Linux Service文件

从service文件说起

之前的linux系统，从开机到启动初始化进程，要经过:

BIOS -> Boot Loader -> 加载系统内核 -> 对内核初始化 -> 启动初始化进程，提供工作环境

现有的systemd采用了并发启动机制，提升了开机速度。它将原来的初始化进程改为了systemd管理下的目标.target，并用systemctl来管理服务。

service服务启动关闭过程

1.设置httpd服务开机启动
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.
可以看到这里是在/etc/下那个目录创建了一个软链接（symbolic link）从/usr/lib/...里，没说反，下面有验证

2.查看服务状态
systemctl status httpd

返回信息
Active:inactive(dead)

3.启动服务
systemctl start httpd

状态更新为
active (running)
相比之前还多了

Main PID:1079(httpd)
  status:"Processing requests..."
   Tasks:6
  Memory:2.9M
  CGroup:...(每个httpd化进程占用的端口号和状态)

4.停止httpd服务
systemctl stop httpd.service

再次查看status变回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"

如果停不下来了，用kill杀掉
systemctl kill httpd.service

5.前面设置了开机启动，现在把它关掉
systemctl disable httpd

返回结果
Removed symlink /etc/systemd/system/multi-user.target.wants/httpd.service.
就是把创建的软链接删除了

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    启动顺序
Documentation=man:httpd(8)
Documentation=man:apachectl(8)

[Service]
Type=notify
EnvironmentFile=/etc/sysconfig/httpd    读取自己的环境参数
ExecStart=/usr/sbin/httpd $OPTIONS -DFOREGROUND    启动进程时执行的命令
ExecReload=/usr/sbin/httpd $OPTIONS -k graceful    $OPTIONS来自上面环境参数文件
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服务所在的Target(服务组)

它分为3个模块:
[Unit]是启动顺序和依赖关系，如果有Wants或Requires字段代表依赖关系
[Service]是启动的行为
[install]是如何安装这个配置文件

.server文件还有很多东西，这里就针对httpd说一下，剩下的遇到了再研究

验证：查看下步骤1.enable里的目录

插一句，简单说下软链接的创建
ln -s file file_ln
-s 创建软链接 没有这个参数就是创建硬链接

接下来先去看下它操作的.service文件

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

显示为lrwxrwxrwx的链接文件，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

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