Chmod

🔐 Command: chmod (Change Mode)

The chmod command is used to change the access permissions of files and directories. In Linux, every file has a set of rules determining who can read it, write to it, or execute it as a program.

1. The "Why"

On Linux system, security is built into the filesystem. You will use chmod when:

• You’ve written a script (like a Bash or Python script) and need to make it "executable" so it can run.
• You want to lock a private folder (like your doctoral research or e-book drafts) so other users on the system can't see it.
• You need to fix a "Permission Denied" error when trying to run a program or edit a file.

2. Understanding Permissions (The "How")

Before using the command, you need to understand the three types of users and the three types of permissions.

The Three Users:

1. u (User/Owner): You.
2. g (Group): A set of users.
3. o (Others): Everyone else.

The Three Permissions:

1. r (Read): Permission to see the contents.
2. w (Write): Permission to edit or delete.
3. x (Execute): Permission to run the file as a program.

3. Two Ways to Use chmod

A. The Symbolic Method (Easier to Remember)

Use letters to add (+) or remove (-) permissions.

• chmod u+x script.sh (Add eXecute for the User)
• chmod g-w file.txt (Remove Write for the Group)
• chmod a+r document.pdf (Add Read for All)

B. The Absolute/Numeric Method (Faster for Professionals)

Each permission is assigned a number:

• 4 = Read
• 2 = Write
• 1 = Execute
• 0 = No permission

You add them together to get a single digit for each user (Owner, Group, Others).

• 7 (4+2+1): Full permissions (Read + Write + Execute)
• 6 (4+2): Read + Write
• 5 (4+1): Read + Execute

4. Practical Examples

A. Making a Script Executable

This is the most common use for developers:

chmod +x my_script.sh

B. Setting Standard Permissions (755)

Common for public folders: Owner can do everything, everyone else can only read and enter the folder.

chmod 755 my_folder

C. Making a File Completely Private (600)

Only the owner can read and write; nobody else can see a thing. Great for SSH keys or passwords.

chmod 600 private_notes.txt

D. Changing an Entire Folder Tree (Recursive)

To change permissions for a folder and everything inside it:

chmod -R 700 my_private_project/

5. Pro-Tips

• View Your Permissions: Use ls -l to see the current permissions. It looks like -rwxr-xr-x. The first three are for the user, the next three for the group, and the last three for others.
• Safety Warning: Never use chmod 777 unless you absolutely have to. It gives everyone permission to do anything to your file, which is a major security risk.
• Arch Linux Context: When you download a script from the AUR or GitHub, you often have to chmod +x it before it will run. Also, your .gnupg and .ssh folders require very specific permissions (usually 700) to function correctly.

6. Summary Reference