add systemd examples

active/software_systemd/systemd.md

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+# Systemd
+
+## Timers
+
+Basic timer:
+
+my_service.timer
+
+```conf
+[Unit]
+Description=Run $my_service every hour
+
+[Timer]
+OnCalendar=hourly
+AccuracySec=10min
+Persistent=true
+Unit=$my_service.service
+
+[Install]
+WantedBy=timers.target
+```
+
+my_service.service
+
+```conf
+[Unit]
+Description=Runs some command
+After=syslog.target network.target auditd.service
+Wants=network-online.target
+
+[Service]
+ExecStart=/usr/bin/command -with -arguments
+
+[Install]
+WantedBy=multi-user.target
+```
+
+Create an on-the-fly timer (will not survive reboots)
+
+```bash
+systemd-run --user --on-calendar '*:0/1' /bin/sh -c "date >> ~/log.txt"
+```