Numeric permission mode (755, 644) in Linux CLI - Mini Project: Build & Apply

Understanding Numeric Permission Modes (755, 644) in Linux

📖 Scenario: You are managing files on a Linux server. You need to set correct permissions so that files and folders are secure but accessible as needed.Numeric permission modes like 755 and 644 are common ways to set these permissions quickly.

🎯 Goal: Learn how to create files and directories, then set their permissions using numeric modes 755 and 644.You will practice creating a directory and a file, then apply these numeric permissions and check the results.

📋 What You'll Learn

Create a directory named project

Create a file named readme.txt inside the project directory

Set the directory permission to numeric mode 755

Set the file permission to numeric mode 644

Use ls -l to display the permissions and verify

💡 Why This Matters

🌍 Real World

Setting correct file and directory permissions is essential for security and proper access control on Linux servers and systems.

💼 Career

System administrators, DevOps engineers, and developers often need to manage permissions to protect data and ensure applications run correctly.

1

Create the directory project

Type the command to create a directory named project in your current folder.

Linux CLI

# Your code here

Hint

Use the mkdir command followed by the directory name.

2

Create the file readme.txt inside project

Type the command to create an empty file named readme.txt inside the project directory.

Linux CLI

mkdir project
# Your code here

Hint

Use the touch command with the path project/readme.txt.

3

Set the directory permission to numeric mode 755

Type the command to set the permission of the project directory to numeric mode 755.

Linux CLI

mkdir project
touch project/readme.txt
# Your code here

Hint

Use the chmod command with 755 and the directory name.

4

Set the file permission to numeric mode 644 and verify

Type the commands to set the permission of the file project/readme.txt to numeric mode 644 and then list the details of the project directory using ls -l project to verify the permissions.

Linux CLI

mkdir project
touch project/readme.txt
chmod 755 project
# Your code here

Hint

Use chmod 644 project/readme.txt to set file permission.

Then use ls -l project to see the permissions.

Practice

1. What does the numeric permission mode 755 mean for a file in Linux?

easy

A. User can read, write, execute; group and others can read and execute

B. User can read and write; group and others can only read

C. User, group, and others can read, write, and execute

D. User can read and execute; group and others have no permissions

Solution

1. Step 1: Understand numeric permission digits

   Each digit represents permissions for user, group, and others respectively. 7 means read(4)+write(2)+execute(1)=7, 5 means read(4)+execute(1)=5.

2. Step 2: Decode 755

   User (7) has read, write, execute; group (5) and others (5) have read and execute only.

3. Final Answer:

   User can read, write, execute; group and others can read and execute -> Option A

4. Quick Check:

   755 = rwx r-x r-x [OK]

Hint: 7 means full rights; 5 means read+execute only [OK]

Common Mistakes:

• Confusing 5 with write permission
• Thinking 7 means read+write only
• Mixing group and others permissions

2. Which of the following is the correct command to set a file named script.sh to permission mode 755?

easy

A. chmod 644 script.sh

B. chmod 700 script.sh

C. chmod 777 script.sh

D. chmod 755 script.sh

Solution

1. Step 1: Identify the command to change permissions

   The command to change file permissions is chmod followed by the numeric mode and filename.

2. Step 2: Match the mode 755 with the command

   To set mode 755, use chmod 755 script.sh.

3. Final Answer:

   chmod 755 script.sh -> Option D

4. Quick Check:

   chmod + numeric mode + filename = chmod 755 script.sh [OK]

Hint: chmod + mode + filename sets permissions [OK]

Common Mistakes:

• Using wrong numeric mode
• Swapping filename and mode
• Using chmod without mode

3. If a file has permissions set to 644, what will be the output of ls -l filename showing the permission part?

medium

A. -rw-r--r--

B. -rwxrwxrwx

C. -rwxr-xr-x

D. -rw-rw-r--

Solution

1. Step 1: Decode numeric mode 644

   6 = read(4)+write(2), 4 = read only, so user has read/write, group and others have read only.

2. Step 2: Translate to symbolic permissions

   Permissions are -rw-r--r-- where '-' means regular file, 'rw-' user, 'r--' group, 'r--' others.

3. Final Answer:

   -rw-r--r-- -> Option A

4. Quick Check:

   644 = rw- r-- r-- [OK]

Hint: 6 = rw-, 4 = r-- in permissions [OK]

Common Mistakes:

• Confusing 4 with execute permission
• Mixing user and group permissions
• Assuming 644 includes execute

4. You ran chmod 755 myfile but ls -l myfile shows -rw-r--r--. What is the likely problem?

medium

A. The chmod command syntax is incorrect

B. You forgot to specify the filename

C. You do not have permission to change the file mode

D. The file is a directory, not a regular file

Solution

1. Step 1: Understand chmod effect and output

   chmod 755 should set execute permission, but output shows no execute bits.

2. Step 2: Identify permission issue

   If permissions did not change, likely user lacks rights to modify file mode (not owner or no sudo).

3. Final Answer:

   You do not have permission to change the file mode -> Option C

4. Quick Check:

   chmod no effect = permission denied [OK]

Hint: No change after chmod? Check your ownership rights [OK]

Common Mistakes:

• Assuming syntax error
• Thinking file type blocks chmod
• Ignoring user permissions

5. You want to make a script file executable only by the owner and readable by everyone else. Which numeric mode should you use?

hard

A. 644

B. 744

C. 700

D. 755

Solution

1. Step 1: Define required permissions

   Owner: executable + read + write (7), group and others: read only (4 each).

2. Step 2: Construct numeric mode

   Owner = 7 (rwx), group = 4 (r--), others = 4 (r--), so mode is 744.

3. Final Answer:

   744 -> Option B

4. Quick Check:

   744 = rwx r-- r-- [OK]

Hint: Owner full, others read only = 744 [OK]

Common Mistakes:

• Choosing 755 which allows execute for others
• Using 700 which denies read for others
• Confusing 644 with execute permission