Introduction to System Administration: Fundamentals of Unix OS System Administration

Users, groups, permissions and ACLs on a real Linux server, taught the way I teach BIT students in Kampala.

I built this course for the students I meet every semester at MUBS and Nkumba who can open a terminal but freeze the moment root access or a broken home directory shows up. It is not a slideshow about Linux history. You work a single case, Nile Creative Studio, a small Kampala design shop that needs a clean multi-user file server. By the last lesson you can create accounts, put people in the right groups, set directory permissions that survive a busy Monday, and apply ACLs when two teams must share the same folder without sharing every secret. We start with what the kernel and the shell actually do for you, then move into first commands, root versus sudo, user accounts, passwords, groups, basic permissions, sticky bits and setgid directories, and ACLs. Each lesson keeps a short story about Kirabo and Joshua at the studio, then the exact commands, then a lab you can run on any Ubuntu or Debian box you control. There is no browser terminal sandbox here. You type on your own machine or a cheap VPS the same way a junior sysadmin does on day one at a helpdesk. I wrote the wording after watching the same mistakes repeat in class: students who chmod 777 everything, who leave unused accounts unlocked, who cannot read ls -l output under pressure. If you are a helpdesk trainee, a BIT student preparing for systems labs, or someone who inherited a Linux box at a Kampala SME, this course gives you the judgement those roles need. Finish it and you will not need to call a senior every time a permission denied error appears. You will know where to look, what to change, and how to leave the system safer than you found it.

Curriculum Outline

Module 1: Fundamentals of Unix OS System Administration

• What is Unix and Linux? (text)

  Time: 40 minutes

  What you will learn

  • Where Unix came from and how Linux is related to it
  • The four layers of a Linux system: hardware, kernel, shell, applications
  • Why servers are multi-user systems, and why that needs an administrator
  • What a distribution is, and why we use Ubuntu Server

  1.1 A short story of Unix

  In 1969, at Bell Labs in the USA, Ken Thompson and Dennis Ritchie built an operating system called Unix. It had a few ideas that were new at the time:

  1. Many people can use one computer at the same time (multi-user). 2. Many programs can run at the same time (multi-tasking). 3. Everything is a file. Documents, folders, even hardware devices are treated like files. 4. Small tools that each do one job well, which you combine to do big jobs.

  In 1991, a Finnish student called Linus Torvalds wrote a free kernel inspired by Unix. He called it Linux. Combined with free tools from the GNU project, it became a complete, free, Unix-like operating system.

  Today Linux runs most of the world's servers, all of the top 500 supercomputers, and every Android phone.

  Think of it like this

  Think of Unix as the original recipe for a famous dish. Linux is a new chef who cooked the same dish from scratch, using the same recipe ideas, and gave it away free. That is why the commands you learn on Linux also work on most Unix systems.

  1.2 The layers of a Linux system

  Layer	What it does	Example
  Hardware	The physical machine	CPU, RAM, disk, network card
  Kernel	The core of the OS. Talks to hardware, manages memory, decides which program runs	The Linux kernel
  Shell	The program that reads your commands and asks the kernel to do them	bash
  Applications	Programs you run	ls, nano, a web server

  Think of it like this

  Think of a restaurant. The kitchen equipment is the hardware. The head chef is the kernel, deciding who uses which stove. The waiter is the shell, taking your order in plain words and passing it to the kitchen. You are the user giving orders.

  1.3 Why servers need an administrator

  A server is shared by many people. At Nile Creative Studio, Kirabo and Joshua share design files. An auditor wants to look at them. A freelancer needs to edit some of them. Nobody should see the manager's payroll files.

  Unix solves this with three ideas you will master in this module:

  1. Users: every person gets their own account. 2. Groups: people who work together are put in the same group. 3. Permissions: every file and folder says who can read it, change it, or open it.

  The system administrator (sysadmin) is the person who sets these up and keeps them correct.

  1.4 Distributions and Ubuntu Server

  Linux comes packaged in distributions (distros). A distro is the Linux kernel plus tools, an installer, and default settings.

  Distro	Common use
  Ubuntu / Ubuntu Server	Very popular, beginner friendly, used a lot in cloud servers
  Debian	Stable base that Ubuntu is built on
  Red Hat Enterprise Linux, Rocky Linux	Banks and large companies
  Fedora	Desktop and developers

  In this module we use Ubuntu Server. It has no graphical desktop. You control it only by typing commands. That is how real servers are run.

  Note

  Almost every command in this module also works on Debian, Red Hat and other Linux systems. A few helper commands such as adduser and deluser are Debian/Ubuntu friendly extras, and we will point those out.

  Quiz: Lesson 1

  Key takeaways

  • Linux is a free, Unix-like operating system.
  • The kernel talks to hardware; the shell talks to you.
  • Servers are shared, so we need users, groups and permissions.
  • We will use Ubuntu Server, controlled only by commands.

  ---

• Your First Steps in the Terminal (text)

  Time: 60 minutes

  What you will learn

  • How to read the command prompt
  • The shape of every Linux command
  • How to find where you are, look around, and move between folders
  • The main folders of a Linux system
  • How to get help without Google

  2.1 Reading the prompt

  When you open the terminal you see something like this:

  ssendi@studio-server:~$

  Part	Meaning
  ssendi	The user you are logged in as
  studio-server	The name of the computer (hostname)
  ~	The folder you are in. ~ is short for your home folder
  $	You are a normal user. If you see #, you are root, the all-powerful administrator

  Careful

  When the prompt ends in #, every command you type has full power over the whole system. Slow down and read before pressing Enter.

  2.2 The shape of a command

  Almost every command follows this pattern:

  command  -options  arguments

  Example	Command	Option	Argument
  ls -l /home	ls (list)	-l (long format)	/home (which folder)
  mkdir -p /srv/studio	mkdir (make directory)	-p (create parents too)	/srv/studio

  Think of it like this

  A command is like a sentence to a helper. The command is the verb ("list"). The options are adverbs ("in detail"). The arguments are the object ("the /home folder"). "List, in detail, the /home folder" becomes ls -l /home.

  Note

  Linux is case sensitive. ls works. LS does not. A user called Kirabo is different from kirabo. We always use lowercase usernames.

  2.3 Where am I? pwd

  pwd means print working directory. It tells you the folder you are standing in.

  $ pwd
  /home/student

  2.4 What is here? ls

  ls lists the contents of a folder.

  Command	What it shows
  ls	Names of files and folders
  ls -l	Long list: permissions, owner, group, size, date
  ls -a	Also shows hidden files (names starting with .)
  ls -la	Both together
  ls -ld /home	Details of the folder itself, not what is inside it
  ls /etc	Contents of another folder without going there

  Pro tip

  Remember ls -ld. You will use it a lot later to check folder permissions. Without the d, ls -l /home shows what is inside /home. With the d, it shows /home itself.

  2.5 Moving around: cd

  cd means change directory.

  Command	Where it takes you
  cd /etc	Straight to /etc (absolute path, starts with /)
  cd Documents	Into Documents inside the current folder (relative path)
  cd ..	Up one level
  cd ~ or just cd	Back to your home folder
  cd -	Back to the previous folder you were in

  2.6 The Linux folder map

  Linux has one big tree that starts at / (called root directory, not to be confused with the root user).

  Folder	What lives there	Why a sysadmin cares
  /	The top of everything
  /home	Personal folders of normal users, e.g. /home/kirabo	New users get a folder here
  /root	Home folder of the root user	Only root can enter
  /etc	System settings (configuration files)	User, password and group files live here
  /srv	Data served by this server	We will build the team folder here
  /var	Changing data such as logs	/var/log holds system logs
  /bin, /usr/bin	Programs (commands)	Where ls, chmod live
  /tmp	Temporary files, anyone can write	Cleaned on reboot

  2.7 Getting help

  You do not need to memorise every option.

  Command	What it does
  man ls	Opens the manual page for ls. Press q to quit, /word to search
  ls --help	Short help printed on screen
  whatis chmod	One-line description
  history	Shows commands you typed before
  clear or Ctrl+L	Clears the screen
  Tab key	Completes a command or file name for you. Press twice to see options
  Up arrow	Brings back your last command
  Ctrl+C	Stops a command that is running or stuck

  Lab 2: Explore the server

  1. Print your current folder. 2. List the contents of /etc in long format. 3. Show the details of the /home folder itself (not what is inside). 4. Go to /srv, then return to your home folder in one short command.

  Quiz: Lesson 2

  Key takeaways

  • pwd = where am I, ls = what is here, cd = go somewhere.
  • ls -ld folder describes the folder itself.
  • /home for users, /etc for settings, /srv for served data.
  • man and --help are your built-in teachers.

  ---

• The Superuser: root and sudo (text)

  Time: 40 minutes

  What you will learn

  • Who root is and why it is dangerous
  • How sudo lets you borrow root's power for one command
  • How to open and leave a root shell
  • The principle of least privilege

  3.1 Meet root

  Every Linux system has one special account called root (user ID 0). Root can read any file, change any password, and delete anything, including the whole system.

  Think of it like this

  Root is the master key of a hostel. It opens every room, the store and the office. You do not carry the master key around all day. You collect it from the office when you need it, use it, and hand it back.

  3.2 sudo: borrow root's power

  On Ubuntu you do not log in as root. Instead, trusted users put sudo (superuser do) in front of a command.

  $ cat /etc/shadow
  cat: /etc/shadow: Permission denied
  
  $ sudo cat /etc/shadow
  [sudo] password for ssendi:
  root:*:19876:0:99999:7:::
  ...

  1. Linux asks for your own password, not root's. 2. It remembers for about 15 minutes, so the next sudo does not ask again. 3. Every sudo command is recorded in the system log. Admins are accountable.

  Note

  Only users in the sudo group can use sudo on Ubuntu. That is why a new user cannot suddenly run admin commands.

  3.3 A root shell: sudo -i

  When you have many admin commands to run, you can open a root shell:

  ssendi@studio-server:~$ sudo -i
  root@studio-server:~# whoami
  root
  root@studio-server:~# exit
  logout
  ssendi@studio-server:~$

  Notice the prompt changes from $ to #. Inside a root shell you do not type sudo. Type exit to go back to your normal account.

  Command	Result
  sudo command	Run one command as root
  sudo -i	Open a root shell (root's environment)
  sudo su	Also opens a root shell
  exit	Leave the root shell

  Careful

  In this course, we write sudo in front of admin commands. If you are already in a root shell (# prompt), leave the sudo out. Both give the same result.

  3.4 Least privilege

  The principle of least privilege says: give every person and program only the access they need, and nothing more. Use sudo only for commands that need it. This one idea explains almost everything else in this module.

  Quiz: Lesson 3

  Key takeaways

  • root can do anything, so we avoid using it directly.
  • sudo command runs one command as root.
  • sudo -i opens a root shell, exit closes it.
  • Least privilege: only the access you need.

  ---

• Creating User Accounts (text)

  Time: 75 minutes

  What you will learn

  • What a Linux user account is made of
  • How to read /etc/passwd
  • How to create users with useradd and adduser
  • How to check that a user was created correctly with id, getent and grep

  4.1 What is a user account?

  Ssendi Samuel's first job at Nile Creative Studio is to create accounts for the two designers, Kirabo and Joshua.

  Every account has:

  Part	Meaning	Example for kirabo
  Username	Login name	kirabo
  UID	User ID number. Linux really uses this number, not the name	1001
  GID	ID of the user's primary group	1001
  Comment (GECOS)	Full name or notes	Kirabo, Graphic Designer
  Home directory	Personal folder	/home/kirabo
  Login shell	Program started at login	/bin/bash

  Think of it like this

  An account is like a staff ID card. The name is printed on the front, but the office system actually uses the ID number on the back (the UID). The card also says which department you belong to (primary group), which desk is yours (home directory), and which door you use to enter (shell).

  4.2 The account book: /etc/passwd

  Linux keeps every account in the text file /etc/passwd. Each line is one account, with 7 fields separated by colons :.

  kirabo:x:1001:1001:Kirabo Designer:/home/kirabo:/bin/bash

  #	Field	Value
  1	Username	kirabo
  2	Password placeholder	x means "the real password is in /etc/shadow"
  3	UID	1001
  4	GID (primary group)	1001
  5	Comment (GECOS)	Kirabo Designer
  6	Home directory	/home/kirabo
  7	Login shell	/bin/bash

  Note

  UIDs below 1000 are system accounts (for services like the web server). Normal human users start at 1000 on Ubuntu.

  4.3 Creating a user with useradd

  useradd is the standard low-level tool that exists on every Linux system.

  sudo useradd -m -s /bin/bash kirabo

  Part	Meaning
  sudo	Creating users needs root
  useradd	The command
  -m	Make the home directory /home/kirabo
  -s /bin/bash	Set the shell to bash
  kirabo	The new username

  Other useful options:

  Option	Meaning	Example
  -c "text"	Comment / full name	-c "Kirabo Designer"
  -G group	Add to extra groups at creation	-G designers
  -u 1500	Choose the UID

  Careful

  Plain sudo useradd kirabo (no -m, no -s) creates a half-finished account on Ubuntu: no home folder, and the shell is /bin/sh, not bash. When Kirabo logs in she gets an error about her home directory. Always use -m -s /bin/bash.

  4.4 The friendly way: adduser

  Ubuntu and Debian also have adduser, a friendly script that asks you questions:

  $ sudo adduser joshua
  info: Adding user `joshua' ...
  info: Adding new group `joshua' (1002) ...
  info: Adding new user `joshua' (1002) with group `joshua (1002)' ...
  info: Creating home directory `/home/joshua' ...
  New password:
  Retype new password:
  passwd: password updated successfully
  Full Name []: Joshua
  ...
  Is the information correct? [Y/n] Y

  adduser creates the home folder, sets bash, and asks for a password, all in one go.

  	useradd	adduser
  Available on	All Linux systems	Debian and Ubuntu
  Home folder	Only with -m	Automatically
  Shell	Only with -s	bash automatically
  Password	Separate passwd step	Asked during creation
  Best for	Scripts, exams on any Linux	Quick interactive work on Ubuntu

  Pro tip

  Learn useradd -m -s /bin/bash well. It works everywhere. Use adduser when you are on Ubuntu and want speed.

  4.5 Checking your work

  A professional never assumes. After creating a user, verify.

  Command	What it shows
  id kirabo	UID, primary group, and all groups
  getent passwd kirabo	The full /etc/passwd line: UID, GID, home, shell
  grep kirabo /etc/passwd	Same line, found by searching the file
  ls -ld /home/kirabo	Proves the home folder exists and who owns it

  $ id kirabo
  uid=1001(kirabo) gid=1001(kirabo) groups=1001(kirabo)
  
  $ getent passwd kirabo
  kirabo:x:1001:1001::/home/kirabo:/bin/bash

  Note

  id shows numbers and groups but not the home folder. Use getent passwd when you need to prove the home directory and shell.

  Think of it like this

  Ubuntu gave Kirabo a group with her own name (kirabo). This is called a user private group. Think of it as a personal locker that only she uses, before she joins any team.

  Lab 4

  Create a user called okello with a home folder, bash shell, and the comment "Okello Video Editor". Verify it with getent passwd okello and ls -ld /home/okello.

  Quiz: Lesson 4

  Key takeaways

  • /etc/passwd has 7 fields: name, x, UID, GID, comment, home, shell.
  • sudo useradd -m -s /bin/bash name creates a complete account.
  • sudo adduser name is the friendly Ubuntu way.
  • Verify with id, getent passwd, and ls -ld /home/name.

  ---

• Passwords and the Shadow File (text)
• Becoming Another User (text)
• Groups: Putting People in Teams (text)
• When Staff Leave: Remove, Lock, Record (text)
• File and Folder Permissions (text)
• Special Permissions: setgid and the Sticky Bit (text)
• Access Control Lists (ACLs) (text)
• Verify Like a Professional (text)
• Capstone Lab: Set Up the Nile Creative Studio Photo Lab (text)
• Final Module Quiz (quiz)