Users and groups: id, whoami, who

Who is running this command?

Linux is a multi-user system to the core. Every file has an owner, every process runs on behalf of a user, and the kernel decides what is allowed by comparing the two. Inside the kernel a user is just a number, the UID. Names such as root or alice exist only for us: a small text file, /etc/passwd, maps names to numbers, and the tools translate back and forth.

Users also belong to groups, identified by a GID. Each user has one *primary* group (new files get it) and any number of *supplementary* groups. Groups are how you say "these three people may read the project folder" without listing them on every file. UID 0 is root, the superuser: the kernel skips almost every permission check for it. In the practice VM you are root the whole time, which is convenient for learning and exactly what you should avoid on a real machine.

whoami and id

whoami prints the name of the effective user of your shell and nothing else. id gives the whole picture: UID, primary GID and every group, each as number and name. With options you can pick out a single piece: -u the UID, -g the GID, -G all group ids, and -n to show names instead of numbers. Give id a username and it describes that user instead of you.

~% whoami
root
~% id
uid=0(root) gid=0(root)
~% id -u
0
~% id -nG
root
~% id nobody
uid=65534(nobody) gid=65534(nogroup)
~% echo $USER $HOME
root /root

Notice the shell also keeps the identity in environment variables, $USER and $HOME. They are convenient in scripts but they are only *copies* set at login; id asks the kernel and is always right.

Who else is here: who, w, last

who lists the users currently logged in, with their terminal and login time. w adds what each one is doing and the load average. last shows the history of logins and reboots. All three read records that the login program writes to /var/run/utmp and /var/log/wtmp. In this VM nobody *logs in*: init starts a root shell directly on the console, so no record is ever written, who prints nothing, w shows only its header and last complains that wtmp does not exist. That is not a bug in the tools, it is a fact about how this tiny system boots. On a normal server all three are full of useful information.