Service Management

# Systemd is PID 1
$ ps -p 1 -o comm=
systemd

# Check if a service is running
$ systemctl status nginx
● nginx.service - A high performance web server and reverse proxy server
     Loaded: loaded (/lib/systemd/system/nginx.service; enabled; vendor preset: enabled)
     Active: active (running) since Thu 2026-04-10 09:05:00 UTC; 2h 15min ago
       Docs: man:nginx(8)
    Process: 1234 ExecStart=/usr/sbin/nginx -g daemon on; master_process on; (code=exited, status=0/SUCCESS)
   Main PID: 1256 (nginx)
      Tasks: 5 (limit: 4915)
     Memory: 5.2M
        CPU: 245ms
     CGroup: /system.slice/nginx.service
             └─1256 nginx: master process /usr/sbin/nginx -g daemon on; master_process on;
               ├─1257 nginx: worker process
               ├─1258 nginx: worker process
               └─1259 nginx: worker process

# Check if service is enabled (starts on boot)
$ systemctl is-enabled nginx
enabled
 
# Check if service is active (currently running)
$ systemctl is-active nginx
active

# Start a service immediately
$ sudo systemctl start nginx
 
# Stop a running service
$ sudo systemctl stop nginx
 
# Restart a service (stop + start)
$ sudo systemctl restart nginx
 
# Reload configuration without interrupting connections
$ sudo systemctl reload nginx
 
# Reload or restart (restarts if reload isn't supported)
$ sudo systemctl reload-or-restart nginx

# Enable service to start on boot
$ sudo systemctl enable nginx
Created symlink /etc/systemd/system/multi-user.target.wants/nginx.service → /lib/systemd/system/nginx.service.
 
# Disable service from starting on boot
$ sudo systemctl disable nginx
Removed /etc/systemd/system/multi-user.target.wants/nginx.service.
 
# Enable AND start immediately
$ sudo systemctl enable --now nginx
 
# Disable AND stop immediately
$ sudo systemctl disable --now nginx

# List all active services
$ systemctl list-units --type=service
  UNIT                        LOAD   ACTIVE SUB     DESCRIPTION
  nginx.service               loaded active running A high performance web server
  ssh.service                 loaded active running OpenSSH server daemon
  docker.service              loaded active running Docker Application Container
  postgresql.service          loaded active running PostgreSQL database server
 
# List only failed services
$ systemctl list-units --type=service --state=failed
  UNIT          LOAD   ACTIVE SUB    DESCRIPTION
  backup.service loaded failed failed Backup Service
 
# List enabled services that are NOT currently active
$ systemctl list-unit-files --type=service --state=enabled

# View all logs for a service
$ journalctl -u nginx.service
-- Journal begins at Thu 2026-04-10 08:00:00 UTC, ends at Thu 2026-04-10 11:30:00 UTC. --
Apr 10 09:05:00 server systemd[1]: Starting A high performance web server...
Apr 10 09:05:00 server nginx[1234]: 2026/04/10 09:05:00 [notice] 1234#1234: using inherited sockets...
Apr 10 09:05:00 server systemd[1]: Started A high performance web server.
 
# Follow logs in real-time (-f = follow)
$ journalctl -u nginx.service -f
 
# View last 50 lines
$ journalctl -u nginx.service -n 50
 
# View since last boot
$ journalctl -u nginx.service -b
 
# View logs from specific time range
$ journalctl -u nginx.service --since "2026-04-10 09:00:00"
$ journalctl -u nginx.service --since "1 hour ago"
$ journalctl -u nginx.service --since "today"
 
# View only error-level and worse
$ journalctl -p err -u nginx.service

# /etc/systemd/system/myapp.service
[Unit]
Description=My Application
Documentation=https://myapp.dev/docs
After=network.target    # Start after network is up
Wants=network.target
 
[Service]
Type=simple             # simple = one process; forking = spawns children
User=myapp              # Run as this user
Group=myapp
WorkingDirectory=/opt/myapp
ExecStart=/opt/myapp/bin/myapp --config /opt/myapp/config.yaml
ExecReload=/bin/kill -HUP $MAINPID    # Graceful reload signal
ExecStop=/bin/kill -TERM $MAINPID      # Graceful stop signal
Restart=on-failure      # Restart on crash
RestartSec=5            # Wait 5s before restarting
StandardOutput=journal   # Log to systemd journal
StandardError=journal
 
# Environment
EnvironmentFile=/opt/myapp/.env
 
[Install]
WantedBy=multi-user.target    # Start at boot (if enabled)

# Reload systemd to recognize new/changed unit files
$ sudo systemctl daemon-reload
 
# Start the service
$ sudo systemctl start myapp
 
# Enable at boot
$ sudo systemctl enable myapp
 
# Check status
$ systemctl status myapp
 
# View logs
$ journalctl -u myapp -f

# Always check status first — it shows the error
$ sudo systemctl status nginx
● nginx.service - A high performance web server
     Loaded: loaded (/lib/systemd/system/nginx.service; enabled)
     Active: failed (Result: exit-code) since Thu 2026-04-10 09:10:00 UTC; 5s ago
    Process: 5678 ExecStart=/usr/sbin/nginx -g daemon on; master_process on; (code=exited, status=1/FAILURE)
   Main PID: 5678 (code=exited, status=1/FAILURE)
 
Apr 10 09:10:00 server nginx[5678]: 2026/04/10 09:10:00 [emerg] 5678#5678: direct

# 1. Configuration error — test before starting
$ sudo nginx -t
nginx: [emerg] directive "worker_processes" is unexpected in /etc/nginx/nginx.conf:15
nginx: configuration file /etc/nginx/nginx.conf test failed
 
# 2. Port already in use
$ sudo systemctl start nginx
Job for nginx.service failed because the control process exited with error code.
See "systemctl status nginx.service" and "journalctl -xe" for details.
 
$ journalctl -u nginx | tail -20
Apr 10 09:10:00 server nginx[5678]: [emerg] 5678#5678: bind() to 0.0.0.0:80 failed (98: Address already in use)
 
# Check what's using the port
$ ss -tlnp | grep :80
 
# 3. Permission denied
$ journalctl -u nginx | grep "Permission denied"

# -xe shows logs with additional context around the error
$ journalctl -xe -u nginx
 
# Check kernel messages (sometimes hardware-related)
$ dmesg | tail -50