systemd — Service Management

# Service lifecycle
systemctl start nginx          # start now
systemctl stop nginx           # stop now
systemctl restart nginx        # stop then start
systemctl reload nginx         # reload config without full restart (if supported)
systemctl enable nginx         # start at boot
systemctl disable nginx        # don't start at boot
systemctl enable --now nginx   # enable + start in one command

# Status and inspection
systemctl status nginx
# ● nginx.service - A high performance web server
#      Loaded: loaded (/lib/systemd/system/nginx.service; enabled)
#      Active: active (running) since Mon 2024-01-15 10:00:00 UTC; 2h 5min ago
#    Main PID: 1234 (nginx)
#     CGroup: /system.slice/nginx.service
#             ├─1234 nginx: master process
#             â””─1235 nginx: worker process

# Is the service active/enabled?
systemctl is-active nginx      # "active" or "inactive"
systemctl is-enabled nginx     # "enabled" or "disabled"

# System-wide
systemctl list-units --type=service          # all loaded services
systemctl list-units --type=service --failed # failed services
systemctl list-unit-files --type=service     # all installed service files

# Boot analysis
systemd-analyze time          # total boot time
systemd-analyze blame         # which services took longest to start
systemd-analyze critical-chain nginx  # dependency chain for nginx

# Basic log queries
journalctl                            # all logs (oldest first, paged)
journalctl -r                         # reverse (newest first)
journalctl -f                         # follow (like tail -f)
journalctl -n 100                     # last 100 lines

# Filter by service
journalctl -u nginx                   # nginx logs only
journalctl -u nginx -f                # follow nginx logs
journalctl -u nginx --since "1 hour ago"
journalctl -u nginx --since "2024-01-15" --until "2024-01-16"

# Filter by priority
journalctl -p err                     # errors and above
journalctl -p warning -u nginx        # warnings+ for nginx

# Boot logs
journalctl -b                         # logs from current boot
journalctl -b -1                      # logs from previous boot
journalctl -b --list-boots            # list all boots

# Follow multiple services
journalctl -u nginx -u postgresql -f

# Disk usage and rotation
journalctl --disk-usage               # how much disk logs use
journalctl --vacuum-size=500M         # keep only 500 MB of logs
journalctl --vacuum-time=30d          # keep only 30 days of logs

# /etc/systemd/system/myapp.service
[Unit]
Description=My Node.js Application
After=network.target postgresql.service     # start after network and DB are up
Requires=postgresql.service                 # fail if postgres is not running

[Service]
Type=simple                     # process stays in foreground (most apps)
User=appuser                    # run as non-root service account
Group=appuser
WorkingDirectory=/opt/myapp

# Environment
Environment=NODE_ENV=production
Environment=PORT=3000
EnvironmentFile=/etc/myapp/env  # load from file (keep secrets out of unit file)

# Start and stop
ExecStart=/usr/bin/node /opt/myapp/server.js
ExecStop=/bin/kill -TERM $MAINPID
ExecReload=/bin/kill -HUP $MAINPID

# Restart policy
Restart=on-failure              # restart if exits with non-zero code
RestartSec=5                    # wait 5 seconds before restarting
StartLimitIntervalSec=60        # within 60 seconds
StartLimitBurst=3               # if it fails 3 times, stop trying

# Resource limits
LimitNOFILE=65536               # max open file descriptors
MemoryMax=512M                  # cgroup memory limit

# Security
NoNewPrivileges=true
PrivateTmp=true                 # isolated /tmp

[Install]
WantedBy=multi-user.target      # enable for normal multi-user boot

# Deploy the unit file:
# sudo cp myapp.service /etc/systemd/system/
# sudo systemctl daemon-reload     ← reload unit files after changes
# sudo systemctl enable --now myapp