Linux – Systemd Service Type (Auto Restart Program)

透過自己寫 Systemd Unit Files 方式來取代傳統的 (crontab) cron table 來啟動程式,執行一段時間卻發生程式莫名死掉了.

main process exited, code=killed, status=9/KILL 

下面使用的 systemd 設定檔可以避免這個問題.

測試環境 CentOS 8 x86_64 (虛擬機)

程式如下.寫了一支會定期 60 秒寫入時間資料到檔案的 c++ 程式.

[root@localhost ~]# vi test.cpp 
#include <fstream>
#include <iostream>
#include <chrono>
#include <ctime> 
#include <unistd.h>

using namespace std;

int main() {
while(true)
{
    ofstream myFile_Handler;
    // File Open
    myFile_Handler.open("/tmp/1.txt", std::ios_base::app);

    auto timenow = chrono::system_clock::to_time_t(chrono::system_clock::now());
  
    // Write to the file
    myFile_Handler << ctime(&timenow) << endl;

    // File Close
    myFile_Handler.close();
   
    // Sleep
    sleep(60);
}
}

編譯成可執行檔.

[root@localhost ~]# g++ test.cpp -o test

程式放到 /sbin/ 路徑.

[root@localhost ~]# cp test /sbin/

開始編輯 Systemd Unit Files (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=simple
ExecStart=/sbin/test
Restart=always
RestartSec=5s

[Install]
WantedBy=multi-user.target

只簡單設定3個區塊

1. [unit]

   Description=Test Job
   

   Description 敘述該 Systemd Unit Files 目的.

2. [Service]

   Type=simple
   ExecStart=/sbin/test
   

   Type=simple – A long-running process that does not background its self and stays attached to the shell.
   ExecStart – 指定執行程式.

   這邊會確保當程式死掉後 5 秒後會自動啟動.
   Restart=always
   RestartSec=5s

3. [Install]

   WantedBy=multi-user.target
   

   指定哪一個 runlevel 執行.

啟動服務

[root@localhost ~]# systemctl enable test.service
Created symlink /etc/systemd/system/multi-user.target.wants/test.service → /etc/systemd/system/test.service.
[root@localhost ~]# systemctl start test.service
[root@localhost ~]# systemctl status test.service
● test.service - Test Job
   Loaded: loaded (/etc/systemd/system/test.service; enabled; vendor preset: disabled)
   Active: active (running) since Fri 2022-11-25 11:02:40 CST; 7s ago
 Main PID: 89850 (test)
    Tasks: 1 (limit: 49322)
   Memory: 292.0K
   CGroup: /system.slice/test.service
           └─89850 /sbin/test

Nov 25 11:02:40 localhost.localdomain systemd[1]: Started Test Job.

檢視 /sbin/test (定期 60 秒寫入時間資料到檔案 /tmp/1.txt) 是否有正常執行.

[root@localhost ~]# cat /tmp/1.txt
Fri Nov 25 11:02:40 2022

Fri Nov 25 11:03:40 2022

Fri Nov 25 11:04:40 2022

測試一下程式自動 restart 功能.手動把程式刪除.

[root@localhost ~]# ps -aux | grep -i test
root       64661  0.0  0.0  13780  1824 ?        Ss   10:51   0:00 /sbin/test
root       64663  0.0  0.0  12136  1108 pts/0    R+   10:51   0:00 grep --color=auto -i test
[root@localhost ~]# kill 64661
[root@localhost ~]# ps -aux | grep -i test
root       64669  0.0  0.0  12136  1144 pts/0    S+   10:51   0:00 grep --color=auto -i test

等待 5 秒後再檢查一下程式是否自動啟動,程式已自動啟動.

[root@localhost ~]# ps -aux | grep -i test
root       64672  0.0  0.0  13780  1948 ?        Ss   10:51   0:00 /sbin/test
root       64674  0.0  0.0  12136  1096 pts/0    S+   10:51   0:00 grep --color=auto -i test