这里我以阿里云轻量应用服务器为例
需要确保当前为CentOS7的最新系统
yum update

因为阿里云自带的DNS总是会带来莫名其妙的错误 所以要先修改一下DNS 以下为临时修改 重启网卡就重置了
vim /etc/resolv.conf

options timeout:2 attempts:3 rotate single-request-reopen
; generated by /usr/sbin/dhclient-script
#nameserver 100.100.2.138
#nameserver 100.100.2.136
#将以上两行注释掉 然后添加下面两行 保存
nameserver 119.29.29.29
nameserver 8.8.4.4

修改完成之后可能出现failure: repodata/repomd.xml from base: [Errno 256] No more mirrors to try.
http://mirrors.cloud.aliyuncs.com/centos/7/os/x86_64/repodata/repomd.xml: [Errno 14] curl#6 - "Could not resolve host: mirrors.cloud.aliyuncs.com; 未知的名称或服务"的错误
所以我们要修改一下repo源 这里改为 USTC 提供的镜像
位置在 /etc/yum.repos.d/CentOS-Base.repo 直接替换为一下内容

# CentOS-Base.repo
#
# The mirror system uses the connecting IP address of the client and the
# update status of each mirror to pick mirrors that are updated to and
# geographically close to the client.  You should use this for CentOS updates
# unless you are manually picking other mirrors.
#
# If the mirrorlist= does not work for you, as a fall back you can try the
# remarked out baseurl= line instead.
#
#

[base]
name=CentOS-$releasever - Base
#mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=os
baseurl=https://mirrors.ustc.edu.cn/centos/$releasever/os/$basearch/
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7

#released updates
[updates]
name=CentOS-$releasever - Updates
# mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=updates
baseurl=https://mirrors.ustc.edu.cn/centos/$releasever/updates/$basearch/
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7

#additional packages that may be useful
[extras]
name=CentOS-$releasever - Extras
# mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=extras
baseurl=https://mirrors.ustc.edu.cn/centos/$releasever/extras/$basearch/
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7

#additional packages that extend functionality of existing packages
[centosplus]
name=CentOS-$releasever - Plus
# mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=centosplus
baseurl=https://mirrors.ustc.edu.cn/centos/$releasever/centosplus/$basearch/
gpgcheck=1
enabled=0
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7

1、安装epel源

yum -y install epel-release

2、安装rpmconf和yum-utils

yum -y install rpmconf yum-utils

3、执行命令来解析RPM软件包

rpmconf -a

4、安装dnf

yum -y install dnf

5、卸载yum和yum-metadata-parser

dnf -y remove yum yum-metadata-parser
rm -Rf /etc/yum

6、升级

dnf upgrade

7、用dnf安装CentOS 8 发行版本

dnf -y upgrade http://mirrors.ustc.edu.cn/centos/8.0.1905/BaseOS/x86_64/os/Packages/centos-release-8.0-0.1905.0.9.el8.x86_64.rpm

8、修改一下repo源 否则会出现错误

# CentOS-Base.repo
#
# The mirror system uses the connecting IP address of the client and the
# update status of each mirror to pick mirrors that are updated to and
# geographically close to the client.  You should use this for CentOS updates
# unless you are manually picking other mirrors.
#
# If the mirrorlist= does not work for you, as a fall back you can try the
# remarked out baseurl= line instead.
#
#

[BaseOS]
name=CentOS-$releasever - Base
#mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=BaseOS&infra=$infra
baseurl=https://mirrors.ustc.edu.cn/centos/$releasever/BaseOS/$basearch/os/
gpgcheck=1
enabled=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-centosofficial

#additional packages that may be useful
[extras]
name=CentOS-$releasever - Extras
#mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=extras&infra=$infra
baseurl=https://mirrors.ustc.edu.cn/centos/$releasever/extras/$basearch/os/
gpgcheck=1
enabled=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-centosofficial

#additional packages that extend functionality of existing packages
[centosplus]
name=CentOS-$releasever - Plus
#mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=centosplus&infra=$infra
baseurl=https://mirrors.ustc.edu.cn/centos/$releasever/centosplus/$basearch/os/
gpgcheck=1
enabled=0
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-centosofficial

[AppStream]
name=CentOS-$releasever - AppStream
#mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=AppStream&infra=$infra
baseurl=https://mirrors.ustc.edu.cn/centos/$releasever/AppStream/$basearch/os/
gpgcheck=1
enabled=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-centosofficial

[PowerTools]
name=CentOS-$releasever - PowerTools
#mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=PowerTools&infra=$infra
baseurl=https://mirrors.ustc.edu.cn/centos/$releasever/PowerTools/$basearch/os/
gpgcheck=1
enabled=0
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-centosofficial

然后运行sudo yum makecache生成缓存

9、升级EPEL存储库

dnf -y upgrade https://dl.fedoraproject.org/pub/epel/epel-release-latest-8.noarch.rpm

10、成功升级EPEL存储库后 删除所有临时文件

dnf clean all

11、删除用于CentOS 7的旧内核内核

rpm -e rpm -q kernel

12、删除有冲突的软件包

rpm -e --nodeps sysvinit-tools

13、启动CentOS 8升级

dnf -y --releasever=8 --allowerasing --setopt=deltarpm=false distro-sync

14、CentOS 8安装新内核

dnf -y install kernel-core

15、安装 CentOS 8最小软件包

dnf -y groupupdate "Core" "Minimal Install"

16、检查版本

cat /etc/redhat-release

沉淪在無盡的深淵中...