Lesson 9: systemd & Services
Lesson 9: systemd & Services
systemd Overview
systemd is the init system (PID 1) and service manager on modern Linux. It manages:
- Services (.service) — daemons
- Sockets (.socket) — IPC/listen
- Timers (.timer) — cron replacement
- Targets (.target) — boot milestones (like runlevels)
- Paths (.path) — filesystem monitoring
Service Management
systemctl status nginx # Status (active, enabled, logs)
systemctl start nginx # Start now
systemctl stop nginx # Stop now
systemctl restart nginx # Stop + start
systemctl reload nginx # Reload config (SIGHUP)
systemctl enable nginx # Auto-start at boot
systemctl disable nginx # No auto-start
systemctl is-enabled nginx # Check enabled state
systemctl mask nginx # Prevent start entirely
systemctl unmask nginx # Unmask
Listing Units
systemctl list-units --type=service # All loaded services
systemctl list-units --type=service --state=running # Running only
systemctl list-unit-files --type=service # All unit files (enabled/disabled)
systemctl list-dependencies nginx # Show dependencies
systemctl list-dependencies --reverse nginx # What depends on nginx
Logs with journalctl
journalctl -u nginx # All logs for nginx
journalctl -f -u nginx # Follow live
journalctl -u nginx -n 50 # Last 50 lines
journalctl -u nginx --since "1 hour ago" # Time range
journalctl -u nginx --since "2024-11-01" --until "2024-11-02"
journalctl -p err -u nginx # Only errors
journalctl -xeu nginx # Verbose with explanations
Boot Targets (Runlevels)
| Target | Description | Runlevel Equivalent |
|---|---|---|
poweroff.target |
Shutdown | 0 |
rescue.target |
Single-user | 1 |
multi-user.target |
Multi-user, no GUI | 3 |
graphical.target |
Multi-user + GUI | 5 |
reboot.target |
Reboot | 6 |
systemctl get-default # Current default target
systemctl set-default multi-user.target # Change default
systemctl isolate multi-user.target # Switch now
Unit File Structure
# /etc/systemd/system/myapp.service
[Unit]
Description=My Application
After=network.target
Requires=network.target
[Service]
Type=simple
User=ubuntu
WorkingDirectory=/home/ubuntu/myapp
ExecStart=/home/ubuntu/myapp/run.sh
Restart=on-failure
RestartSec=5
Environment=NODE_ENV=production
[Install]
WantedBy=multi-user.target
Key Directives
| Directive | Purpose |
|---|---|
Type=simple |
Default; process runs in foreground |
Type=forking |
Daemon forks; PID file needed |
Type=oneshot |
Runs once, exits (for timers) |
Restart=always\|on-failure\|on-abnormal |
Auto-restart policy |
User=ubuntu |
Run as specific user |
WorkingDirectory= |
Set cwd |
Environment= |
Set env vars |
ExecStartPre= |
Command before start |
ExecStartPost= |
Command after start |
Creating Custom Service
# 1. Create unit file
sudo tee /etc/systemd/system/myapp.service > /dev/null <<'EOF'
[Unit]
Description=My App
After=network.target
[Service]
Type=simple
User=ubuntu
WorkingDirectory=/home/ubuntu/myapp
ExecStart=/home/ubuntu/myapp/venv/bin/python app.py
Restart=on-failure
RestartSec=5
[Install]
WantedBy=multi-user.target
EOF
# 2. Reload systemd
sudo systemctl daemon-reload
# 3. Enable & start
sudo systemctl enable --now myapp
# 4. Check
systemctl status myapp
journalctl -u myapp -f
Timers (Cron Replacement)
# /etc/systemd/system/backup.timer
[Unit]
Description=Daily backup timer
[Timer]
OnCalendar=daily
Persistent=true
RandomizedDelaySec=1h
[Install]
WantedBy=timers.target
sudo systemctl enable --now backup.timer
systemctl list-timers
Practice
# 1. Check nginx status
systemctl status nginx
# 2. View recent nginx logs
journalctl -u nginx -n 20
# 3. List running services
systemctl list-units --type=service --state=running
# 4. Create a simple timer service
cat > /tmp/test-timer.service <<'EOF'
[Unit]
Description=Test Timer
[Service]
Type=oneshot
ExecStart=/bin/date >> /tmp/timer.log
EOF
cat > /tmp/test-timer.timer <<'EOF'
[Unit]
Description=Run test every minute
[Timer]
OnCalendar=*:*:00
Persistent=true
[Install]
WantedBy=timers.target
EOF
sudo cp /tmp/test-timer.* /etc/systemd/system/
sudo systemctl daemon-reload
sudo systemctl enable --now test-timer.timer
systemctl list-timers
# Wait a minute, check /tmp/timer.log
💻 实战命令速查
systemctl status nginx
Show service status
$ systemctl status nginx
● nginx.service - A high performance web server
Loaded: loaded (/lib/systemd/system/nginx.service; enabled; vendor preset: enabled)
Active: active (running) since Fri 2024-11-01 10:00:00 UTC; 5min agosystemctl start nginx
Start service now
$ sudo systemctl start nginxsystemctl stop nginx
Stop service now
$ sudo systemctl stop nginxsystemctl restart nginx
Restart service
$ sudo systemctl restart nginxsystemctl reload nginx
Reload config without restart
$ sudo systemctl reload nginxsystemctl enable nginx
Enable auto-start at boot
$ sudo systemctl enable nginx
Created symlink /etc/systemd/system/multi-user.target.wants/nginx.service → /lib/systemd/system/nginx.servicesystemctl disable nginx
Disable auto-start at boot
$ sudo systemctl disable nginxsystemctl is-enabled nginx
Check if enabled
$ systemctl is-enabled nginx
enabledsystemctl list-units --type=service --state=running
List running services
$ systemctl list-units --type=service --state=running | head -10
UNIT LOAD ACTIVE SUB DESCRIPTION
nginx.service loaded active running A high performance web server
ssh.service loaded active running OpenBSD Secure Shell serversystemctl list-unit-files --type=service
List all service unit files (enabled/disabled)
$ systemctl list-unit-files --type=service | grep nginx
nginx.service enabledjournalctl -u nginx
View logs for specific service
$ journalctl -u nginx
-- Logs begin at Fri 2024-11-01 10:00:00 UTC --
Nov 01 10:00:00 server systemd[1]: Starting nginx...
Nov 01 10:00:00 server nginx[1234]: nginx: configuration test is successfuljournalctl -f -u nginx
Follow service logs live
$ journalctl -f -u nginxjournalctl --since '1 hour ago'
Logs from last hour
$ journalctl --since '1 hour ago' --until '30 min ago'systemctl daemon-reload
Reload systemd after unit file changes
$ sudo systemctl daemon-reloadsystemctl cat nginx
Show unit file content
$ systemctl cat nginx
# /lib/systemd/system/nginx.service
[Unit]
Description=A high performance web server
After=network.target