Linux – Systemd Timers

測試環境為 CentOS 8 x86_64 (虛擬機)

參考文章 – https://wiki.archlinux.org/title/Systemd/Timers

Systemd unit configuration 檔案以 .timer 為結尾的就是用於控制 .service 文件或事件,建議透過這種方式來取代傳統的 (crontab) cron table,下面來看一下要如何編寫 .service 與 .timer 檔案.

參考範例 – https://unix.stackexchange.com/questions/198444/run-script-every-30-min-with-systemd

以下程式需要每 10 秒執行一次.

[root@localhost ~]# vi /tmp/1.sh
#!/bin/bash
echo `date` >> /tmp/2

首先編輯 Systemd unit configuration 檔案 (一般使用者寫的 service 檔案放在 /etc/systemd/system/ ,系統的放在 /usr/lib/systemd/system)

[root@localhost ~]# vi /etc/systemd/system/test.service
[Unit]
Description=test job

[Service]
Type=oneshot
ExecStart=/bin/bash /tmp/1.sh

說明:
[Unit]

1. Description 敘述該 Systemd Unit Files 目的.

[Service]

1. Type – oneshot : This type is similar to simple, but the process exits before starting consequent units.
2. ExecStart – 指定執行程式.

Note:
在 [Service] 區間可以透過 RuntimeMaxSec=8h 來指定程式需執行多久的時間後停止,時間單位請參考 https://www.freedesktop.org/software/systemd/man/latest/systemd.time.html#Parsing%20Time%20Spans 說明.

剛程式不會定期執行,還需編輯 .timer 檔案.

[root@localhost ~]# vi /etc/systemd/system/test.timer
[Unit]
Description=test

[Timer]
OnUnitActiveSec=10s
OnBootSec=10s

[Install]
WantedBy=timers.target

說明:

1. OnUnitActiveSec : Defines a timer relative to when the unit the timer unit is activating was last activated.
2. OnBootSec : Defines a timer relative to when the machine was booted up.
3. WantedBy : A list of units that weakly depend on the unit, 這邊就是須等待 timers.target 服務啟動後才會啟動.

   [root@localhost ~]# systemctl list-dependencies --before test.timer
   test.timer
   ● ├─test.service
   ● ├─shutdown.target
   ● └─timers.target
   

啟動 test.timer 服務.

[root@localhost ~]# systemctl daemon-reload

[root@localhost ~]# systemctl start test.timer

[root@localhost ~]# systemctl enable test.timer

檢視服務狀態.

[root@localhost ~]# systemctl status test
● test.service - test job
   Loaded: loaded (/etc/systemd/system/test.service; static; vendor preset: disabled)
   Active: activating (start) since Fri 2022-08-19 01:37:26 PDT; 2ms ago
 Main PID: 28234 (bash)
    Tasks: 0 (limit: 49280)
   Memory: 0B
   CGroup: /system.slice/test.service
           ├─28234 /bin/bash /tmp/1.sh
           └─28235 date

Aug 19 01:37:26 localhost.localdomain systemd[1]: Starting test job...

[root@localhost ~]# systemctl status test.timer
● test.timer - test
   Loaded: loaded (/etc/systemd/system/test.timer; disabled; vendor preset: disabled)
   Active: active (running) since Thu 2022-08-18 23:36:35 PDT; 2h 1min ago
  Trigger: n/a

Aug 18 23:36:35 localhost.localdomain systemd[1]: Started test.

檢視程式是否有正確執行.

[root@localhost ~]# systemctl list-timers --all
NEXT                         LEFT       LAST                         PASSED       UNIT                         ACTIVATES
Fri 2022-08-19 02:22:52 PDT  42min left Fri 2022-08-19 00:42:36 PDT  57min ago    dnf-makecache.timer          dnf-makecache.service
Sat 2022-08-20 00:00:00 PDT  22h left   Fri 2022-08-19 00:00:36 PDT  1h 39min ago unbound-anchor.timer         unbound-anchor.service
Sat 2022-08-20 01:33:36 PDT  23h left   Fri 2022-08-19 01:33:36 PDT  6min ago     systemd-tmpfiles-clean.timer systemd-tmpfiles-clean.service
n/a                          n/a        Fri 2022-08-19 01:39:53 PDT  4ms ago      test.timer                   test.service

4 timers listed.

資料的確有寫入到 /tmp/2 檔案.

[root@localhost ~]# cat /tmp/2
Thu Aug 18 23:36:35 PDT 2022
Thu Aug 18 23:36:56 PDT 2022
Thu Aug 18 23:37:56 PDT 2022
Thu Aug 18 23:38:56 PDT 2022
Thu Aug 18 23:39:36 PDT 2022
Thu Aug 18 23:39:56 PDT 2022
Thu Aug 18 23:40:56 PDT 2022