The Security Audit Vector
When assuming administrative control of a legacy Ubuntu server, your absolute first priority must be a comprehensive security audit. You must determine exactly who possesses cryptographic access to the machine. You cannot rely on graphical control panels; they frequently hide critical system-level accounts. To guarantee you see every single user capable of executing code on the hardware, you must directly interrogate the kernel’s foundational master authentication database.
How to List All Users
Ubuntu stores the definitive list of all user accounts within a highly protected, plain-text configuration file located at `/etc/passwd`.
1. Open your terminal application or connect to the server via SSH.
2. You need to print the contents of the file to the screen. Because the `/etc/passwd` file is structurally complex, containing home directory paths and shell assignments, printing it raw is difficult to read. You must use the `cut` utility to surgically extract just the usernames.
3. Type the following complex command pipeline exactly as written:
cut -d: -f1 /etc/passwd
4. Command Breakdown:
* cut: Invokes the text-slicing utility.
* -d: Defines the “delimiter.” The `/etc/passwd` file separates data using colons. This tells the utility to look for colons.
* -f1: Defines the “field.” The username is always the very first field before the first colon.
* /etc/passwd: The target file.
5. Press Enter.
6. Analyzing the Output: The terminal will instantly output a massive vertical list of names. Do not panic. The first thirty names (like `root`, `daemon`, `sys`, `www-data`) are vital system accounts created by the operating system itself to run background processes. Scroll to the absolute bottom of the list. Human users (like `johndoe` or `admin`) are always appended to the very bottom of the file.