write = 4 + 2 = 6.

Permission Representation

Linux permissions are shown with ls -l. Example:

-rwxr-xr--

This means:

• File type: - (regular file)
• Owner: rwx (read, write, execute)
• Group: r-x (read, execute)
• Others: r-- (read only)

Ownership and Groups

Every file or directory has:

• Owner (user who created it)
• Group (user group associated with the file)
• Others (all other users on the system)

Permissions are defined separately for each of these.

Permission Commands and Syntax

chmod – Change file or directory permissions

Symbolic mode example:

chmod u+x script.sh

Octal mode example:

chmod 755 script.sh

chown – Change ownership (user or user:group)

chown alice file.txt
chown alice:devs file.txt

chgrp – Change group ownership

chgrp devs file.txt

Command Summary Table