Linux Service文件

從service文件說起

之前的linux系統，從開機到啟動初始化進程，要經過:

BIOS -> Boot Loader -> 加載系統內核 -> 對內核初始化 -> 啟動初始化進程，提供工作環境

現有的systemd采用了並發啟動機制，提升了開機速度。它將原來的初始化進程改為了systemd管理下的目標.target，並用systemctl來管理服務。

service服務啟動關閉過程

1.設置httpd服務開機啟動
systemctl enable httpd

返回信息
Created symlink from /etc/systemd/system/multi-user.target.wants/httpd.service to /usr/lib/systemd/system/httpd.service.
可以看到這里是在/etc/下那個目錄創建了一個軟鏈接（symbolic link）從/usr/lib/...里，沒說反，下面有驗證

2.查看服務狀態
systemctl status httpd

返回信息
Active:inactive(dead)

3.啟動服務
systemctl start httpd

狀態更新為
active (running)
相比之前還多了

Main PID:1079(httpd)
  status:"Processing requests..."
   Tasks:6
  Memory:2.9M
  CGroup:...(每個httpd化進程占用的端口號和狀態)

4.停止httpd服務
systemctl stop httpd.service

再次查看status變回inactive了

● httpd.service - The Apache HTTP Server
   Loaded: loaded (/usr/lib/systemd/system/httpd.service; enabled; vendor preset: disabled)
   Active: inactive (dead) since Sat 2020-07-25 09:23:48 CST; 2s ago
     Docs: man:httpd(8)
           man:apachectl(8)
  Process: 3578 ExecStop=/bin/kill -WINCH ${MAINPID} (code=exited, status=0/SUCCESS)
  Process: 1079 ExecStart=/usr/sbin/httpd $OPTIONS -DFOREGROUND (code=exited, status=0/SUCCESS)
 Main PID: 1079 (code=exited, status=0/SUCCESS)
   Status: "Total requests: 0; Current requests/sec: 0; Current traffic:   0 B/sec"

如果停不下來了，用kill殺掉
systemctl kill httpd.service

5.前面設置了開機啟動，現在把它關掉
systemctl disable httpd

返回結果
Removed symlink /etc/systemd/system/multi-user.target.wants/httpd.service.
就是把創建的軟鏈接刪除了

6.[.server]文件里面是什么
cat httpd.server

[Unit]
Description=The Apache HTTP Server
After=network.target remote-fs.target nss-lookup.target    啟動順序
Documentation=man:httpd(8)
Documentation=man:apachectl(8)

[Service]
Type=notify
EnvironmentFile=/etc/sysconfig/httpd    讀取自己的環境參數
ExecStart=/usr/sbin/httpd $OPTIONS -DFOREGROUND    啟動進程時執行的命令
ExecReload=/usr/sbin/httpd $OPTIONS -k graceful    $OPTIONS來自上面環境參數文件
ExecStop=/bin/kill -WINCH ${MAINPID}
# We want systemd to give httpd some time to finish gracefully, but still want
# it to kill httpd after TimeoutStopSec if something went wrong during the
# graceful stop. Normally, Systemd sends SIGTERM signal right after the
# ExecStop, which would kill httpd. We are sending useless SIGCONT here to give
# httpd time to finish.
KillSignal=SIGCONT
PrivateTmp=true

[Install]
WantedBy=multi-user.target    表示httpd服務所在的Target(服務組)

它分為3個模塊:
[Unit]是啟動順序和依賴關系，如果有Wants或Requires字段代表依賴關系
[Service]是啟動的行為
[install]是如何安裝這個配置文件

.server文件還有很多東西，這里就針對httpd說一下，剩下的遇到了再研究

驗證：查看下步驟1.enable里的目錄

插一句，簡單說下軟鏈接的創建
ln -s file file_ln
-s 創建軟鏈接 沒有這個參數就是創建硬鏈接

接下來先去看下它操作的.service文件

cd /etc/systemd/system/multi-user.target.wants
ll

顯示為lrwxrwxrwx的鏈接文件，原文件在/usr/lib/systemd/system/里
再到/usr/lib/systemd/system/里，找到了httpd.service
-rw-r--r-- 1 root root 752 Nov 27 2019 httpd.service

參考博客
https://blog.csdn.net/Mr_Yang__/article/details/84133783Centos7之Systemd(Service文件)詳解