linux的服务管理

启动进程对比：

开机的自动启动
    init
    systemd
运行过程中的手动启动
    service
    systemctl
修改和查看开始的自动启动项
    chkconfig
    systemctl
启动脚本：
    /etc/init.d
    /usr/lib/systemd/system
启动级别：
    /etc/rc3.d
    /etc/systemd/system
默认运行级别：
    /etc/inittab
    /etc/systemd/system/default.target
切换级别：
    init 3
    systemctl isolate multi-user.target

systemctl管理

runlevel
init 0（systemctl poweroff）
init 6（systemctl reboot）
init 3（systemctl isolate multi-user.target）

systemctl get-default
systemctl set-default multi-user.target

systemctl enable httpd.service
systemctl disable httpd.service

systemctl start httpd.service
systemctl stop httpd.service
systemctl restart httpd.service

systemctl is-active httpd.service
systemctl is-enabled httpd.service
systemctl status httpd.service

systemctl list-units（针对的是汇总的服务）
systemctl list-unit-files | grep enabled（针对的是涉及到的所有文件）
systemctl list-unit-files --type=service | grep enabled（type为service,socket,target）

systemctl cat sshd.service
systemctl list-dependencies default.target