chmod (change permissions) in Linux CLI - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

What is the output of ls -l after running chmod 754 file.txt?

You have a file named file.txt. You run the command chmod 754 file.txt. What will be the permission string shown by ls -l file.txt?

Linux CLI

chmod 754 file.txt
ls -l file.txt

A -rwxr-xr-- 1 user group 0 date file.txt

B -rwxr-xr-x 1 user group 0 date file.txt

C -rwxr--r-x 1 user group 0 date file.txt

D -rwxr--r-- 1 user group 0 date file.txt

💡 Hint

Remember the digits in chmod represent owner, group, and others permissions in octal.

The number 7 means read, write, execute (rwx) for owner. 5 means read and execute (r-x) for group. 4 means read only (r--) for others. So the permission string is '-rwxr-xr--'.

💻 Command Output

intermediate

What error occurs when running chmod 999 file.txt?

You try to set permissions with chmod 999 file.txt. What happens?

Linux CLI

chmod 999 file.txt

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

B Permission changed successfully

C chmod: invalid mode: ‘999’

D chmod: invalid argument ‘999’

💡 Hint

Check if the mode digits are valid octal numbers.

Digits in chmod must be octal (0-7). '9' is invalid, so chmod reports 'invalid mode'.

📝 Syntax

advanced

Which command correctly adds execute permission for the group on script.sh?

You want to add execute permission for the group only, without changing other permissions. Which command is correct?

A chmod g+x+ script.sh

B chmod +x g script.sh

C chmod x+g script.sh

D chmod g+x script.sh

💡 Hint

The syntax is chmod [who]+[permission] file.

The correct syntax to add execute permission for group is 'chmod g+x file'. Other options are invalid syntax.

🚀 Application

advanced

How to set permissions so owner has full access, group has read-only, and others have no access on data.txt?

You want data.txt to have these permissions: owner can read, write, execute; group can only read; others have no permissions. Which command achieves this?

A chmod 740 data.txt

B chmod 700 data.txt

C chmod 754 data.txt

D chmod 744 data.txt

💡 Hint

Convert permissions to octal: rwx=7, r--=4, ---=0.

Owner rwx=7, group r--=4, others ---=0 gives 740.

🧠 Conceptual

expert

What is the effect of chmod u=rwx,g=rx,o= file.txt?

You run chmod u=rwx,g=rx,o= file.txt. What permissions does file.txt have?

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

B Owner has read, write, execute; group has read and execute; others have no permissions

C Owner has full permissions; group and others have full permissions

D Owner has no permissions; group has read and execute; others have read only

💡 Hint

Look at each user class and the permissions assigned explicitly.

The command sets owner (u) to rwx, group (g) to rx, and others (o) to no permissions.

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