su (switch user) in Linux CLI - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

What is the output of this command?

You run the command su -c 'whoami' as a normal user. What will be the output if you enter the root password correctly?

Linux CLI

su -c 'whoami'

A root

B normal user name

C su: Authentication failure

D command not found

💡 Hint

The -c option runs a command as the switched user.

The command su -c 'whoami' runs whoami as the root user after successful authentication, so the output is root.

💻 Command Output

intermediate

What happens if you run su without a username?

You type su and press Enter. What user will you switch to after entering the password?

Linux CLI

su

A Switches to root user

B Switches to current user again

C Switches to nobody user

D Shows an error message

💡 Hint

By default, su switches to the superuser if no username is given.

When no username is specified, su switches to the root user after password authentication.

📝 Syntax

advanced

Which command correctly switches to user 'alice' and starts a login shell?

Choose the correct su command to switch to user 'alice' with a login shell.

A su alice --login

B su alice -l

C su - alice

D su -l alice

💡 Hint

The dash - before the username starts a login shell.

The correct syntax is su - alice to start a login shell as user 'alice'.

🔧 Debug

advanced

Why does this command fail with 'Authentication failure'?

You run su - bob but get 'Authentication failure' even though you typed the correct password. What could be the reason?

Linux CLI

su - bob

A You typed the root password instead of bob's password

B User 'bob' does not have a password set

C The 'su' command is not installed

D You are not allowed to run 'su' as a normal user

💡 Hint

Check if the target user has a valid password.

If user 'bob' has no password set, authentication will fail even with the correct password typed.

🚀 Application

expert

How to run a command as another user without switching shell interactively?

You want to run ls /root as root from your normal user account without entering an interactive shell. Which command achieves this?

A su -c ls /root

B su - root -c 'ls /root'

C su -c 'ls /root'

D su root -c 'ls /root'

💡 Hint

Use the dash to start a login shell and the -c option to run a command.

The command su root -c 'ls /root' runs ls /root as root without starting a login shell, without interactive switching.

Practice

1. What does the su command do in Linux?

easy

A. Updates the system packages

B. Shows the current user's groups

C. Switches the current user to another user

D. Displays the current directory contents

Solution

1. Step 1: Understand the purpose of su

   The su command is used to switch from the current user to another user in the terminal.

2. Step 2: Compare options

   Updating system packages uses apt or yum, showing user's groups uses 'groups', displaying directory contents uses 'ls'. These are unrelated to user switching.

3. Final Answer:

   Switches the current user to another user -> Option C

4. Quick Check:

   su = switch user [OK]

Hint: Remember: su means switch user [OK]

Common Mistakes:

• Confusing su with commands like ls or groups
• Thinking su updates software
• Assuming su shows files

2. Which of these is the correct syntax to switch to user alice and load her full environment?

easy

A. su - alice

B. su alice

C. su --user alice

D. su /alice

Solution

1. Step 1: Recall the syntax for full environment loading

   Using su - username switches user and loads their full environment.

2. Step 2: Check options

   su alice changes user without full environment, su --user alice uses invalid flag, su /alice is invalid syntax.

3. Final Answer:

   su - alice -> Option A

4. Quick Check:

   Use dash (-) to load full environment [OK]

Hint: Use dash (-) after su for full environment [OK]

Common Mistakes:

• Omitting the dash (-) to load environment
• Using wrong flags like --user
• Adding slashes before username

3. What will be the output of the following commands if you run su - bob and enter the correct password?

whoami

medium

A. root

B. Permission denied

C. current user name

D. bob

Solution

1. Step 1: Understand what su - bob does

   This command switches the current user to 'bob' and loads his environment after password verification.

2. Step 2: Check the output of whoami

   After switching, whoami shows the current active user, which will be 'bob'.

3. Final Answer:

   bob -> Option D

4. Quick Check:

   After su, whoami = new user [OK]

Hint: whoami shows switched user after su [OK]

Common Mistakes:

• Expecting root instead of target user
• Thinking it shows original user
• Assuming permission denied without password

4. You try to switch to user john using su john but get the error: Authentication failure. What is the most likely cause?

medium

A. You entered the wrong password for john

B. User john does not exist

C. You forgot to use sudo before su

D. You need to be root to use su

Solution

1. Step 1: Understand the error message

   Authentication failure means the password entered was incorrect for the target user.

2. Step 2: Analyze other options

   User existence error would say 'unknown user'. Using sudo is not required for su. You do not need to be root to switch users if you know the password.

3. Final Answer:

   You entered the wrong password for john -> Option A

4. Quick Check:

   Authentication failure = wrong password [OK]

Hint: Authentication failure means wrong password [OK]

Common Mistakes:

• Assuming user does not exist
• Thinking sudo is required for su
• Believing only root can switch users

5. You want to run a script as user deploy without fully switching to their environment. Which command is best to use?

hard

A. su -c deploy ./script.sh

B. su deploy -c './script.sh'

C. su deploy ./script.sh

D. su - deploy ./script.sh

Solution

1. Step 1: Understand running commands as another user

   Using su username -c 'command' runs the command as that user without opening a full shell.

2. Step 2: Check options for correct syntax

   su -c deploy ./script.sh wrong flag order, su - deploy ./script.sh starts login shell, su deploy ./script.sh starts non-login shell. Only su deploy -c './script.sh' runs directly.

3. Final Answer:

   su deploy -c './script.sh' -> Option B

4. Quick Check:

   Use -c to run command as user [OK]

Hint: Use su -c 'command' to run without full switch [OK]

Common Mistakes:

• Placing - before username incorrectly
• Omitting -c flag for command
• Confusing argument order