Ubuntu 26.04 Tutorial / Change Service Configuration with Validation and Drop-ins
Change service configuration by recording the original and effective state, introducing a separate override, validating it, and checking real behavior. Reloading systemd units, reloading application configuration, and restarting a process are different operations.
Locate the original unit and drop-ins
Identify the real unit and inspect it with systemctl cat and systemctl show -p FragmentPath -p DropInPaths. Direct edits to packaged units under /usr/lib/systemd/system can be replaced during updates; separate local changes under /etc/systemd/system.
Create an independent lab service
This dedicated-VM lab leaves real server applications alone. If ubuntu-course-lab.service exists, choose a different name consistently. Noclobber prevents overwriting the existing file; stop if creation fails.
(
set -eu
sudo sh -c 'set -C; cat > /etc/systemd/system/ubuntu-course-lab.service' <<'EOF'
[Unit]
Description=Ubuntu course configuration lab
[Service]
Type=oneshot
Environment=LAB_MODE=baseline
ExecStart=/usr/bin/printenv LAB_MODE
EOF
sudo systemd-analyze verify /etc/systemd/system/ubuntu-course-lab.service
sudo systemctl daemon-reload
sudo systemctl start ubuntu-course-lab.service
journalctl -u ubuntu-course-lab.service --since '5 minutes ago' --no-pager
)Check for baseline in the journal and a successful result. A Type=oneshot unit may become inactive after successful completion; inactive alone is not a failure. Inspect systemctl show ubuntu-course-lab.service -p Result as well.
Change one value through a drop-in
(
set -eu
sudo mkdir -p /etc/systemd/system/ubuntu-course-lab.service.d
sudo sh -c 'set -C; cat > /etc/systemd/system/ubuntu-course-lab.service.d/10-lab.conf' <<'EOF'
[Service]
Environment=LAB_MODE=changed
EOF
sudo systemd-analyze verify /etc/systemd/system/ubuntu-course-lab.service
sudo systemctl daemon-reload
sudo systemctl start ubuntu-course-lab.service
journalctl -u ubuntu-course-lab.service --since '5 minutes ago' --no-pager
)Leave the original unit value intact and override it with changed. Compare merged configuration and the new execution log. Unit verification does not validate every application-specific setting, file access, or network response.
Distinguish three reload operations
| Command | Role |
|---|---|
| systemctl daemon-reload | Reloads systemd unit and dependency definitions |
| systemctl reload unit | Invokes the application’s supported reload operation |
| systemctl restart unit | Stops and starts the process |
For example, validate Nginx configuration with nginx -t before its supported reload. Reloading systemd alone does not change an existing process’s environment. Choose the application operation after reviewing downtime and fresh-connection tests.
The systemd.unit reference explains unit lookup paths and drop-in precedence.
Reverse only this change
(
set -eu
sudo rm -- /etc/systemd/system/ubuntu-course-lab.service.d/10-lab.conf
sudo systemctl daemon-reload
sudo systemctl start ubuntu-course-lab.service
journalctl -u ubuntu-course-lab.service --since '5 minutes ago' --no-pager
)Confirm baseline appears again. This rm removes only the dedicated drop-in created for the exercise. Avoid a broad systemctl revert that also removes another administrator’s changes. To clean up the unit itself, verify its ownership and inactive state, remove only that lab file, and reload systemd.
