What does 'effective' user ID mean?

That's a fantastic and insightful question!

In Linux, an Effective User ID (EUID) is the identity the system uses to determine what files and resources you are allowed to access at this very moment.

To make it simple, think of it like this:

1. Real User ID (RUID): This is who you "actually" are (like your ID card). When you log in as labex, your Real User ID is labex.
2. Effective User ID (EUID): This is the identity that determines your permissions.

Why is there a difference?

Sometimes, a normal user needs to perform a task that usually requires "Superuser" (Admin) powers—like changing a password or installing a program.

When you use a special command (like sudo), your Real ID stays as labex, but your Effective ID temporarily becomes root (the administrator). This allows the computer to say: "I know you are labex, but for this specific task, I will give you the permissions of an admin."

In short:

• Real ID: Who you logged in as.
• Effective ID: Who the system "sees" you as when checking if you're allowed to do something.

Since you just ran whoami and it said labex, it means your Effective User ID matches your login—you are currently acting with the standard permissions of the labex user!