The chmod command in Linux changes the permissions of files or directories, allowing you to control who can read, write, or execute them. This command is essential for managing access rights in a multi-user environment.

Permission Structure

Permissions in Linux are typically represented in three sets for:

• User (Owner)
• Group
• Others

Each set has three permissions:

• Read (r): View file content.
• Write (w): Modify file content.
• Execute (x): Run the file as a program or enter a directory.

Permissions are displayed as a string in the format rwxr-xr--, where:

• The first character (- or d) represents the type (file or directory).
• The following sets of three characters represent permissions for the User, Group, and Others respectively.

Basic Syntax

chmod [options] mode file_or_directory

Examples with Output

Example 1: Setting Permissions Using Symbolic Mode

Symbolic mode allows specifying permissions using symbols like +, -, and =.

Command

chmod u+x file.txt
ls -l file.txt

Output

-rwxr--r-- 1 user user 1024 Oct 29 10:00 file.txt

In this example:

• chmod u+x file.txt adds execute permission (x) for the user (owner) only.
• The ls -l command confirms that file.txt is now executable by the user (rwx for user).

Example 2: Removing Write Permission for Group

The following example removes write permission from the group.

Command

chmod g-w file.txt
ls -l file.txt

Output

-rwxr--r-- 1 user user 1024 Oct 29 10:00 file.txt

Here, chmod g-w file.txt removes the write permission (w) from the group, so only the user has write access.

Example 3: Setting Permissions Using Numeric (Octal) Mode

Numeric mode uses numbers to represent each permission type:

• 4 for read (r)
• 2 for write (w)
• 1 for execute (x)

The three-digit number represents permissions for User, Group, and Others in that order.

Command

chmod 754 file.txt
ls -l file.txt

Output

-rwxr-xr-- 1 user user 1024 Oct 29 10:00 file.txt

Explanation:

• 7 = 4+2+1 (read, write, execute) for User
• 5 = 4+1 (read, execute) for Group
• 4 = 4 (read) for Others

Example 4: Granting Read and Write Permissions to Everyone

To give read and write permissions to all users, you can use either symbolic or numeric mode.

Command

chmod a+rw file.txt
ls -l file.txt

Output

-rw-rw-rw- 1 user user 1024 Oct 29 10:00 file.txt

Explanation:

• chmod a+rw file.txt grants read (r) and write (w) permissions to all (user, group, and others).
• The ls -l output shows that everyone now has read and write access.

Example 5: Setting Special Permissions (SUID, SGID, and Sticky Bit)

1. SUID (Set User ID): Allows a file to be executed with the permissions of the file owner.
2. SGID (Set Group ID): Sets a directory so that new files inside inherit the directory’s group.
3. Sticky Bit: Commonly used on directories, it restricts deletion of files to the file owner only.

Command: Setting SUID, SGID, and Sticky Bit

chmod 4755 file.txt  # SUID
chmod 2755 directory # SGID
chmod 1755 directory # Sticky bit

Output

-rwsr-xr-x 1 user user 1024 Oct 29 10:00 file.txt
drwxr-sr-x 2 user user 4096 Oct 29 10:00 directory
drwxrwxr-t 2 user user 4096 Oct 29 10:00 directory

Summary of Common Commands

• chmod u+x file.txt: Adds execute permission for the user.
• chmod 644 file.txt: Sets read and write for the user, read-only for group and others.
• chmod a+rw file.txt: Grants read and write permissions to everyone.

The chmod command is a powerful tool for managing access rights in Linux, allowing you to control file and directory permissions securely.