What Linux is

Almost every server you will touch as a cloud engineer runs Linux. Web servers, databases, containers and the machines that build your code all run it, and you work on them by typing commands. This topic gets a Linux shell running on your own computer and takes you through your first commands.

By the end you can open a shell, read the prompt, run a command, find out what a command does, and save its output to a file.

Linux, the kernel and distributions

Linux is strictly the name of a kernel: the program that starts first when a machine boots and that controls the hardware. It decides which program gets the CPU next, hands out memory, and talks to disks and network cards. Programs never touch the hardware themselves; they ask the kernel.

A kernel on its own is not much use, so it is packaged with everything else you need: a shell, the basic commands, a package manager, and system services. That package is a distribution, or distro. The ones you will meet most often:

The commands in this skill work the same on all of them. Where they differ, mostly in how software is installed, the topic says so.

Servers run Linux because it is free to run on as many machines as you like, it is stable enough to run for years without a restart, and it can be run with no screen at all and managed over the network.

The shell

You do not talk to the kernel directly. You type into a shell, a program that reads a line of text, works out which program you mean, runs it, and shows you what it printed. The shell on most Linux machines is Bash. A terminal is the window the shell runs in.

When the shell is ready for a command it shows a prompt, for example:

sam@web-01:~$

Read it from left to right:

• sam is the user you are logged in as.
• web-01 is the name of the machine. This matters when you have several machines open.
• ~ is the directory you are in. ~ is short for your home directory.
• $ means you are an ordinary user. A # instead means you are root, the administrator, and every command can change anything on the machine.

In this skill, a line starting with $ is a command for you to type, without the $. Lines without it are what the command prints.

Get a Linux shell

You need a Linux shell to follow along. Pick the option for your computer.

Any computer with Docker. This starts a fresh Ubuntu machine in a container and puts you in its shell. When you type exit the container is deleted, so nothing you do can break your computer.

$ docker run -it --rm --hostname lab ubuntu:24.04

Windows. Install the Windows Subsystem for Linux. Open PowerShell as an administrator, run the command below, restart, then open Ubuntu from the Start menu.

wsl --install

macOS. The macOS Terminal runs a shell, and most commands look the same, but macOS is not Linux and some commands behave differently. Use Docker, or run an Ubuntu virtual machine with Multipass:

$ brew install --cask multipass
$ multipass launch --name lab
$ multipass shell lab

Linux. Open the Terminal app. You are already there.

The Docker container logs you in as root, so your prompt ends in #. That is fine for practice. On a real server you log in as an ordinary user and only become root when you need to, which the Users and permissions topic covers.

Your first commands

A command is a program name, sometimes followed by options that change what it does and arguments that tell it what to work on. Separate each part with spaces.

$ whoami
root
$ hostname
lab
$ date
Sat Oct 24 09:12:31 UTC 2026
$ pwd
/

• whoami prints the user you are logged in as.
• hostname prints the machine's name.
• date prints the date and time.
• pwd (print working directory) prints the directory you are in. The Docker container starts you in /, the top of the filesystem. Your home directory, /root, is where you keep your own files; the Moving around topic explains both.

echo prints its arguments back. It sounds useless, but you will use it constantly in scripts:

$ echo Hello from Linux
Hello from Linux

uname describes the kernel. With the -r option it prints only the kernel's version, and with -a it prints everything:

$ uname -r
6.12.54-linuxkit
$ uname -a
Linux lab 6.12.54-linuxkit #1 SMP Tue Nov  4 21:21:47 UTC 2025 aarch64 aarch64 aarch64 GNU/Linux

Your version numbers will differ. To see which distribution you are on, read the file that describes it:

$ cat /etc/os-release
PRETTY_NAME="Ubuntu 24.04.5 LTS"
NAME="Ubuntu"
VERSION_ID="24.04"
...

Linux is case-sensitive: Date and date are different names, and only date is a command. If you mistype a command, the shell tells you:

$ Date
bash: Date: command not found

Some keys that save a lot of typing:

history lists the commands you have run in this shell.

Getting help

Nobody remembers every option. Two ways to look them up:

$ date --help
Usage: date [OPTION]... [+FORMAT]
Display date and time in the given FORMAT.
...

Most commands accept --help and print a short summary. For the full manual, use man:

$ man date

man opens the manual in a reader: use the arrow keys or Space to scroll, / to search, and q to quit. The Docker Ubuntu image leaves out manual pages to stay small. If man says the system has been minimized, run unminimize and answer y, or use --help.

Saving output to a file

Everything a command prints normally goes to your screen. The > sign sends it to a file instead:

$ date > today.txt
$ cat today.txt
Sat Oct 24 09:15:02 UTC 2026

> creates the file, or replaces what was in it if it exists. To add to the end of a file instead, use >>:

$ echo "Machine: $(hostname)" >> today.txt
$ cat today.txt
Sat Oct 24 09:15:02 UTC 2026
Machine: lab

$(hostname) runs hostname and puts what it printed into the line. The Pipes and text topic goes much further with this.

Tasks

Do these in your Linux shell.

Task 1: Describe the machine

Create a file called machine.txt in your home directory that holds, in this order, one line each:

1. the user you are logged in as
2. the machine's name
3. the kernel version
4. the PRETTY_NAME line from /etc/os-release

Use > for the first line and >> for the rest. You can find a single line of a file with grep PRETTY_NAME /etc/os-release; the Finding things topic explains grep.

$ cd ~
$ whoami > machine.txt
$ hostname >> machine.txt
$ uname -r >> machine.txt
$ grep PRETTY_NAME /etc/os-release >> machine.txt
$ cat machine.txt
root
lab
6.12.54-linuxkit
PRETTY_NAME="Ubuntu 24.04.5 LTS"

Task 2: Find an option yourself

Using date --help or man date, find out how to make date print only the year, as four digits. Then add a line to machine.txt that reads Year: followed by the year.

$ date +%Y
2026
$ echo "Year: $(date +%Y)" >> machine.txt
$ tail -n 1 machine.txt
Year: 2026

Quiz

1. Your prompt reads admin@db-02:/var/log#. Which machine are you on, which directory are you in, and what should make you careful?

2. What is the difference between Linux and Ubuntu?

3. You run hostname > info.txt and then whoami > info.txt. What is in info.txt?

4. You log in to a server you have never seen before. Which three commands tell you who you are, where you are, and which distribution it runs?