• 將二個單元檔（unit）置於 /lib/systemd/system
• 其中之一是 .service，是描述執行目的
• 另一是 .timer，是用以觸發該 service
• 此二者的名稱必須相同
• 描述時（被 systemd 解析時），必須使用絕對路徑
• 創建時，存取權限須留意
• 如下範例：
• 每隔 30 秒將當時時間寫入 time_records.log
• 而 service 是去執行 write_current_time.sh
• write_current_time.sh：

#! /usr/bin/env bash
echo "$(date)" >> /home/ken/tasks/time_records.log

• time_records.service：當啟用時，就會先被執行一次

[Unit]
    Description=now time service
[Service]
    ExecStart=/bin/bash /home/ken/tasks/write_current_time.sh
[Install]
    WantedBy=multi-user.target

• time_records.timer：

[Unit]
    Description=run service at every 30s
[Timer]
    OnUnitActiveSec=30s
    Unit=time_records
[Install]
    WantedBy=timers.target

• 備註：
  複合指令的用法例：
  ExecStart=/bin/sh -c ‘until ping -c1 google.com; do sleep 1; done;’
• 參考資料
• https://wiki.archlinux.org/title/Systemd/Timers
• https://man.archlinux.org/man/systemd.timer.5
• https://man.archlinux.org/man/systemd.service.5
• https://man.archlinux.org/man/systemd.1.zh_TW
• http://www.ruanyifeng.com/blog/2018/03/systemd-timer.html
• https://www.cnblogs.com/operationhome/p/10720067.html
• https://systemd.io/
• https://www.freedesktop.org/software/systemd/man/systemd.html
• https://www.freedesktop.org/software/systemd/man/systemd.directives.html
• https://www.freedesktop.org/software/systemd/man/systemd.unit.html
• https://www.freedesktop.org/software/systemd/man/systemd.service.html
• https://www.freedesktop.org/software/systemd/man/systemd.exec.html
• http://www.jinbuguo.com/systemd/systemd.service.html
• https://cloud.tencent.com/developer/article/1516125
• https://www.ruanyifeng.com/blog/2016/03/systemd-tutorial-commands.html
• https://www.ruanyifeng.com/blog/2016/03/systemd-tutorial-part-two.html
• https://www.digitalocean.com/community/tutorials/systemd-essentials-working-with-services-units-and-the-journal
• https://www.digitalocean.com/community/tutorials/understanding-systemd-units-and-unit-files
• https://www.cloudnull.io/2019/04/running-services-in-network-name-spaces-with-systemd/