Systemd

Notes for managing services on RHEL/Linux systems.

Systemd is used to start, stop, restart, enable, disable, and troubleshoot services.

Basic service commands

Check service status

systemctl status SERVICE_NAME

Example:

systemctl status sshd

Start a service

sudo systemctl start SERVICE_NAME

Stop a service

sudo systemctl stop SERVICE_NAME

Restart a service

sudo systemctl restart SERVICE_NAME

Use restart carefully on production systems because it stops and starts the service.

Reload a service

sudo systemctl reload SERVICE_NAME

Reload is safer than restart when supported because it reloads configuration without fully stopping the service.

Not every service supports reload.

Enable service at boot

sudo systemctl enable SERVICE_NAME

Disable service at boot

sudo systemctl disable SERVICE_NAME

Check if service is enabled

systemctl is-enabled SERVICE_NAME

Check if service is active

systemctl is-active SERVICE_NAME

Failed services

Show failed services

systemctl --failed

Reset failed state

sudo systemctl reset-failed

For one service:

sudo systemctl reset-failed SERVICE_NAME

This does not fix the service. It only clears the failed state.

Service logs

Show logs for a service

journalctl -u SERVICE_NAME

Show latest service logs

journalctl -u SERVICE_NAME -n 100

Follow service logs live

journalctl -u SERVICE_NAME -f

Show service logs from current boot

journalctl -u SERVICE_NAME -b

Show service errors only

journalctl -u SERVICE_NAME -p err

List services

List running services

systemctl list-units --type=service --state=running

List all services

systemctl list-units --type=service --all

List service unit files

systemctl list-unit-files --type=service

Search for a service

systemctl list-unit-files | grep SERVICE_NAME

Example:

systemctl list-unit-files | grep ssh

Unit files

Show unit file content

systemctl cat SERVICE_NAME

Example:

systemctl cat sshd

Show service properties

systemctl show SERVICE_NAME

Show important service properties

systemctl show SERVICE_NAME -p ActiveState -p SubState -p MainPID -p ExecMainStatus

Reload systemd after unit changes

After creating or editing a systemd unit file:

sudo systemctl daemon-reload

Then restart or start the service:

sudo systemctl restart SERVICE_NAME

Common service states

active/running = service is running
inactive/dead = service is stopped
failed = service failed
activating = service is starting
deactivating = service is stopping
enabled = starts at boot
disabled = does not start at boot
masked = blocked from starting

Mask and unmask

Mask a service

sudo systemctl mask SERVICE_NAME

This prevents the service from being started manually or automatically.

Unmask a service

sudo systemctl unmask SERVICE_NAME

Restart a stuck service

Basic flow:

systemctl status SERVICE_NAME
journalctl -u SERVICE_NAME -n 100
sudo systemctl restart SERVICE_NAME
systemctl status SERVICE_NAME

If it fails again:

journalctl -u SERVICE_NAME -p err
journalctl -xe

Check what changed recently

Useful commands:

journalctl -u SERVICE_NAME --since "1 hour ago"
journalctl -u SERVICE_NAME --since "today"
systemctl status SERVICE_NAME

Also check configuration files if the service was edited recently.

Create a simple service file

Example service file:

[Unit]
Description=Example custom service
After=network.target

[Service]
Type=simple
User=randy
WorkingDirectory=/home/randy/example
ExecStart=/home/randy/example/start.sh
Restart=on-failure

[Install]
WantedBy=multi-user.target

Save it as:

/etc/systemd/system/example.service

Then run:

sudo systemctl daemon-reload
sudo systemctl enable example.service
sudo systemctl start example.service
systemctl status example.service

Common troubleshooting checklist

When a service fails:

1. Am I checking the correct service name?
2. Is the service active, inactive, or failed?
3. What does systemctl status show?
4. What do the last 100 journal lines show?
5. Did the config file change?
6. Is a required file missing?
7. Are permissions correct?
8. Is the port already in use?
9. Is the disk full?
10. Did the service fail after reboot?

First commands for service issues

systemctl status SERVICE_NAME
journalctl -u SERVICE_NAME -n 100
journalctl -u SERVICE_NAME -p err
systemctl --failed
df -h
ss -tulpn

Notes

Add service-specific notes here over time.