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風險：
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系統：
ssh、yum不可用，需按文末步驟安裝
數據：
從實驗來看，不會清空 /opt、/home、/var 目錄下的用戶數據，部分應用(如Jenkins)會自動更新版本
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詳細步驟：
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查看版本對應關系：
https://www.cnblogs.com/emanlee/p/13594584.html
下載 centos 7.8 ISO
https://mirrors.tuna.tsinghua.edu.cn/centos/7.8.2003/isos/x86_64/CentOS-7-x86_64-DVD-2003.iso
1、查看當前CentOS版本
cat /etc/redhat-release
CentOS release 6.4 (Final)
2、更新源
vim /etc/yum.repos.d/upgrade.repo
並輸入以下內容：
[upgrade]
name=upgrade
baseurl=https://buildlogs.centos.org/centos/6/upg/x86_64/
enable=1
gpgcheck=0
3、卸載6.x自帶的較新的助手，並安裝老版[否則會報錯]
yum erase openscap -y
4、安裝助手
yum install redhat-upgrade-tool preupgrade-assistant-contents -y
5、檢測版本升級的風險，如果控制台輸出了錯誤信息，則需要查詢下解決方案並解決
preupg -s CentOS6_7
Do you want to continue? y/n
Assessment finished (time 01:50s)
I/O warning : failed to load external entity "/usr/share/openscap/xsl/security-guide.xsl"
compilation error: file /usr/share/preupgrade/xsl/preup.xsl line 40 element import
xsl:import : unable to load /usr/share/openscap/xsl/security-guide.xsl
I/O warning : failed to load external entity "/usr/share/openscap/xsl/oval-report.xsl"
compilation error: file /usr/share/preupgrade/xsl/preup.xsl line 41 element import
xsl:import : unable to load /usr/share/openscap/xsl/oval-report.xsl
I/O warning : failed to load external entity "/usr/share/openscap/xsl/sce-report.xsl"
compilation error: file /usr/share/preupgrade/xsl/preup.xsl line 42 element import
xsl:import : unable to load /usr/share/openscap/xsl/sce-report.xsl
OpenSCAP Error:: Could not parse XSLT file '/usr/share/preupgrade/xsl/preup.xsl' [oscapxml.c:416]
Unable to open file /root/preupgrade/result.html
Usage: preupg [options]
preupg: error: [Errno 2] No such file or directory: '/root/preupgrade/result.html'
6、導入CentOS7的key
rpm --import http://mirror.centos.org/centos/RPM-GPG-KEY-CentOS-7
7、開始升級（升級失敗）
centos-upgrade-tool-cli --network 7 --instrepo=http://vault.centos.org/centos/7.7.1908/os/x86_64/
本地更新：
centos-upgrade-tool-cli --iso=/home/soft_sda4/CentOS-7-x86_64-DVD-2003.iso
錯誤提示
setting up repos...
YumRepo Error: All mirror URLs are not using ftp, http[s] or file.
Eg. Invalid release/repo/arch combination/
removing mirrorlist with no valid mirrors: /var/tmp/system-upgrade/base/mirrorlist.txt
YumRepo Error: All mirror URLs are not using ftp, http[s] or file.
Eg. Invalid release/repo/arch combination/
removing mirrorlist with no valid mirrors: /var/tmp/system-upgrade/extras/mirrorlist.txt
YumRepo Error: All mirror URLs are not using ftp, http[s] or file.
Eg. Invalid release/repo/arch combination/
removing mirrorlist with no valid mirrors: /var/tmp/system-upgrade/updates/mirrorlist.txt
No upgrade available for the following repos: base extras updates
.treeinfo | 353 B 00:00
getting boot images...
Downloading failed: invalid data in .treeinfo: No section: 'checksums'
8、國內服務器需經過漫長的等待
9、更新完后，重啟服務器
先看下面的問題，再重啟
reboot
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升級完成后遇到的問題：
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1、ssh、yum不可用問題：
vi /root/start.sh #輸入以下內容：
#!/bin/bash
ln -s /usr/lib64/libsasl2.so.3.0.0 /usr/lib64/libsasl2.so.2
ln -s /usr/lib64/libpcre.so.1.2.0 /usr/lib64/libpcre.so.0
service sshd restart
rm -rf /etc/rc.d/rc.local
mv /etc/rc.d/rc.local.bak /etc/rc.d/rc.local #恢復原始文件
rm -rf /root/start.sh #刪除自身
#執行以下命令
chmod +x start.sh
chmod +x /etc/rc.d/rc.local
cp /etc/rc.d/rc.local /etc/rc.d/rc.local.bak #創建備份
echo 'bash /root/start.sh' >>/etc/rc.d/rc.local #添加腳本為開機自啟動
#重啟，后看下ssh是否可以正常連接
reboot
2、 ps工具不可用問題：
yum upgrade -y
yum downgrade grep
yum upgrade python
yum update
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REF
https://blog.csdn.net/hjnth/article/details/82946845
https://blog.csdn.net/QingKong999/article/details/79114925
https://blog.csdn.net/u011643449/article/details/85560273