Numeric permission mode (755, 644) in Linux CLI - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

What is the permission string for mode 755?

You run ls -l on a file with numeric permission mode 755. What is the permission string shown in the output?

A -rw-r--r--

B -rwxr-xr-x

C -rwxrwxr-x

D -rwxr-----

💡 Hint

Think about what each digit means: owner, group, others.

Mode 755 means owner has read, write, execute (7), group has read and execute (5), others have read and execute (5). This corresponds to '-rwxr-xr-x'.

💻 Command Output

intermediate

What is the permission string for mode 644?

You set a file's permission to 644. What permission string will ls -l show?

A -rw-r--r--

B -rwxr-xr-x

C -rw-rw-r--

D -rwxr-----

💡 Hint

Remember 6 means read and write, 4 means read only.

Mode 644 means owner has read and write (6), group has read only (4), others have read only (4). This matches '-rw-r--r--'.

🚀 Application

advanced

Set permissions to 755 using chmod

You want to set a directory's permissions to 755. Which command will do this correctly?

A chmod 777 directory_name

B chmod 644 directory_name

C chmod 755 directory_name

D chmod u=rwx,g=rw,o=r directory_name

💡 Hint

755 means owner full, group and others read and execute.

The numeric mode 755 sets owner to rwx, group and others to r-x. Option C uses the correct numeric mode.

💻 Command Output

advanced

What error occurs with invalid numeric mode 999?

You run chmod 999 file.txt. What happens?

A chmod: cannot access 'file.txt': No such file or directory

B Permissions set to rwxrwxrwx

C No error, permissions set to 777

D chmod: invalid mode: ‘999’

💡 Hint

Numeric modes must be between 0 and 7 for each digit.

Each digit in numeric mode must be 0-7. 9 is invalid, so chmod reports an invalid mode error.

🧠 Conceptual

expert

How many files have execute permission with mode 755 in a list?

You have 3 files with permissions set as: file1=755, file2=644, file3=700. How many files have execute permission for the owner?

A 2

B 3

C 0

D 1

💡 Hint

Check the owner's execute bit in each mode.

755 owner has execute, 644 owner does not, 700 owner has execute. So 2 files have execute permission for owner.

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