新发布的CentOS 7 中使用systemd服务代替了之前版本的SysV服务，对比下两种启动方式的不同。
修改系统启动级别
旧版
编辑配置文件/etc/inittab，设置启动级别为3 (多用户文字界面)，修改initdefault前面的数字为3，保存重启
新版
修改默认启动级别为3
systemctl enable multi-user.target
这个命令实际则是在目录 /etc/systemd/system 下创建了一个软链接
ln -s ‘/usr/lib/systemd/system/multi-user.target‘ ‘/etc/systemd/system/default.target‘
若修改默认启动级别为5，需要将之前的启动级别disable
systemctl disable multi-user.target
该命令实际删除了软链接default.target
rm ‘/etc/systemd/system/default.target‘
然后再启用启动级别5
systemctl enable graphical.target
实际创建了新的软链接
ln -s ‘/usr/lib/systemd/system/graphical.target‘ ‘/etc/systemd/system/default.target‘
当然你也可以跳过命令直接以创建软链接的方式来改变启动级别
应用程序的自启动项
当通过yum安装了httpd服务后，准备将其添加到自启动项里，
旧版
chkconfig httpd on
新版
systemctl enable httpd
实际是创建了软链接
ln -s ‘/usr/lib/systemd/system/httpd.service‘ ‘/etc/systemd/system/multi-user.target.wants/httpd.service‘
关掉httpd的自启动
systemctl disable httpd
实际软链接被删除
rm ‘/etc/systemd/system/multi-user.target.wants/httpd.service‘
关于启动级别3下面的服务启动项都在/etc/systemd/system/multi-user.target.wants目录下，而级别5的则在目录graphical.target.wants下面
我们打开这个链接文件 httpd.service 可以看到内容，就是有关httpd的启动脚本
[Unit]
Description=The Apache HTTP Server
After=network.target remote-fs.target nss-lookup.target
[Service]
Type=notify
EnvironmentFile=/etc/sysconfig/httpd
ExecStart=/usr/sbin/httpd $OPTIONS -DFOREGROUND
ExecReload=/usr/sbin/httpd $OPTIONS -k graceful
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关于服务的启动/关闭/重启
旧版
service httpd {start|stop|restart}新版
systemctl {start|stop|restart} httpd查看当前httpd的运行状态
systemctl status httpd
输出结果
httpd.service - The Apache HTTP Server
Loaded: loaded (/usr/lib/systemd/system/httpd.service; enabled)
Active: active (running) since Thu 2014-07-17 15:12:50 CST; 4s ago
Process: 2762 ExecStop=/bin/kill -WINCH ${MAINPID} (code=exited, status=0/SUCCESS)
Main PID: 2769 (httpd)
Status: "Processing requests..."
CGroup: /system.slice/httpd.service
?..2769 /usr/sbin/httpd -DFOREGROUND
?..2770 /usr/sbin/httpd -DFOREGROUND
?..2771 /usr/sbin/httpd -DFOREGROUND
?..2772 /usr/sbin/httpd -DFOREGROUND
?..2773 /usr/sbin/httpd -DFOREGROUND
?..2774 /usr/sbin/httpd -DFOREGROUND
Jul 17 15:12:50 localhost.localdomain systemd[1]: Starting The Apache HTTP Server...
Jul 17 15:12:50 localhost.localdomain httpd[2769]: AH00558: httpd: Could not reliably determine the server‘s fully qualified domain name, using localhost.localdomain. Set the ...his message
Jul 17 15:12:50 localhost.localdomain systemd[1]: Started The Apache HTTP Server.
Hint: Some lines were ellipsized, use -l to show in full查看所有服务自启动状态
旧版
chkconfig --list
新版
systemctl list-unit-files
以上是我对systemd的一个初步印象，需要一个逐步适应的过程，希望大家多多讨论交流。
本文出自 “努力为之” 博客，请务必保留此出处http://carllai.blog.51cto.com/1664997/1439562
初次体验CentOS 7的systemd,布布扣,bubuko.com
原文地址：http://carllai.blog.51cto.com/1664997/1439562
