1. 按下电源
2. BIOS自检
3. MBR引导
4. 进入GRUB菜单
5. 加载内核 Kernel
6. systemd init进程
7. 读取运行级别
[root@qls ~]# ll /etc/systemd/system/default.target
lrwxrwxrwx 1 root root 37 Jul 6 02:16 /etc/systemd/system/default.target -> /lib/systemd/system/multi-user.target
[root@qls ~]# ll /usr/lib/systemd/system/runlevel*.target
lrwxrwxrwx. 1 root root 15 Jul 6 02:14 /usr/lib/systemd/system/runlevel0.target -> poweroff.target
lrwxrwxrwx. 1 root root 13 Jul 6 02:14 /usr/lib/systemd/system/runlevel1.target -> rescue.target
lrwxrwxrwx. 1 root root 17 Jul 6 02:14 /usr/lib/systemd/system/runlevel2.target -> multi-user.target
lrwxrwxrwx. 1 root root 17 Jul 6 02:14 /usr/lib/systemd/system/runlevel3.target -> multi-user.target
lrwxrwxrwx. 1 root root 17 Jul 6 02:14 /usr/lib/systemd/system/runlevel4.target -> multi-user.target
lrwxrwxrwx. 1 root root 16 Jul 6 02:14 /usr/lib/systemd/system/runlevel5.target -> graphical.target
lrwxrwxrwx. 1 root root 13 Jul 6 02:14 /usr/lib/systemd/system/runlevel6.target -> reboot.target
8. 初始化系统
[root@qls ~]# ll /usr/lib/systemd/system/sysinit.target
-rw-r--r--. 1 root root 518 Oct 31 2018 /usr/lib/systemd/system/sysinit.target
9. 并行启动开机自启动的服务 串行启动
10. 运行getty文件，显示登陆界面
[root@qls ~]# ll /usr/lib/systemd/system/getty.target
-rw-r--r--. 1 root root 460 Oct 31 2018 /usr/lib/systemd/system/getty.target
Centos-6 和 Centos-7的区别
init管理进程 systemd管理进程
串行启动服务 并行启动服务
启动效率慢，容易故障 启动效率高，不容易出现故障
[root@qls ~]# ll /usr/lib/systemd/system/runlevel*.target
lrwxrwxrwx. 1 root root 15 Jul 6 02:14 /usr/lib/systemd/system/runlevel0.target -> poweroff.target
lrwxrwxrwx. 1 root root 13 Jul 6 02:14 /usr/lib/systemd/system/runlevel1.target -> rescue.target
lrwxrwxrwx. 1 root root 17 Jul 6 02:14 /usr/lib/systemd/system/runlevel2.target -> multi-user.target
lrwxrwxrwx. 1 root root 17 Jul 6 02:14 /usr/lib/systemd/system/runlevel3.target -> multi-user.target
lrwxrwxrwx. 1 root root 17 Jul 6 02:14 /usr/lib/systemd/system/runlevel4.target -> multi-user.target
lrwxrwxrwx. 1 root root 16 Jul 6 02:14 /usr/lib/systemd/system/runlevel5.target -> graphical.target
lrwxrwxrwx. 1 root root 13 Jul 6 02:14 /usr/lib/systemd/system/runlevel6.target -> reboot.target
0 poweroff #关机
1 rescue #单用户模式
2 multi-user #多用户模式
3 multi-user #多用户模式
4 multi-user #多用户模式
5 graphical #图形界面模式 桌面模式
6 reboot #重启
#如何查看系统运行级别
Centos-6
[root@qls ~]# runlevel
N 3
[root@qls ~]# init 5
[root@qls ~]# runlevel
3 5
[root@qls ~]# init 3
Centos-7
[root@qls ~]# systemctl get-default
multi-user.target
#修改运行级别
[root@qls ~]# systemctl set-default graphical.target
Removed symlink /etc/systemd/system/default.target.
Created symlink from /etc/systemd/system/default.target to /usr/lib/systemd/system/graphical.target.
[root@qls ~]# systemctl get-default
graphical.target
[root@qls ~]# systemctl set-default multi-user.target
Removed symlink /etc/systemd/system/default.target.
Created symlink from /etc/systemd/system/default.target to /usr/lib/systemd/system/multi-user.target.
[root@qls ~]# systemctl get-default
multi-user.target
systemd system daemon
#主配置文件目录
[root@qls ~]# ll /etc/systemd/system/ -d
drwxr-xr-x. 12 root root 4096 Aug 5 09:05 /etc/systemd/system/
#系统服务配置文件
[root@qls ~]# ll /usr/lib/systemd/system/ -d
drwxr-xr-x. 26 root root 12288 Jul 30 19:51 /usr/lib/systemd/system/
#程序，进程运行时产生的一些文件
[root@qls ~]# ll /run/systemd/system/ -d
drwxr-xr-x 5 root root 160 Aug 5 09:30 /run/systemd/system/
#启动
[root@qls ~]# systemctl start nginx
#停止
[root@qls ~]# systemctl stop nginx
#状态
[root@qls ~]# systemctl status nginx
#重启
[root@qls ~]# systemctl restart nginx
#平滑重启
[root@qls ~]# systemctl reload nginx
#禁用某个服务
[root@qls ~]# systemctl mask nginx
#解除禁用
[root@qls ~]# systemctl unmask nginx
#开机自启动
[root@qls ~]# systemctl enable nginx
#取消开机自启动
[root@qls ~]# systemctl disable nginx
status状态
active (running) #运行
Active: inactive (dead) #不在运行
disabled #开机不自启
enabled #开机自启
failed #失败
#显示当前运行级别下开机自启动的服务
[root@qls ~]# systemctl list-unit-files
#使用system进行管理关机重启操作
[root@qls ~]# systemctl reboot
[root@qls ~]# systemctl poweroff
#如何设置主机名
[root@qls ~]# hostname
qls
[root@qls ~]# hostnamectl
Static hostname: qls
Icon name: computer-vm
Chassis: vm
Machine ID: 6b11e94f1fe04eb89579c1c41f6f5307
Boot ID: 783be43c5a44420ead4bd26792a6d22c
Virtualization: vmware
Operating System: CentOS Linux 7 (Core)
CPE OS Name: cpe:/o:centos:centos:7
Kernel: Linux 3.10.0-957.el7.x86_64
Architecture: x86-64
[root@qls ~]# hostnamectl set-hostname web01
[root@qls ~]# logout
Connection closing...Socket close.
Connection closed by foreign host.
Disconnected from remote host(虚拟机-10.0.0.100) at 09:54:12.
Type `help‘ to learn how to use Xshell prompt.
[C:\~]$
Connecting to 10.0.0.100:22...
Connection established.
To escape to local shell, press ‘Ctrl+Alt+]‘.
Last login: Wed Aug 5 17:51:59 2020 from 10.0.0.1
[root@web01 ~]#
[root@web01 ~]# cat /etc/hostname
web01
Centos-6修改 临时生效
[root@web01 ~]# hostname qls
[root@web01 ~]# logout
Connection closing...Socket close.
Connection closed by foreign host.
Disconnected from remote host(虚拟机-10.0.0.100) at 09:55:10.
Type `help‘ to learn how to use Xshell prompt.
[C:\~]$
Connecting to 10.0.0.100:22...
Connection established.
To escape to local shell, press ‘Ctrl+Alt+]‘.
Last login: Wed Aug 5 09:54:16 2020 from 10.0.0.1
[root@qls ~]# cat /etc/hostname
web01
#如何修改字符集
[root@qls ~]# echo $LANG
en_US.UTF-8
[root@qls ~]# localectl set-locale LANG=zh_CN.UTF-8
[root@qls ~]# logout
Connection closing...Socket close.
Connection closed by foreign host.
Disconnected from remote host(虚拟机-10.0.0.100) at 09:57:21.
Type `help‘ to learn how to use Xshell prompt.
[C:\~]$
Connecting to 10.0.0.100:22...
Connection established.
To escape to local shell, press ‘Ctrl+Alt+]‘.
Last login: Wed Aug 5 09:55:13 2020 from 10.0.0.1
[root@qls ~]#
[root@qls ~]# cat /etc/locale.conf
LANG=zh_CN.UTF-8
[root@qls ~]# yum install -y sl
已加载插件：fastestmirror
Loading mirror speeds from cached hostfile
* base: mirrors.aliyun.com
* extras: mirrors.aliyun.com
* updates: mirrors.aliyun.com
软件包 sl-5.02-1.el7.x86_64 已安装并且是最新版本
无须任何处理
#临时修改变量
[root@qls ~]# export LANG=en_US.UTF-8
[root@qls ~]# echo $LANG
en_US.UTF-8
#系统日志管理
[root@qls ~]# journalctl -xe #查看当前服务的日志
[root@qls ~]# tail /var/log/messages
Aug 5 10:01:32 qls systemd: Stopped The nginx HTTP and reverse proxy server.
Aug 5 10:01:32 qls systemd: Starting The nginx HTTP and reverse proxy server...
Aug 5 10:01:32 qls nginx: nginx: [emerg] unknown directive "isten" in /etc/nginx/nginx.conf:39
Aug 5 10:01:32 qls nginx: nginx: configuration file /etc/nginx/nginx.conf test failed
Aug 5 10:01:32 qls systemd: nginx.service: control process exited, code=exited status=1
Aug 5 10:01:32 qls systemd: Failed to start The nginx HTTP and reverse proxy server.
Aug 5 10:01:32 qls systemd: Unit nginx.service entered failed state.
Aug 5 10:01:32 qls systemd: nginx.service failed.
Aug 5 10:02:01 qls systemd: Started Session 19 of user root.
Aug 5 10:03:01 qls systemd: Started Session 20 of user root.
#查看日志的最后20行内容
[root@qls ~]# journalctl -n 20
#实时查看日志的输出
[root@qls ~]# journalctl -f
#显示日志的级别日志
[root@qls ~]# journalctl -p
alert crit debug emerg err info notice warning
[root@qls ~]# journalctl -p err
-- Logs begin at Wed 2020-08-05 17:51:37 CST, end at Wed 2020-08-05 10:05:01 CST. --
Aug 05 17:51:41 qls kernel: sd 2:0:0:0: [sda] Assuming drive cache: write through
Aug 05 17:51:41 qls kernel: sd 2:0:1:0: [sdb] Assuming drive cache: write through
Aug 05 17:51:41 qls kernel: sd 2:0:2:0: [sdc] Assuming drive cache: write through
Aug 05 17:51:44 qls kernel: piix4_smbus 0000:00:07.3: SMBus Host Controller not enabled!
Aug 05 09:54:13 web01 sshd[7162]: error: Received disconnect from 10.0.0.1 port 50011:0:
Aug 05 09:55:11 qls sshd[7333]: error: Received disconnect from 10.0.0.1 port 50033:0:
Aug 05 09:57:22 qls sshd[7369]: error: Received disconnect from 10.0.0.1 port 50041:0:
Aug 05 09:59:29 qls sshd[7419]: error: Received disconnect from 10.0.0.1 port 50067:0:
Aug 05 10:00:51 qls sshd[7473]: error: Received disconnect from 10.0.0.1 port 50097:0:
Aug 05 10:01:32 qls systemd[1]: Failed to start The nginx HTTP and reverse proxy server.
#只查看某个服务的日志
[root@qls ~]# journalctl -u sshd
-- Logs begin at Wed 2020-08-05 17:51:37 CST, end at Wed 2020-08-05 10:06:01 CST. --
Aug 05 17:51:56 qls systemd[1]: Starting OpenSSH server daemon...
Aug 05 17:51:56 qls sshd[6981]: Server listening on 0.0.0.0 port 22.
Aug 05 17:51:56 qls sshd[6981]: Server listening on :: port 22.
Aug 05 17:51:56 qls systemd[1]: Started OpenSSH server daemon.
Aug 05 17:51:59 qls sshd[7162]: Accepted password for root from 10.0.0.1 port 50011 ssh2
Aug 05 09:54:16 web01 sshd[7333]: Accepted password for root from 10.0.0.1 port 50033 ssh2
Aug 05 09:55:12 qls sshd[7369]: Accepted password for root from 10.0.0.1 port 50041 ssh2
Aug 05 09:57:26 qls sshd[7419]: Accepted password for root from 10.0.0.1 port 50067 ssh2
Aug 05 09:59:31 qls sshd[7473]: Accepted password for root from 10.0.0.1 port 50097 ssh2
Aug 05 10:00:52 qls sshd[7525]: Accepted password for root from 10.0.0.1 port 50111 ssh2
忘记root密码
开机在grub界面输入 e
在linux16开头的行的最后面输入 init=/bin/bash enforcing=0
按 ctrl键 + x
重新挂载根分区 mount -o rw,remount /
修改密码
重启 exec /sbin/init
#运行级别被设置为重启
[root@web01 ~]# systemctl set-default reboot.target
Removed symlink /etc/systemd/system/default.target.
Created symlink from /etc/systemd/system/default.target to /usr/lib/systemd/system/reboot.target.
[root@web01 ~]# systemctl get-default
reboot.target
开机在grub界面输入 e
在linux16开头的行的最后面输入 rd.break
按 ctrl键 + x
重新挂载根分区 mount -o rw,remount /sysroot
赋予权限 chroot /sysroot
修改运行级别
exit
reboot
#系统损坏了，数据很重要 需要把数据拷贝出来
#MBR引导坏掉
dd if=/dev/zero of=/dev/sda bs=1 count=446
#修复MBR
grub2-install /dev/sda
grub2-mkconfig -o /boot/grub2/grub.cfg
biosdevname=0
net.ifnames=1 设置为0
规则1：如果Firmware或者BIOS提供的设备索引信息可用就用此命名。比如eno1。否则使用规则2
规则2：如果Firmware或Bios的PCI-E扩展插槽可用就用此命名。比如ens1，否则使用规则3
规则3：如果硬件接口的位置信息可用就用此命名。比如enp2s0
规则4：根据MAC地址命名，比如enx7d3e9f。默认不开启。
规则5：上述均不可用时回归传统命名方式
上面的所有命名规则需要依赖于一个安装包：biosdevname
#命令行修改网卡命名规则
mv /etc/sysconfig/network-scripts/ifcfg-ens33 /etc/sysconfig/network-scripts/ifcfg-eth0
NAME=ens33
DEVICE=ens33
修改
NAME=eth0
DEVICE=eth0
grub2-mkconfig -o /boot/grub2/grub.cfg
reboot
[root@qls ~]# ifconfig eth1 172.16.1.100/24
[root@qls ~]# cp /etc/sysconfig/network-scripts/ifcfg-eth0 /etc/sysconfig/network-scripts/ifcfg-eth1
[root@qls ~]# cat /etc/sysconfig/network-scripts/ifcfg-eth1
TYPE=Ethernet
BOOTPROTO=none
NAME=eth1
DEVICE=eth1
ONBOOT=yes
IPADDR=172.16.1.100
PREFIX=24
#更新源
[root@qls ~]# rm -rf /etc/yum.repos.d/*
[root@qls ~]# curl -o /etc/yum.repos.d/CentOS-Base.repo https://mirrors.aliyun.com/repo/Centos-7.repo
[root@qls ~]# curl -o /etc/yum.repos.d/epel.repo http://mirrors.aliyun.com/repo/epel-7.repo
[root@qls ~]# ll /etc/yum.repos.d/
total 8
-rw-r--r--. 1 root root 2523 Aug 5 20:09 CentOS-Base.repo
-rw-r--r--. 1 root root 664 Aug 5 20:09 epel.repo
#关闭firewalld及selinux
[root@qls ~]# systemctl stop firewalld
[root@qls ~]# systemctl disable firewalld
[root@qls ~]# setenforce 0
[root@qls ~]# vi /etc/sysconfig/selinux
[root@qls ~]# grep ‘SELINUX‘ /etc/sysconfig/selinux
# SELINUX= can take one of these three values:
SELINUX=disabled
#关闭NetworkManager网络
[root@qls ~]# systemctl stop NetworkManager
[root@qls ~]# systemctl disable NetworkManager
#同步时间
[root@qls ~]# crontab -l
* * * * * /usr/bin/ntpdate ntp.aliyun.com &> /dev/null
#配置文件介绍
<domain> <type> <item> <value>
<domain>表示要限制的用户
<type>设定类型
<item>表示可选的资源
<value>表示要限制的值
#加大文件描述符
[root@qls ~]# echo ‘* - nofile 65535 ‘ >>/etc/security/limits.conf
#检查结果
[root@qls ~]# tail -1 /etc/security/limits.conf
原文：https://www.cnblogs.com/future1/p/14060607.html