su (switch user) in Linux CLI - Commands & Configuration

Introduction

Sometimes you need to do tasks as another user on your Linux system. The su command lets you switch to a different user account temporarily without logging out.

When you need to install software as the root user but are logged in as a regular user.

When you want to test commands or scripts as another user without logging out.

When you need to access files or settings that only another user can access.

When you want to switch to a less privileged user for safer testing.

When you want to run a command as another user without opening a new session.

Commands

Switch to the root user with a login shell, which loads root's environment variables and settings.

Terminal

su -

Expected Output

Password:

root@hostname:~# 

→

- - Starts a login shell for the target user, loading their environment.

Switch to user 'alice' with a login shell to use her environment and permissions.

Terminal

su - alice

Expected Output

Password:

alice@hostname:~$ 

→

- - Starts a login shell for the specified user.

Run the command 'whoami' as user 'alice' without switching to an interactive shell.

Terminal

su -c "whoami" alice

Expected Output

alice

→

-c - Runs a single command as the specified user.

Exit from the switched user session and return to your original user.

Terminal

exit

Expected Output

user@hostname:~$ 

Key Concept

If you remember nothing else from this pattern, remember: su lets you temporarily become another user to run commands or access files with their permissions.

Common Mistakes

Running 'su' without specifying '-' or '-l' and expecting the full environment of the target user.

Without '-', su keeps your current environment, which can cause unexpected behavior or permission issues.

Always use 'su -' or 'su -l' to switch to a full login shell of the target user.

Trying to switch to root without knowing the root password.

su requires the target user's password, so without root's password you cannot switch to root.

Use 'sudo' if you have sudo privileges or ask the administrator for the root password.

Not exiting the su session and getting confused about which user you are logged in as.

You might run commands with unintended permissions, risking system changes or security issues.

Use 'exit' to return to your original user when done.

Summary

Use 'su -' to switch to root with a full login environment.

Use 'su - username' to switch to another user with their environment.

Use 'su -c "command" username' to run a single command as another user.

Use 'exit' to return to your original user after 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