whoami and id commands in Linux CLI - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the whoami command do in Linux?

The whoami command shows the username of the current user who is logged in and running the shell.

beginner

What information does the id command provide?

The id command shows the user ID (UID), group ID (GID), and the groups the current user belongs to.

intermediate

How can id help in understanding user permissions?

By showing the UID, GID, and groups, id helps you know what permissions the user has based on group memberships.

intermediate

What is the difference between whoami and id -un?

whoami and id -un both show the current username, but id -un is part of the id command options.

intermediate

Can id show information about other users?

Yes, by adding a username after id, like id username, it shows that user's UID, GID, and groups.

What does the whoami command display?

A List of all users

B Current user's password

C Current user's username

D Current user's home directory

whoami simply prints the username of the user running the command.

Which command shows the user ID (UID) and group ID (GID)?

A id

B whoami

C pwd

D groups

id shows UID, GID, and groups of the user.

How do you use id to see information about another user named 'alice'?

A id alice

B id -u alice

C whoami alice

D groups alice

Typing id alice shows UID, GID, and groups for user 'alice'.

Which command would you use to find your current username quickly?

A id -un

B whoami

C id -g

D Both A and B

Both whoami and id -un show the current username.

What does the id command NOT show by default?

A Group ID (GID)

B User's home directory

C User ID (UID)

D Groups the user belongs to

id does not show the user's home directory; it shows UID, GID, and groups.

Explain what the whoami and id commands do and when you might use each.

Think about checking who you are vs. what groups you belong to.

Describe how you would find out the groups a user belongs to using Linux commands.

Focus on commands that reveal group info.

Practice

1. What does the whoami command display when run in a Linux terminal?

easy

A. The current logged-in username

B. The current directory path

C. The list of running processes

D. The system's IP address

Solution

1. Step 1: Understand the purpose of whoami

   The whoami command is designed to show the username of the current user logged into the shell.

2. Step 2: Compare with other options

   Options A, B, and C describe other commands or outputs unrelated to whoami.

3. Final Answer:

   The current logged-in username -> Option A

4. Quick Check:

   whoami = username [OK]

Hint: Remember: whoami = "Who am I?" shows your username [OK]

Common Mistakes:

• Confusing whoami with pwd (current directory)
• Thinking whoami shows system info
• Mixing up with commands showing processes

2. Which of the following is the correct syntax to display your user ID and group information in Linux?

easy

A. user id

B. id

C. whoami -u

D. showid

Solution

1. Step 1: Identify the correct command syntax

   The command id without arguments shows user ID, group ID, and group memberships.

2. Step 2: Check other options for validity

   Options A, B, and D are invalid commands or incorrect syntax for this purpose.

3. Final Answer:

   id -> Option B

4. Quick Check:

   id shows user and group info [OK]

Hint: Use simple id to see user and group IDs [OK]

Common Mistakes:

• Adding wrong flags to whoami
• Typing commands with spaces like 'user id'
• Using non-existent commands like showid

3. What will be the output of the command id -u if the current user has user ID 1001?

medium

A. 1001

B. uid=1001(user)

C. user=1001

D. id: invalid option

Solution

1. Step 1: Understand the -u option of id

   The -u option prints only the numeric user ID (UID) of the current user.

2. Step 2: Match output format

   The output will be just the number 1001, not a string with 'uid=' or other text.

3. Final Answer:

   1001 -> Option A

4. Quick Check:

   id -u = UID number only [OK]

Hint: id -u prints only your numeric user ID [OK]

Common Mistakes:

• Expecting full id output with -u
• Confusing uid= prefix with plain number
• Thinking -u is invalid option

4. You run the command id -u but get an error: "id: invalid option -- 'U'". What is the likely cause?

medium

A. You ran the command as root user

B. Your Linux system does not support the id command

C. You used a space between id and -u

D. You typed a capital U instead of lowercase u

Solution

1. Step 1: Check the option case sensitivity

   Linux commands are case sensitive. The option -u is lowercase. Using uppercase -U instead of lowercase -u can cause errors.

2. Step 2: Analyze the error message

   The error "invalid option -- 'U'" suggests the option letter was not recognized, likely due to uppercase letter typed instead of lowercase.

3. Final Answer:

   You typed a capital U instead of lowercase u -> Option D

4. Quick Check:

   Linux options are case sensitive [OK]

Hint: Check option letter case carefully; Linux is case sensitive [OK]

Common Mistakes:

• Typing uppercase instead of lowercase option
• Assuming id command is missing
• Adding spaces incorrectly in command

5. You want to write a script that prints your username and all group names you belong to. Which combination of commands will achieve this?

hard

A. id -un && id -G

B. whoami -g && id -u

C. echo $(whoami) && id -Gn

D. whoami && id -G

Solution

1. Step 1: Identify command to print username

   whoami prints the current username. Using echo $(whoami) prints it clearly.

2. Step 2: Identify command to print group names

   id -Gn prints all group names the user belongs to, separated by spaces.

3. Step 3: Evaluate options

   echo $(whoami) && id -Gn correctly combines username and group names. whoami -g && id -u uses invalid whoami -g. id -un && id -G uses id -un (which prints username) and id -G (which prints numeric group IDs, not names). whoami && id -G prints username and numeric group IDs, not names.

4. Final Answer:

   echo $(whoami) && id -Gn -> Option C

5. Quick Check:

   Use whoami + id -Gn for username and group names [OK]

Hint: Use whoami for username and id -Gn for group names [OK]

Common Mistakes:

• Using invalid flags with whoami
• Confusing numeric IDs with names
• Not combining commands properly