chmod (change permissions) in Linux CLI - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the chmod command do in Linux?

The chmod command changes the permissions of files or directories, controlling who can read, write, or execute them.

beginner

What do the letters r, w, and x stand for in file permissions?

r means read permission, w means write permission, and x means execute permission.

beginner

How do you give execute permission to the owner of a file named script.sh?

Use the command: chmod u+x script.sh. Here, u means user (owner), and +x adds execute permission.

intermediate

What does the numeric mode 755 mean when used with chmod?

It means: owner can read, write, and execute (7), group can read and execute (5), others can read and execute (5).

beginner

How would you remove write permission for others on a file named data.txt?

Use the command: chmod o-w data.txt. Here, o means others, and -w removes write permission.

What does chmod 644 file.txt do?

A Owner can read/write; group and others can read only

B Owner can read/write/execute; group and others can read only

C Owner can read only; group and others can read/write

D Owner, group, and others can read/write/execute

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

Which command adds execute permission for everyone on run.sh?

A chmod g+x run.sh

B chmod u+x run.sh

C chmod o+x run.sh

D chmod +x run.sh

chmod +x adds execute permission for user, group, and others.

What does chmod u=rwx,g=rx,o= file do?

A Owner has read only; group has read and write; others have execute only

B Owner has all permissions; group can read and execute; others have no permissions

C Owner has execute only; group has write only; others have read only

D Owner, group, and others have full permissions

It sets explicit permissions: user (rwx), group (rx), others (none).

Which symbol removes write permission from the group?

A -x

B +w

C -w

D +x

-w removes write permission.

If a file has permissions -rwxr-xr--, what is the numeric mode?

A 754

B 755

C 644

D 700

Owner: rwx (7), group: r-x (5), others: r-- (4) → 754.

Explain how to use symbolic modes with chmod to add and remove permissions.

Think about how you tell a friend to allow or block certain actions.

Describe the difference between symbolic and numeric modes in chmod.

One is like writing a sentence, the other is like a code number.

Practice

1. What does the chmod 755 filename command do in Linux?

easy

A. Removes all permissions from everyone

B. Sets read and write for owner; execute for group and others

C. Sets read, write, execute for owner; read and execute for group and others

D. Only changes the file owner

Solution

1. Step 1: Understand numeric permission 755

   The digits 7, 5, 5 represent permissions for owner, group, and others respectively. 7 means read(4)+write(2)+execute(1)=7, 5 means read(4)+execute(1)=5.

2. Step 2: Apply permissions to file

   Owner gets full permissions (rwx), group and others get read and execute (r-x). This matches the description in Sets read, write, execute for owner; read and execute for group and others.

3. Final Answer:

   Sets read, write, execute for owner; read and execute for group and others -> Option C

4. Quick Check:

   chmod 755 = rwxr-xr-x [OK]

Hint: Remember 7=rwx, 5=rx, 6=rw [OK]

Common Mistakes:

• Confusing numeric codes with letters
• Thinking 755 removes permissions
• Assuming chmod changes ownership

2. Which of the following is the correct syntax to add execute permission for the user on a file named script.sh?

easy

A. chmod +x u script.sh

B. chmod script.sh u+x

C. chmod x+u script.sh

D. chmod u+x script.sh

Solution

1. Step 1: Recall chmod symbolic syntax

   The correct format is chmod [who][operator][permission] filename. Here, 'u' means user, '+' adds permission, 'x' is execute.

2. Step 2: Match syntax to options

   chmod u+x script.sh matches the correct syntax: chmod u+x script.sh. Other options have incorrect order or misplaced arguments.

3. Final Answer:

   chmod u+x script.sh -> Option D

4. Quick Check:

   chmod u+x filename adds execute for user [OK]

Hint: Use chmod [u/g/o][+/-][r/w/x] filename [OK]

Common Mistakes:

• Placing permissions before user
• Putting filename before options
• Using spaces inside symbolic mode

3. What will be the output of ls -l file.txt after running chmod 640 file.txt on a file owned by user 'alice' and group 'staff'?

medium

A. -rw-r----- 1 alice staff 0 date file.txt

B. -rwxr-xr-x 1 alice staff 0 date file.txt

C. -rw-rw-r-- 1 alice staff 0 date file.txt

D. -r--r----- 1 alice staff 0 date file.txt

Solution

1. Step 1: Decode chmod 640 permissions

   6 = read(4)+write(2) for owner, 4 = read(4) for group, 0 = no permissions for others.

2. Step 2: Translate to ls -l format

   Owner: rw-, Group: r--, Others: ---. So the permission string is '-rw-r-----'. The rest shows ownership and file info.

3. Final Answer:

   -rw-r----- 1 alice staff 0 date file.txt -> Option A

4. Quick Check:

   chmod 640 = rw-r----- [OK]

Hint: Match digits to rwx: 6=rw-, 4=r--, 0=--- [OK]

Common Mistakes:

• Mixing group and others permissions
• Assuming execute permission is set
• Confusing numeric values

4. You ran chmod 777 myfile but ls -l myfile still shows -rw-r--r--. What is the most likely reason?

medium

A. You forgot to run the command as root or with sudo

B. The file system is mounted as read-only

C. The chmod command syntax is incorrect

D. The file is a directory, not a file

Solution

1. Step 1: Understand chmod effect and file system state

   chmod 777 should set full permissions. If permissions don't change, it may be due to file system restrictions like read-only mount.

2. Step 2: Evaluate other options

   Running as root is not always required for own files. Syntax is correct. File type does not prevent permission changes. Read-only file system blocks changes.

3. Final Answer:

   The file system is mounted as read-only -> Option B

4. Quick Check:

   Read-only FS blocks chmod changes [OK]

Hint: Check if filesystem is read-only if chmod fails [OK]

Common Mistakes:

• Assuming sudo always needed
• Blaming syntax without checking
• Ignoring file system mount options

5. You want to set permissions so that the owner can read and write, the group can only read, and others have no permissions on a file named data.txt. Which command achieves this?

hard

A. chmod 640 data.txt

B. chmod 750 data.txt

C. chmod 664 data.txt

D. chmod 600 data.txt

Solution

1. Step 1: Translate desired permissions to numeric code

   Owner: read+write = 6, Group: read only = 4, Others: no permissions = 0. So numeric code is 640.

2. Step 2: Match numeric code to options

   chmod 640 data.txt is chmod 640 data.txt, which matches the desired permissions exactly.

3. Final Answer:

   chmod 640 data.txt -> Option A

4. Quick Check:

   Owner=rw, Group=r, Others=none = 640 [OK]

Hint: Use chmod 640 for rw-r----- permissions [OK]

Common Mistakes:

• Using 750 which gives execute permission
• Using 664 which allows others read
• Using 600 which denies group read