Rocky Linux Core Commands: Page 4

This page covers commands for managing file permissions and ownership, crucial for system security.

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16. chmod - Change File Permissions

Purpose: The chmod command (change mode) is used to change the permissions (read, write, execute) of files and directories.

Permissions are defined for three categories: user (owner), group, and others.

Syntax: chmod [OPTIONS] MODE FILE...

Permission Modes (Octal):

• Read (r): 4
• Write (w): 2
• Execute (x): 1
• No permission: 0

Combine numbers for desired permissions (e.g., 7 = rwx, 6 = rw-, 5 = r-x, 4 = r--).

Permission Modes (Symbolic):

• u: user (owner)
• g: group
• o: others
• a: all (user, group, others)
• +: add permission
• -: remove permission
• =: set exact permission
• r, w, x: read, write, execute

Examples:

chmod 755 script.sh      # rwx for user, r-x for group and others
chmod u+x my_program     # Add execute permission for the owner
chmod go-w shared_file   # Remove write permission for group and others
chmod 644 document.txt   # rw- for user, r-- for group and others

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17. chown - Change File Ownership

Purpose: The chown command (change owner) is used to change the owner of files and directories. This typically requires root privileges.

Syntax: chown [OPTIONS] USER[:GROUP] FILE...

Key Options:

• -R: (recursive) Changes ownership recursively for directories and their contents.

Examples:

sudo chown newuser file.txt
sudo chown newuser:newgroup folder/
sudo chown -R webadmin:webgroup /var/www/html/

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18. chgrp - Change File Group Ownership

Purpose: The chgrp command (change group) is used to change the group owner of files and directories. You must be the owner of the file or have root privileges.

Syntax: chgrp [OPTIONS] GROUP FILE...

Key Options:

• -R: (recursive) Changes group ownership recursively.

Examples:

chgrp developers project_file.py
sudo chgrp -R www-data /var/www/

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19. whoami - Display Current Username

Purpose: The whoami command prints the effective username of the current user. It's a quick way to verify which user account you are currently operating as.

Syntax: whoami

Example:

whoami

Output might be your_username or root if you're logged in as root.

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20. id - Print User and Group Information

Purpose: The id command displays detailed information about the current user, including their user ID (UID), primary group ID (GID), and all supplementary group memberships.

Syntax: id [USERNAME]

Examples:

id
id johndoe

Output includes UID, GID, and groups the user belongs to.