CIS120 Book

CIS120 Linux Fundamentals by Scott Shaper

systemctl Basics

Think of systemctl as the control panel for system services. Just as you might use a remote to turn a TV on or off or change channels, systemctl lets you start, stop, restart, and check the status of services. It also lets you enable or disable services so they start (or don't start) automatically when the computer boots. Most systemctl commands that change something—like starting or stopping a service—require root privileges, so you'll often use sudo systemctl. This lesson covers the basics you need for everyday use on an Ubuntu Server system.

Quick Reference

Command	What It Does	Common Use
`systemctl start name`	Start a service	Starting a web server or other service
`systemctl stop name`	Stop a service	Stopping a service cleanly
`systemctl restart name`	Restart a service	Applying config changes or recovering from a problem
`systemctl status name`	Show service status and recent log	Checking if a service is running or why it failed
`systemctl enable name`	Enable service to start at boot	Making a service start automatically
`systemctl disable name`	Disable service from starting at boot	Stopping a service from auto-starting
`systemctl list-units --type=service`	List loaded services	Seeing what services are running

Note: Starting, stopping, enabling, and disabling services usually require root. Use sudo systemctl .... Checking status often works without sudo.

When to Use systemctl

You need to start, stop, or restart a service (e.g. after changing its config)
You want a service to start automatically when the system boots
You want to see whether a service is running and read its recent messages
You're troubleshooting why something (SSH, web server, etc.) isn't working

Starting, Stopping, and Restarting Services

You use the service name (without .service) with systemctl. On Ubuntu Server, common service names include ssh (SSH server), nginx or apache2 (web server), and cron (scheduled tasks). You can say systemctl start ssh and systemd will find the matching unit (e.g. ssh.service).

Command	What It Does	When to Use It
`start`	Start the service now	When the service isn't running and you want it to run
`stop`	Stop the service	When you want to shut it down cleanly
`restart`	Stop and then start the service	After changing config or to "reset" the service
`reload`	Reload config without full restart	When the service supports reloading config without dropping connections

Practical Examples

# Start the SSH service (on Ubuntu the service is named ssh)
sudo systemctl start ssh

# Stop a service
sudo systemctl stop nginx

# Restart a service (e.g. after editing its config file)
sudo systemctl restart nginx

# Reload config without stopping the service (if supported)
sudo systemctl reload nginx

Checking Service Status

systemctl status servicename shows whether the service is running, stopped, or failed, and prints a few recent log lines. You usually don't need sudo to run status.

Status Examples

# Check if SSH service is running (on Ubuntu the service is named ssh)
systemctl status ssh

# Check a service by full name
systemctl status nginx.service

# Status shows: active (running), inactive (dead), failed, etc.
# It also shows recent log output for that service

Enabling and Disabling Services (Start at Boot)

"Enable" means "start this service when the system boots into its default target." "Disable" means "don't start it at boot." Enabling or disabling doesn't start or stop the service right now—it only affects what happens at the next boot. You often need sudo to enable or disable.

Command	What It Does	When to Use It
`enable`	Start this service at boot	When you want the service to run whenever the system starts
`disable`	Don't start this service at boot	When you don't want it to auto-start
`is-enabled name`	Show whether the service is enabled	Checking if a service will start at boot

Enable and Disable Examples

# Enable SSH to start at boot (on Ubuntu the service is named ssh)
sudo systemctl enable ssh

# Disable a service from starting at boot
sudo systemctl disable nginx

# Check if a service is enabled
systemctl is-enabled ssh
# Output: enabled  or  disabled

Listing Services

To see which services exist and whether they're running, use systemctl list-units. You can filter by type (e.g. only services) and by state (e.g. only active).

Command	What It Does	When to Use It
`list-units --type=service`	List loaded service units	See what services are currently loaded
`list-units --type=service --all`	List all service units (including inactive)	Find a service name or see everything installed
`list-unit-files --type=service`	List unit files and enable state	See which services are enabled at boot

Listing Examples

# List currently loaded/active services
systemctl list-units --type=service

# List all service units (active and inactive)
systemctl list-units --type=service --all

# List service unit files and whether they're enabled
systemctl list-unit-files --type=service

# Filter with grep (e.g. find ssh)
systemctl list-units --type=service --all | grep ssh

Tips for Success

Use systemctl status servicename first when something isn't working—it shows state and recent errors
Remember: enable/disable affect boot; start/stop affect the current run
Service names on Ubuntu are often the program or package name (e.g. ssh, nginx, apache2, cron); use list-units --all to find the exact name
After changing a unit file, run sudo systemctl daemon-reload before start or restart

Common Mistakes to Avoid

Forgetting sudo when starting, stopping, enabling, or disabling—you'll get "Permission denied" or similar
Confusing enable (start at boot) with start (start now)
Using the full filename (e.g. ssh.service) when the short name (ssh) works and is simpler
Editing unit files and then starting the service without running daemon-reload first

Best Practices

Use systemctl stop and systemctl start (or restart) instead of kill for services
Check status after starting or restarting to confirm the service is running and healthy
Enable only services you need at boot to keep startup fast and reduce exposure
Use reload instead of restart when the service supports it and you only changed config