Linux Reference

Section 01

CLI & Navigation

Linux organises everything in a single tree starting at / (root). Your home folder is /home/james, shortened to ~. You move through this tree with a handful of commands.

Where am I / Moving around

Command	What it does
pwd	Print Working Directory — shows your current location
ls	List files and folders in the current directory
ls -la	Long listing with hidden files, sizes, and permissions
cd /path/to/dir	Change to an absolute path
cd ~	Jump to your home directory from anywhere
cd ..	Go up one level to the parent directory
cd -	Jump back to the previous directory (like a back button)
tree	Show directory structure as a visual tree (sudo apt install tree)

Tip: Tab completion is your best friend. Start typing a path and press Tab — the shell fills it in. Press Tab twice to see all matches when there are multiple options.

Viewing files

Command	What it does
cat filename	Print the entire contents of a file to the terminal
less filename	View a file one screen at a time (q to quit, space for next page)
head -20 filename	Show the first 20 lines of a file
tail -50 filename	Show the last 50 lines of a file
tail -f filename	Follow a file live — new lines appear as they're written (great for logs)

Finding files

Command	What it does
find /path -name "*.txt"	Search for files by name pattern under a directory
find /path -name file 2>/dev/null	Search and suppress Permission denied errors
which program	Show the full path to a command (e.g. which python3)
locate filename	Fast filename search using a pre-built index (sudo updatedb to refresh)

Copying, moving, deleting

Command	What it does
cp file.txt /dest/	Copy a file to a destination
cp -r folder/ /dest/	Copy a folder and all its contents recursively
mv file.txt /dest/	Move a file (also used to rename: mv old.txt new.txt)
rm file.txt	Delete a file permanently — no Recycle Bin
rm -rf folder/	Delete a folder and everything inside it — use with extreme care
mkdir newfolder	Create a new directory
mkdir -p a/b/c	Create nested directories all at once, no error if they already exist

Warning: rm -rf is permanent and instant. There is no undo. Always double-check your path before running it.

Searching file contents with grep

grep searches for text patterns inside files. It's one of the most useful tools you'll use — especially piped with other commands.

Command	What it does
grep 'word' file.txt	Search for a word inside a file, print matching lines
grep -r 'word' /path/	Search recursively through all files in a directory
grep -i 'word' file.txt	Case-insensitive search
grep -n 'word' file.txt	Show line numbers alongside matches
grep -v 'word' file.txt	Show lines that do NOT contain the word (invert)
ps aux \| grep steam	Pipe: filter output of another command — like we did earlier

Pipes & redirection

The pipe character | sends the output of one command into another. Redirection operators control where output goes.

Command	What it does
cmd1 \| cmd2	Send output of cmd1 as input to cmd2
cmd > file.txt	Redirect output to a file (overwrites existing content)
cmd >> file.txt	Append output to a file without overwriting
cmd 2>/dev/null	Discard error messages (send stderr to /dev/null)
cmd 2>&1	Merge stderr into stdout — see all output together

Keyboard shortcuts

TabAutocomplete commands and paths

↑ / ↓Scroll through command history

Ctrl+RReverse search — type part of a past command

Ctrl+CKill the current running command

Ctrl+LClear the terminal screen

Ctrl+A / EJump to start / end of the current line

Ctrl+WDelete the previous word

!!Repeat last command — e.g. sudo !! to re-run as root

Section 02

Processes & System

A process is any running program. Every process has a PID (Process ID) — a unique number the system uses to track and control it. Understanding processes lets you see what's running, why something is slow, and how to stop things that misbehave.

Viewing processes

Command	What it does
ps aux	Snapshot of all running processes from all users
ps aux \| grep steam	Filter the process list — find processes matching a name
top	Live interactive process viewer, updates every few seconds (q to quit)
htop	Better version of top with colours and mouse support (sudo apt install htop)
pgrep steam	List just the PIDs of processes matching a name

ps aux — what the columns mean

Column	Meaning
USER	Who owns the process
PID	Process ID — the number you use to target it with kill
%CPU	CPU usage right now
%MEM	Memory (RAM) usage right now
STAT	Process state: S = sleeping, R = running, Z = zombie, D = waiting on disk
COMMAND	The actual command being run

Stopping processes

Command	What it does
kill PID	Ask a process to stop gracefully (sends SIGTERM)
kill -9 PID	Force-kill a process immediately — use when kill alone doesn't work
pkill steam	Kill all processes matching a name — no PID needed
pkill -f steam	Match against the full command line, not just the process name
pkill -9 steam	Force-kill all matching processes
pkill -f steam && flatpak run com.valvesoftware.Steam	Kill orphaned Steam processes then relaunch — what fixed it today

Tip: Always try graceful kill before -9. Force-killing can leave lock files or corrupted state behind that causes problems on the next launch.

Background & foreground jobs

Command	What it does
command &	Run a command in the background — shell stays usable
Ctrl + C	Stop (interrupt) the foreground process
Ctrl + Z	Pause the foreground process and send it to the background
jobs	List paused/background jobs in the current shell session
fg	Bring the most recent background job back to the foreground
bg	Resume a paused job in the background
nohup command &	Run in background, keep running even after you close the terminal

Resource information

Command	What it does
free -h	Show RAM usage in human-readable format (MB/GB)
df -h	Show disk space usage for all mounted drives
du -sh folder/	Show total size of a specific folder
lscpu	CPU info — cores, architecture, speed
lsblk	List all storage devices and partitions
lspci	List PCI devices (GPU, network card, etc.)
uname -a	Show kernel version and system architecture
uptime	How long the system has been running and current load

System logs

Logs are your first stop when something breaks. Linux stores most logs in /var/log/ and via journald — which is what we used to debug Steam today.

Command	What it does
journalctl -xe	Recent system journal entries with explanations
journalctl -u servicename	Logs for a specific systemd service
journalctl --since today	Only show logs from today
tail -f /var/log/syslog	Watch the system log live
dmesg \| tail -30	Kernel messages — useful after hardware changes or crashes

Permissions

Every file has an owner and permission bits controlling who can read (r), write (w), and execute (x) it. ls -la shows these as: -rwxr-xr-- (owner / group / everyone else).

Command	What it does
chmod +x script.sh	Make a file executable
chmod 755 file	Owner: rwx, Group: r-x, Others: r-x
chown james:james file	Change file owner and group
sudo command	Run a command as root — needed for system-level changes

Warning: sudo runs commands as root — the most powerful user on the system. Understand what a command does before running it with sudo.

Section 03

Package Management

On Kubuntu you have three ways to install software: apt (system packages from Ubuntu repos), flatpak (sandboxed apps from Flathub — like Steam), and snap (Canonical's containerised packages). Each has its use case.

apt — system packages

apt is the main package manager for Ubuntu/Debian-based systems. It installs software into the system directly.

Command	What it does
sudo apt update	Refresh the package list from repositories (always do this first)
sudo apt upgrade	Upgrade all installed packages to latest versions
sudo apt install package	Install a package
sudo apt remove package	Remove a package (keeps config files)
sudo apt purge package	Remove a package and its config files
sudo apt autoremove	Remove packages that are no longer needed by anything
apt search keyword	Search for packages by keyword
apt show package	Show details about a package — version, description, dependencies
dpkg -l \| grep package	List installed packages matching a name
sudo apt --fix-broken install	Fix broken package dependencies

Tip: Always run sudo apt update before installing anything. Without it you might install an outdated version or hit dependency errors.

flatpak — sandboxed apps

Flatpak runs apps in a sandbox, isolated from the rest of your system. Steam is installed this way on your machine. Apps come from Flathub.

Command	What it does
flatpak list	List all installed Flatpak apps
flatpak search appname	Search Flathub for an app
flatpak install flathub com.app.Name	Install an app from Flathub
flatpak run com.app.Name	Run a Flatpak app (note: case-sensitive app ID)
flatpak update	Update all installed Flatpak apps
flatpak update com.app.Name	Update a specific app
flatpak uninstall com.app.Name	Remove a Flatpak app
flatpak uninstall --unused	Remove unused runtimes to free disk space

App IDs are case-sensitive: com.valvesoftware.Steam not com.valvesoftware.steam — this is what tripped you up earlier today.

snap — Canonical packages

Snap is another containerised package system from Canonical. It's not installed by default on Kubuntu but can be added. Less common than apt or flatpak for most use cases.

Command	What it does
sudo apt install snapd	Install snap support if it's not already there
snap find appname	Search the Snap Store for an app
sudo snap install appname	Install a snap package
snap list	List installed snaps
sudo snap refresh	Update all snaps
sudo snap remove appname	Remove a snap

Which one to use?

Situation	Use
System tools, libraries, dev tools (python, git, htop, tree)	apt
Desktop apps (Steam, browsers, media players)	flatpak
App only available as snap and not in apt/flatpak	snap
.deb file downloaded manually	sudo dpkg -i file.deb

Section 04

Networking & Troubleshooting

Linux has a solid set of tools for inspecting network interfaces, checking connections, testing DNS, and transferring files. Especially relevant given your UniFi setup.

Network interfaces & IP addresses

Command	What it does
ip a	Show all network interfaces and their IP addresses (modern replacement for ifconfig)
ip link	Show link layer info for all interfaces (up/down state, MAC)
ip route	Show the routing table — which interface handles which traffic
ip route show default	Show just the default gateway
nmcli	NetworkManager CLI — manage connections, Wi-Fi, VPNs
nmcli device status	Show all network devices and their connection state
nmcli con show	List all saved network connections

Testing connectivity

Command	What it does
ping 8.8.8.8	Test basic connectivity to an IP address (Ctrl+C to stop)
ping -c 4 google.com	Send exactly 4 pings and show results
traceroute google.com	Show every hop your traffic takes to reach a destination
mtr google.com	Live traceroute with packet loss stats (sudo apt install mtr)
curl -I https://example.com	Fetch just the HTTP headers from a URL — test if a site is up
wget -q -O /dev/null url	Test download without saving anything — check if URL is reachable

DNS

Command	What it does
nslookup google.com	Simple DNS lookup — shows what IP a hostname resolves to
dig google.com	Full DNS query with detailed response (sudo apt install dnsutils)
dig google.com @1.1.1.1	Query a specific DNS server (useful for testing your UniFi DNS filtering)
dig +short google.com	Just the answer, no extra output
resolvectl status	Show current DNS configuration per interface
cat /etc/resolv.conf	Show which DNS servers the system is configured to use

Ports & connections

Command	What it does
ss -tulnp	List all listening ports and which process owns them (modern replacement for netstat)
ss -tnp	Show active TCP connections with process names
ss -s	Summary of socket statistics
nc -zv host port	Test if a specific port is open on a remote host

ss flags: -t TCP, -u UDP, -l listening only, -n no hostname resolution, -p show process

Downloading & transferring files

Command	What it does
curl -O https://url/file	Download a file keeping its original name
curl -L -o name.file https://url	Download following redirects, save with a custom name
wget https://url/file	Download a file (simpler than curl for basic downloads)
scp file.txt user@host:/dest/	Securely copy a file to a remote machine over SSH
rsync -avz src/ user@host:/dest/	Sync files efficiently — only transfers what changed

Wi-Fi

Command	What it does
nmcli device wifi list	Scan and list available Wi-Fi networks
nmcli device wifi connect "SSID" password "pass"	Connect to a Wi-Fi network
iwconfig	Show wireless interface info including signal strength
iw dev wlan0 link	Show connection details for a specific wireless interface

Section 05

Shell Scripting

Shell scripts let you automate repetitive tasks. A script is just a text file with commands — the same ones you type in the terminal — that run in sequence. Bash is the default shell on Kubuntu.

Your first script

Create a file, make it executable, run it. That's the whole workflow.

# Create the file
nano myscript.sh

# Inside the file — always start with this line:
#!/bin/bash
echo "Hello, James"

# Save and exit nano: Ctrl+O, Enter, Ctrl+X
# Make it executable:
chmod +x myscript.sh

# Run it:
./myscript.sh

#!/bin/bash is called a shebang. It tells the system which interpreter to use. Always put it on the first line of every script.

Variables

# No spaces around the = sign
NAME="James"
COUNT=5

# Use a variable with $
echo "Hello, $NAME"
echo "Count is: ${COUNT}"   # braces optional but clearer

# Capture command output into a variable
TODAY=$(date +%Y-%m-%d)
echo "Today is $TODAY"

# Special variables
echo $0   # Script name
echo $1   # First argument passed to the script
echo $#   # Number of arguments
echo $?   # Exit code of the last command (0 = success)

Conditionals

# Basic if/else
if [ $COUNT -gt 3 ]; then
    echo "Count is greater than 3"
elif [ $COUNT -eq 3 ]; then
    echo "Count is exactly 3"
else
    echo "Count is less than 3"
fi

# Comparison operators
# -eq  equal        -ne  not equal
# -gt  greater than  -lt  less than
# -ge  >=            -le  <=

# String comparison
if [ "$NAME" = "James" ]; then
    echo "Hi James"
fi

# Check if a file exists
if [ -f /path/to/file ]; then
    echo "File exists"
fi

# Check if a directory exists
if [ -d /path/to/dir ]; then
    echo "Directory exists"
fi

Loops

# For loop — iterate over a list
for item in one two three; do
    echo "Item: $item"
done

# For loop — iterate over files
for file in /home/james/*.txt; do
    echo "Found: $file"
done

# For loop — numeric range
for i in {1..5}; do
    echo "Number: $i"
done

# While loop
COUNT=0
while [ $COUNT -lt 5 ]; do
    echo "Count: $COUNT"
    COUNT=$(( COUNT + 1 ))
done

Functions

# Define a function
greet() {
    echo "Hello, $1"   # $1 is the first argument to the function
}

# Call it
greet "James"
greet "world"

# Function with return value (via exit code)
is_running() {
    pgrep -f "$1" > /dev/null 2>&1
}

if is_running "steam"; then
    echo "Steam is running"
else
    echo "Steam is not running"
fi

Aliases — shortcuts for long commands

Add aliases to ~/.bashrc so they persist across sessions. Run source ~/.bashrc after editing to apply them immediately.

# Open ~/.bashrc
nano ~/.bashrc

# Add aliases at the bottom:
alias steam='pkill -f steam; flatpak run com.valvesoftware.Steam'
alias update='sudo apt update && sudo apt upgrade'
alias ll='ls -la'
alias ports='ss -tulnp'
alias myip='ip a | grep inet'

# Apply without restarting terminal:
source ~/.bashrc

Useful real-world patterns

# Run command, and if it fails run something else
ping -c1 google.com && echo "Online" || echo "Offline"

# Kill orphaned processes and relaunch (what fixed Steam today)
pkill -f steam && flatpak run com.valvesoftware.Steam

# Log output of a command to a file with timestamp
echo "[$(date)] Starting backup" >> ~/backup.log
rsync -av ~/Documents/ /backup/ >> ~/backup.log 2>&1

# Watch a command every 2 seconds (like a live dashboard)
watch -n 2 free -h

# Find largest files in a directory
du -ah /home/james/ | sort -rh | head -20

# Check if a package is installed
dpkg -l | grep htop