Files and Directory Permissions (chmod)⚓︎

Understanding file permissions is crucial for protecting your environment, files, and directories from unauthorized access or accidental deletion. In UNIX-based systems like Linux, which are multi-user environments, every file and directory in your account can be protected or made accessible by changing its access permissions.

🔍 Types of Permissions⚓︎

In UNIX, file and directory permissions are categorized into three main types:

Read (r): Allows the user to view the contents of a file.
Write (w): Allows the user to modify or delete the contents of a file.
Execute (x): Allows the user to run a file, if it's a program or script.

⚙️ Levels of Permissions⚓︎

Permissions can be set at three different levels:

User (u): The owner of the file.
Group (g): Users who belong to the same group as the file owner.
Others (o): All other users on the system.

👀 Viewing File Permissions⚓︎

To see the permissions of a file or directory, you can use the ls -l command. Here’s an example of what you might see when running ls -l:

$ ls -l
-rw-r--r-- 1 user1 user1 1048576 Dec 17 10:00 example.txt

In this output: - The first column shows the file type and permissions. - The first character (-) indicates it is a file (a d would indicate a directory). - The next three characters (rw-) indicate that the owner has read and write permissions. - The second set of three characters (r--) shows that the group has read permission only. - The final set (r--) shows that others also have read permission.

-rw-r--r--
|  |  |  |
|  |  |  +-- Others: Read
|  |  +----- Group: Read
|  +-------- User: Read & Write
+----------- File Type

The chmod command is used to modify the permissions of files and directories. It works by specifying a permission (read, write, or execute) for the user, group, or others.

Example 1: Removing Write Permission from the Group⚓︎

# Command
chmod g-w example.txt

# Verify the change
ls -l example.txt

Example 2: Removing Read Permission for Everyone⚓︎

# Command
chmod a-r example.txt

# Verify the change
ls -l example.txt

Example 3: Granting Read and Write Permissions to User and Group⚓︎

# Command
chmod ug+rw example.txt

# Verify the change
ls -l example.txt

Example 4: Granting Execute Permission⚓︎

To allow a script to be executed, you would add execute (x) permission.

# Command
chmod u+x script.sh

# Verify the change
ls -l script.sh

If you want everyone to be able to execute the file, use:

chmod a+x script.sh

Example 5: Changing Permissions Recursively⚓︎

If you want to change the permissions of all files and subdirectories within a directory, you can use the -R (recursive) option:

chmod -R u+rw directory/

⚠️ Warning: Use chmod -R carefully, as it applies changes to all subdirectories and files.

📂 Directory Permissions⚓︎

The execute permission for directories is particularly important. It allows you to enter (i.e., cd) into the directory. For example, if a directory has execute permission for the user, you can use cd to navigate into it. If not, you will get a permission denied message.

Example: Removing Execute Permission from a Directory⚓︎

# Command
chmod a-x projects

# Verify the change
ls -ld projects

To restore the permission:

chmod a+x projects

✅ Verifying Permission Changes⚓︎

After making changes to a file or directory’s permissions, always verify by running the ls -l command to ensure the changes took effect.

ls -l example.txt

🔢 Numerical Representation of Permissions⚓︎

Each permission type is represented by a specific numerical value. These values are added together to create the numeric mode for each user (owner), group, and others.

Permission Type	Numerical Value	Symbol
No permission	0	---
Execute	1	--x
Write	2	-w-
Write + Execute	3	-wx
Read	4	r--
Read + Execute	5	r-x
Read + Write	6	rw-
Read + Write + Execute	7	rwx

You add these values together to assign multiple permissions. For example: - 4 (Read) + 2 (Write) = 6 (Read + Write) - 4 (Read) + 1 (Execute) = 5 (Read + Execute)

📂 Structure of Permissions⚓︎

A file or directory’s permissions are divided into three groups: 1. 👤 User (Owner): The first digit in the numeric mode. 2. 👥 Group: The second digit. 3. 🌍 Others (Everyone else): The third digit.

Each digit is the sum of the permission values (Read = 4, Write = 2, Execute = 1). For example, if you want the owner to have read and write permissions, the numeric value would be 6 (4 for read + 2 for write).

🛠️ Using `chmod` with Numeric Mode⚓︎

To assign permissions using numerical values, use the chmod command followed by the numeric representation and the file or directory name.

📋 Examples:⚓︎

Example 1: Assign read, write, and execute to the owner; read and write to the group; and read to others

chmod 764 filename

7: read (4), write (2), and execute (1) for the owner.
6: read (4) and write (2) for the group.
4: read (4) for others.