Numeric permission mode (755, 644) in Linux CLI - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the numeric permission mode 755 mean in Linux?

755 means the owner has read, write, and execute permissions (7), while the group and others have read and execute permissions (5 each).

beginner

Explain the numeric permission mode 644 in Linux.

644 means the owner has read and write permissions (6), while the group and others have only read permissions (4 each).

beginner

How do you calculate the numeric permission for read, write, and execute?

Read = 4, Write = 2, Execute = 1. Add the values for each permission you want to set.

beginner

What command changes a file's permission to 755?

Use the command chmod 755 filename to set the file's permissions to 755.

beginner

Why would you use 644 permission mode for a file?

Because it allows the owner to edit the file, but others can only read it, which is common for configuration or text files.

What does the '7' in permission mode 755 represent?

A Read permission only

B Read and write permissions only

C Execute permission only

D Read, write, and execute permissions

7 means read (4) + write (2) + execute (1) permissions combined.

Which numeric permission mode allows the owner to read and write, but others only to read?

A 644

B 700

C 755

D 777

644 means owner can read/write (6), others can only read (4).

What command sets a file's permission to 644?

A chmod 755 filename

B chmod 777 filename

C chmod 644 filename

D chmod 700 filename

chmod 644 filename sets owner read/write and others read only.

If a file has permission 755, what can the group do?

A Read, write, and execute

B Read and execute

C Write only

D No permissions

5 means read (4) + execute (1) permissions for group.

What numeric value represents write permission alone?

A 2

B 7

C 4

D 1

Write permission is represented by 2.

Describe how to interpret the numeric permission mode 755 in Linux.

Think about what each digit means for owner, group, and others.

Explain why 644 is a common permission mode for files.

Consider who needs to change the file and who only needs to see it.

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