What Is Linux? Kernel, Shell, Distributions and Userland Explained for Beginners

By Ravindra Bagale, trainer · LinkedIn · Last updated: 25 October 2026

1. An everyday example: a restaurant kitchen

Why do we need this?

At a restaurant in Pune, you give your order to the waiter. The waiter tells the chef. The chef uses the stove and knives. You never walk into the kitchen. If every customer cooked for themselves, the kitchen would be chaos.

What is it?

You are the user. The waiter is the shell. The chef is the kernel. The stove and knives are the hardware (CPU, RAM, disk, network). The menu, plates and cutlery are userland: the programs and tools you use every day.

How to do it (step by step)

2. What is Linux?

Why do we need this?

Over 90% of cloud servers, all top 500 supercomputers and every Android phone run Linux. If you work in IT, DevOps, cloud, data or security, you will use it every day.

What is it?

Linux is a free, open-source operating system kernel. Linus Torvalds started it in 1991. People add tools around the kernel and call the whole thing a Linux system. It is multi-user and multi-tasking.

How to do it (step by step)

1. Check that you are on Linux. Run uname.

uname

Output:

Linux

What it means: The kernel name is Linux.

2. See all kernel details with -a.

uname -a

Output:

Linux web01 6.1.0-18-amd64 #1 SMP Debian 6.1.76-1 x86_64 GNU/Linux

What it means: Host name web01, kernel 6.1, 64-bit CPU (x86_64).

3. See how long the machine has been up.

uptime

Output:

10:42:01 up 12 days,  3:10,  2 users,  load average: 0.15, 0.10, 0.05

What it means: Running for 12 days. 2 users logged in. Load is low.

3. The kernel: the heart of Linux

Why do we need this?

Programs must not fight over the CPU, RAM or disk. Someone must share them fairly and safely. That job belongs to the kernel.

What is it?

The kernel is the core program. It loads first and stays in memory. It manages processes, memory, devices (drivers), files and networking. Programs ask it for help through system calls like open, read, write and fork.

How to do it (step by step)

1. Print only the kernel release.

uname -r

Output:

5.15.0-91-generic

What it means: Kernel version 5.15, build 91.

2. See the kernel messages from boot.

sudo dmesg | head -3

Output:

[    0.000000] Linux version 5.15.0-91-generic ...
[    0.000000] Command line: BOOT_IMAGE=/boot/vmlinuz-5.15.0-91-generic root=UUID=...
[    0.004000] BIOS-provided physical RAM map:

What it means: These lines come straight from the kernel.

3. List loaded kernel modules (drivers).

lsmod | head -3

Output:

Module                  Size  Used by
ext4                  925696  1
xfs                  2002944  0

What it means: ext4 and xfs file system drivers are loaded.

4. Watch system calls of a command with strace.

strace -c ls > /dev/null

Output:

% time     seconds  usecs/call     calls    errors syscall
 ...  openat  read  write  close ...

What it means: ls asks the kernel to open, read and write. That is a system call.

4. The shell: your waiter

Why do we need this?

You cannot talk to the kernel in plain words. You need a translator that reads your command and starts the right program.

What is it?

The shell is a program that reads commands and runs them. bash is the most common. Others are sh (dash), zsh, ksh and fish. The shell also has variables, loops and scripts. That is shell scripting.

How to do it (step by step)

1. See your current shell.

echo $SHELL

Output:

/bin/bash

What it means: Your login shell is bash.

2. See all shells installed.

cat /etc/shells

Output:

/bin/sh
/bin/bash
/usr/bin/bash
/bin/dash
/usr/bin/zsh

What it means: These shells can be used as a login shell.

3. Check if a command is built into the shell or is a file.

type cd ls

Output:

cd is a shell builtin
ls is aliased to `ls --color=auto'

What it means: cd lives inside bash. ls is a separate program (with an alias).

4. Find the file for a program.

which python3

Output:

/usr/bin/python3

What it means: The shell will run /usr/bin/python3.

5. Userland: everything outside the kernel

Why do we need this?

The kernel alone cannot list files or edit text. You need tools. These tools make Linux useful.

What is it?

Userland (user space) is all programs that run outside the kernel: GNU coreutils (ls, cp, cat), the C library (glibc or musl), systemd, the shell, editors, web servers and your apps. Kernel space is protected. User space cannot touch hardware directly.

How to do it (step by step)

1. See which package owns ls (Debian/Ubuntu).

dpkg -S /usr/bin/ls

Output:

coreutils: /usr/bin/ls

What it means: ls comes from the coreutils package. It is userland.

2. Check the C library version.

ldd --version | head -1

Output:

ldd (Ubuntu GLIBC 2.35-0ubuntu3.6) 2.35

What it means: glibc 2.35 links almost every program to the kernel.

3. See process 1, the first userland program.

ps -p 1 -o pid,comm

Output:

  PID COMMAND
    1 systemd

What it means: systemd is PID 1. It starts all other services.

6. Distributions: Ubuntu, RHEL, Amazon Linux and friends

Why do we need this?

Nobody installs a bare kernel. You want a ready package: kernel plus tools plus installer plus updates. Companies pick one based on support, cost and habit.

What is it?

A distribution (distro) is the kernel bundled with userland, a package manager and a release plan. Two big families exist: Debian (Debian, Ubuntu, Mint) uses .deb and apt. Red Hat (RHEL, Rocky, AlmaLinux, Fedora, Amazon Linux) uses .rpm and dnf/yum. Alpine is tiny and uses apk.

How to do it (step by step)

1. Read the distro name and version.

cat /etc/os-release

Output:

NAME="Amazon Linux"
VERSION="2023"
ID="amzn"
ID_LIKE="fedora"
PRETTY_NAME="Amazon Linux 2023.4.20240401"

What it means: This is Amazon Linux 2023. It is like Fedora, so use dnf.

2. Short form with hostnamectl.

hostnamectl

Output:

   Static hostname: app01
  Operating System: Ubuntu 22.04.4 LTS
            Kernel: Linux 5.15.0-91-generic
      Architecture: x86-64

What it means: One command shows host, OS, kernel and CPU type.

3. On Debian/Ubuntu use lsb_release.

lsb_release -a

Output:

Distributor ID: Ubuntu
Description:    Ubuntu 22.04.4 LTS
Release:        22.04
Codename:       jammy

What it means: Ubuntu 22.04, code name jammy.

7. Package managers: apt, dnf, yum, apk

Why do we need this?

Downloading programs from random websites is unsafe and hard to update. A package manager installs, updates and removes software from trusted repos in one command.

What is it?

A package is a ready-made program with its files and a list of what it needs (dependencies). The package manager fetches it from a repository and installs the dependencies too.

How to do it (step by step)

1. Refresh the package list on Ubuntu.

sudo apt update

Output:

Hit:1 http://archive.ubuntu.com/ubuntu jammy InRelease
Reading package lists... Done
12 packages can be upgraded.

What it means: The list of available versions is now fresh.

2. Install tree with apt.

sudo apt install -y tree

Output:

Setting up tree (2.0.2-1) ...

What it means: tree is installed. -y says yes to the prompt.

3. Install the same on Amazon Linux.

sudo dnf install -y tree

Output:

Installed:
  tree-1.8.0-6.amzn2023.x86_64
Complete!

What it means: dnf did the same job on a Red Hat style distro.

4. Install in an Alpine container.

apk add --no-cache curl

Output:

OK: 12 MiB in 20 packages

What it means: Alpine is tiny, so images stay small.

8. Users, root and sudo (first look)

Why do we need this?

If everyone was all-powerful, one typo could delete the whole server. Linux keeps normal users limited and gives power only when asked.

What is it?

root (UID 0) can do anything. Normal users own only their files. sudo runs one command as root after checking /etc/sudoers. On cloud images the default user is ubuntu, ec2-user or admin.

How to do it (step by step)

1. Who am I?

whoami

Output:

ec2-user

What it means: You are logged in as ec2-user.

2. See your user id and groups.

id

Output:

uid=1000(ec2-user) gid=1000(ec2-user) groups=1000(ec2-user),4(adm),10(wheel)

What it means: UID 1000. The wheel group can use sudo.

3. Run one command as root.

sudo whoami

Output:

root

What it means: For that one command you were root.

9. Real examples from many industries

Why do we need this?

The same Linux skill shows up in every kind of company. Seeing it in many places helps you remember it.

What is it?

Each row is a made-up team. It shows who uses this and how.

How to do it (step by step)

10. Common errors and how to fix them

Why do we need this?

Everyone hits these errors. Knowing the cause saves hours.

What is it?

Read the exact message first. Then match it to a row below.

How to do it (step by step)

11. Troubleshooting step by step

Why do we need this?

When a command fails on a new server, you first need to know where you are: which distro, which shell, which user.

What is it?

Go top to bottom. Stop at the first step that shows the problem.

How to do it (step by step)

1. Check the distro family.

grep -E '^(ID|ID_LIKE)=' /etc/os-release

2. Check the CPU type.

uname -m

3. Check your user and groups.

id

4. Check which program will run.

type -a python3

5. Check the shell running your script.

head -1 script.sh; ls -l /bin/sh

6. Check network for package installs.

ping -c 2 8.8.8.8

12. Lab 1: Identify any Linux server in 2 minutes

Why do we need this?

You join Zara Finance in London. Your lead gives you SSH to a server and asks: what is this box? Write a short report.

What is it?

A full hands-on lab. Run every step on a test Linux machine (an EC2 instance, a VM, or WSL).

How to do it (step by step)

1. Log in.

ssh -i key.pem ec2-user@10.0.1.15

Output:

[ec2-user@ip-10-0-1-15 ~]$

2. Find distro and version.

grep PRETTY_NAME /etc/os-release

Output:

PRETTY_NAME="Amazon Linux 2023.4.20240401"

3. Find kernel and CPU type.

uname -rm

Output:

6.1.82-99.168.amzn2023.x86_64 x86_64

4. Count CPUs and RAM.

nproc; free -h | head -2

Output:

2
               total        used        free
Mem:           3.8Gi       410Mi       2.9Gi

5. Check disk space.

df -h /

Output:

Filesystem      Size  Used Avail Use% Mounted on
/dev/nvme0n1p1   20G  2.1G   18G  11% /

6. Save the report to a file.

{ grep PRETTY_NAME /etc/os-release; uname -rm; nproc; } > ~/server-report.txt; cat ~/server-report.txt

Output:

PRETTY_NAME="Amazon Linux 2023.4.20240401"
6.1.82-99.168.amzn2023.x86_64 x86_64
2

13. Lab 2: Same job on two distros (Ubuntu and Alpine in Docker)

Why do we need this?

See with your own eyes that distros differ in tools, not in the kernel.

What is it?

A full hands-on lab. Run every step on a test Linux machine (an EC2 instance, a VM, or WSL).

How to do it (step by step)

1. Start Ubuntu.

docker run --rm -it ubuntu:22.04 bash

Output:

root@3f2a1b:/#

2. Check kernel inside.

uname -r

Output:

6.1.82-99.168.amzn2023.x86_64

3. Install curl with apt and exit.

apt update && apt install -y curl; exit

Output:

Setting up curl ...

4. Start Alpine.

docker run --rm -it alpine sh

Output:

/ #

5. Check kernel again.

uname -r

Output:

6.1.82-99.168.amzn2023.x86_64

6. Install curl with apk.

apk add --no-cache curl

Output:

OK: 12 MiB in 20 packages

14. Frequently asked questions

15. Interview questions and answers

Beginner level

Mid level level

Senior level

16. Related chapters

• Linux Ops Refresh for DevOps
• Bash Scripting for Automation
• Linux grep and find Commands with Examples
• How to Set Up Passwordless SSH Between Linux Servers

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