systemd and Services

systemctl start nginx           # start service
systemctl stop nginx            # stop service
systemctl restart nginx         # stop + start
systemctl reload nginx          # reload config (no downtime if supported)
systemctl status nginx          # status + recent log lines
systemctl enable nginx          # start at boot
systemctl disable nginx         # don't start at boot
systemctl enable --now nginx    # enable + start immediately
systemctl disable --now nginx   # disable + stop immediately
systemctl is-active nginx       # active / inactive (exit 0/1)
systemctl is-enabled nginx      # enabled / disabled
systemctl is-failed nginx       # failed / other

systemctl list-units                        # all active units
systemctl list-units --type=service         # services only
systemctl list-units --state=failed         # failed units
systemctl list-units --state=running        # running services
systemctl list-unit-files                   # all installed units + state
systemctl list-unit-files --type=service    # service unit files
systemctl list-dependencies nginx           # dependency tree

systemctl poweroff          # power off
systemctl reboot            # reboot
systemctl suspend           # suspend to RAM
systemctl hibernate         # suspend to disk
systemctl rescue            # single-user rescue mode
systemctl emergency         # minimal emergency shell
systemctl daemon-reload     # reload unit files after editing them
systemctl reset-failed      # clear failed state from all units
systemctl reset-failed nginx  # clear failed state for nginx

journalctl                          # all logs (oldest first)
journalctl -r                       # reverse (newest first)
journalctl -f                       # follow (tail -f equivalent)
journalctl -n 50                    # last 50 lines
journalctl -u nginx                 # logs for nginx only
journalctl -u nginx -f              # follow nginx logs
journalctl -u nginx --since today
journalctl --since "2024-01-15 10:00" --until "2024-01-15 11:00"
journalctl -p err                   # priority: emerg alert crit err warning notice info debug
journalctl -p err..crit             # range of priorities
journalctl -k                       # kernel messages only (dmesg)
journalctl -b                       # current boot
journalctl -b -1                    # previous boot
journalctl --list-boots             # list all boots
journalctl -o json-pretty           # JSON output
journalctl --disk-usage             # journal disk size
journalctl --vacuum-size=500M       # trim journal to 500 MB
journalctl --vacuum-time=2weeks     # delete logs older than 2 weeks

# /etc/systemd/system/myapp.service
[Unit]
Description=My Application
After=network.target
Requires=postgresql.service

[Service]
Type=simple
User=deploy
WorkingDirectory=/opt/myapp
ExecStart=/usr/bin/node server.js
ExecReload=/bin/kill -HUP $MAINPID
Restart=on-failure
RestartSec=5s
StandardOutput=journal
StandardError=journal
Environment=NODE_ENV=production
EnvironmentFile=/etc/myapp/env

[Install]
WantedBy=multi-user.target

systemctl daemon-reload             # required after creating/editing unit files
systemctl enable --now myapp        # enable + start

# /etc/systemd/system/backup.timer
[Unit]
Description=Daily Backup Timer

[Timer]
OnCalendar=daily
Persistent=true

[Install]
WantedBy=timers.target

# /etc/systemd/system/backup.service
[Unit]
Description=Backup Service

[Service]
Type=oneshot
ExecStart=/usr/local/bin/backup.sh

systemctl enable --now backup.timer
systemctl list-timers                   # list all active timers

systemctl edit nginx            # create drop-in override (recommended)
# Creates /etc/systemd/system/nginx.service.d/override.conf

systemctl edit --full nginx     # edit full unit copy
systemctl cat nginx             # show effective unit file + overrides

systemctl get-default                       # show default target
systemctl set-default multi-user.target     # boot into multi-user (no GUI)
systemctl isolate rescue.target             # switch target now

systemctl --user status myservice       # user-scoped service
systemctl --user enable --now myservice
journalctl --user -u myservice
loginctl enable-linger alice            # run user services after logout

systemd-analyze                         # total boot time
systemd-analyze blame                   # per-unit time, sorted
systemd-analyze critical-chain          # critical path
systemd-analyze plot > boot.svg         # visual timeline