runlevel和init及Ubuntu20.04修改启动runlevel

程序员文章站 2022-07-14 13:38:38

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runlevel

linux操作系统自从开始启动至启动完毕需要经历几个不同的阶段，这几个阶段就叫做runlevel

 "Runlevels" are an obsolete way to start and stop groups of services used in SysV init.

使用runlevel查看当前系统运行的

[email protected]:~$ runlevel --help
runlevel [OPTIONS...]

Prints the previous and current runlevel of the init system.

Options:
     --help      Show this help

See the runlevel(8) man page for details.

Mapping between runlevels and systemd targets
       ┌─────────┬───────────────────┐
       │Runlevel │ Target            │
       ├─────────┼───────────────────┤
       │0        │ poweroff.target   │
       ├─────────┼───────────────────┤
       │1        │ rescue.target     │
       ├─────────┼───────────────────┤
       │2, 3, 4  │ multi-user.target │
       ├─────────┼───────────────────┤
       │5        │ graphical.target  │
       ├─────────┼───────────────────┤
       │6        │ reboot.target     │
       ├─────────┼───────────────────┤

简介

0 停机
1 单用户模式
2 多用户，没有 NFS
3 完全多用户模式（命令行）
4 没有用到
5 图形界面
6 重新启动
S s Single user mode
用到最多的时3和5还有0
系统默认安装后，若有图形界面则，默认进入 level 5
level 3 是最常用的命令行模式
关机时为level 0
更改密码或修复系统用到单用户模式level 0

init

init命令是Linux下的进程初始化工具，init进程是所有Linux进程的父进程，它的进程号为1。init命令是Linux操作系统中不可缺少的程序之一，init进程是Linux内核引导运行的，是系统中的第一个进程。

 systemd, init - systemd system and service manager

使用init切换用户模式

init 0 执行关机动作，需root权限
init 3 切换至命令行界面
init 5 切换至图形界面
init -s 切换至单用户模式

设置Linux启动时的runlevel

CentOS

[22:09:38 [email protected] ~]#cat /etc/inittab
# inittab is no longer used.
#
# ADDING CONFIGURATION HERE WILL HAVE NO EFFECT ON YOUR SYSTEM.
#
# Ctrl-Alt-Delete is handled by /usr/lib/systemd/system/ctrl-alt-del.target
#
# systemd uses 'targets' instead of runlevels. By default, there are two main targets:
#
# multi-user.target: analogous to runlevel 3
# graphical.target: analogous to runlevel 5
#
# To view current default target, run:
# systemctl get-default
#
# To set a default target, run:
# systemctl set-default TARGET.target

Ubuntu

[email protected]:~$ cat /run/utmp
~~~reboot5.4.0-42-generic{?Y`	35~~~runlevel5.4.0-42-genericGY`®7¤tty1tty1tty1LOGIN¤?Y`=	¶pts/0ts/[email protected]`^® 
Epts/1ts/[email protected]`

上面那个就是看看，配置文件在哪我暂时没找到，等找到了再发过来。

设置开机命令行

sudo systemctl enable multi-user.target
sudo systemctl set-default multi-user.target
或者
sudo systemctl set-default runlevel3.target

-设置开机图形

sudo systemctl enable graphical.target
sudo systemctl set-default graphical.target
或者
sudo systemctl set-default graphical.target

Ubuntu 20.04 设置开机启动由图形界面改为直接进命令行

[email protected]:~$ cat /etc/init/lightdm.conf 
cat: /etc/init/lightdm.conf: No such file or directory
[email protected]:~$ sudo systemctl enable multi-user.target
[sudo] password for u20: 
The unit files have no installation config (WantedBy=, RequiredBy=, Also=,
Alias= settings in the [Install] section, and DefaultInstance= for template
units). This means they are not meant to be enabled using systemctl.
 
Possible reasons for having this kind of units are:
• A unit may be statically enabled by being symlinked from another unit's
  .wants/ or .requires/ directory.
• A unit's purpose may be to act as a helper for some other unit which has
  a requirement dependency on it.
• A unit may be started when needed via activation (socket, path, timer,
  D-Bus, udev, scripted systemctl call, ...).
• In case of template units, the unit is meant to be enabled with some
  instance name specified.
[email protected]:~$ sudo systemctl set-default multi-user.target
Created symlink /etc/systemd/system/default.target → /lib/systemd/system/multi-user.target. ##实际上是更改了软链接
[email protected]:~$ runlevel
N 3

经过实验发现，Ubuntu 20启动runlevel修改方式并不保存在配置文件中，而是通过更改软连接的方式实现了启动runlevel的修改。