systemd

centos7 编写测试服务
bak.sh脚本内容：

cp -a /etc/sysconfig /app/sysconfig-bak >> /dev/null

bak脚本要加执行权限

服务Unit文件示例

[Unit]
Description=backup /etc
Requires=atd.service
[Service]
Type=simple
ExecStart=/bin/bash -c "echo /lib/systemd/system/bak.sh|at now"
[Install]
WantedBy=multi-user.target

运行级别

ls /usr/lib/systemd/system/*.target

`运行级别：
    0 ==> runlevel0.target, poweroff.target
    1 ==> runlevel1.target, rescue.target
    2 ==> runlevel2.target, multi-user.target
    3 ==> runlevel3.target, multi-user.target
    4 ==> runlevel4.target, multi-user.target
    5 ==> runlevel5.target, graphical.target
    6 ==> runlevel6.target, reboot.target
    `

设置默认运行级别

systemctl set-default multi-user.target    
等同于建立软连接：
ln  -s  /usr/lib/systemd/system/multi-user.target   /etc/systemd/system/default.target

删除软连接使用原来默认运行级别
rm  -f   /etc/systemd/system/default.target