我平时都在VirtualBox上装虚拟机，方法是导入ova镜像文件，比如导入从网上下载的CentOS7-base.ova，该ova为CentOS 7.0版本，而现在最新版本为7.7，为此进入系统后第一件事就是系统升级：
# yum update
可是该命令居然异常退出了！
重新进入系统后检查：
# uname -r 3.10.0-123.9.2.el7.x86_64 # cat /proc/version Linux version 3.10.0-123.9.2.el7.x86_64 (builder@kbuilder.dev.centos.org) (gcc version 4.8.2 20140120 (Red Hat 4.8.2-16) (GCC) ) #1 SMP Tue Oct 28 18:05:26 UTC 2014 # cat /etc/redhat-release CentOS Linux release 7.7.1908 (Core)
可见Linux发行版本号已改为7.7，但核心仍然是旧版本，并且再次执行yum update命令失败，说明系统未升级成功。经常一段时间摸索找到两种从7.0升级到7.7的方法。
方法一：升级失败后手工解决错误并重新升级
yum update命令异常退出后重新登录系统，然后执行：
清理上次失败的命令
# yum-complete-transaction --cleanup-only
查看重复的包
# package-cleanup --dupes kbd-misc-1.15.5-10.el7.noarch kbd-misc-1.15.5-15.el7.noarch nss-tools-3.44.0-7.el7_7.x86_64 nss-tools-3.16.2-7.el7_0.x86_64 ... gzip-1.5-10.el7.x86_64 gzip-1.5-7.el7.x86_64
该命令本应无输出，但由于上次yum update命令失败导致大量重复的新旧包。
删除重复的包
# package-cleanup --cleandupes ... Complete! Warning: Some duplicates were not removed because they are required by installed packages. You can try --cleandupes with --removenewestdupes, or review them with --dupes and remove manually.
该命令虽然删除了很多重复包，但提示未完全清除。
尝试重新升级
# yum update ... --> Finished Dependency Resolution Error: Package: avahi-libs-0.6.31-13.el7.x86_64 (@anaconda) Requires: avahi = 0.6.31-13.el7 Removing: avahi-0.6.31-13.el7.x86_64 (@anaconda) avahi = 0.6.31-13.el7 Updated By: avahi-0.6.31-19.el7.x86_64 (base) avahi = 0.6.31-19.el7 Error: firewalld conflicts with 1:NetworkManager-0.9.9.1-28.git20140326.4dba720.el7_0.2.x86_64 Error: avahi-libs conflicts with avahi-0.6.31-19.el7.x86_64 You could try using --skip-broken to work around the problem ...
仍升级失败！
根据上面的错误信息删除旧包
# rpm -e avahi-libs-0.6.31-13.el7.x86_64 error: Failed dependencies: avahi-libs = 0.6.31-13.el7 is needed by (installed) avahi-0.6.31-13.el7.x86_64 avahi-libs = 0.6.31-13.el7 is needed by (installed) avahi-autoipd-0.6.31-13.el7.x86_64
该命令失败，原因是存在依赖项。为此需同时删除依赖项：
# rpm -e avahi-libs-0.6.31-13.el7.x86_64 > avahi-0.6.31-13.el7.x86_64 > avahi-autoipd-0.6.31-13.el7.x86_64 error: Failed dependencies: avahi-autoipd is needed by (installed) NetworkManager-1:0.9.9.1-28.git20140326.4dba720.el7_0.2.x86_64
仍有其它级联依赖项，一并删除：
# rpm -e avahi-libs-0.6.31-13.el7.x86_64 > avahi-0.6.31-13.el7.x86_64 > avahi-autoipd-0.6.31-13.el7.x86_64 > NetworkManager-1:0.9.9.1-28.git20140326.4dba720.el7_0.2.x86_64 error: Failed dependencies: NetworkManager = 1:0.9.9.1-28.git20140326.4dba720.el7_0.2 is needed by (installed) NetworkManager-tui-1:0.9.9.1-28.git20140326.4dba720.el7_0.2.x86_64
继续一并删除：
# rpm -e avahi-libs-0.6.31-13.el7.x86_64 > avahi-0.6.31-13.el7.x86_64 > avahi-autoipd-0.6.31-13.el7.x86_64 > NetworkManager-1:0.9.9.1-28.git20140326.4dba720.el7_0.2.x86_64 > NetworkManager-tui-1:0.9.9.1-28.git20140326.4dba720.el7_0.2.x86_64
成功！
继续尝试升级
# yum update ... Replaced: grub2.x86_64 1:2.02-0.2.10.el7.centos.1 Complete!
成功！成功升级后请重启系统：
# reboot
再次查看并删除重复的包
# package-cleanup --dupes ... kbd-misc-1.15.5-10.el7.noarch kbd-misc-1.15.5-15.el7.noarch libdb-5.3.21-25.el7.x86_64 libdb-5.3.21-17.el7_0.1.x86_64 ... libteam-1.27-9.el7.x86_64 libteam-1.9-15.el7.x86_64
仍有一些重复的包，删除之：
# package-cleanup --cleandupes ... --> Finished Dependency Resolution Error: Rpmdb checksum is invalid: pkg checksums: grub2-tools-1:2.02-0.2.10.el7.centos.1.x86_64 You could try using --skip-broken to work around the problem ...
不知何故居然报错了，但再次执行就成功了：
# package-cleanup --cleandupes ... Removed: audit-libs.x86_64 0:2.3.3-4.el7 bind-license.noarch 32:9.9.4-14.el7 device-mapper.x86_64 7:1.02.84-14.el7 device-mapper-event-libs.x86_64 7:1.02.84-14.el7 device-mapper-libs.x86_64 7:1.02.84-14.el7 dracut.x86_64 0:033-161.el7 e2fsprogs-libs.x86_64 0:1.42.9-4.el7 kbd-misc.noarch 0:1.15.5-10.el7 libcom_err.x86_64 0:1.42.9-4.el7 libcurl.x86_64 0:7.29.0-19.el7 libdb.x86_64 0:5.3.21-17.el7_0.1 libss.x86_64 0:1.42.9-4.el7 libteam.x86_64 0:1.9-15.el7 systemd.x86_64 0:208-11.el7_0.4 systemd-libs.x86_64 0:208-11.el7_0.4 Complete!
再检查一下是否有重复的包：
# package-cleanup --dupes
没有重复的包了。
重启系统并再次检查新版本号
# reboot
# uname -r 3.10.0-1062.18.1.el7.x86_64
# cat /proc/version
Linux version 3.10.0-1062.18.1.el7.x86_64 (mockbuild@kbuilder.bsys.centos.org) (gcc version 4.8.5 20150623 (Red Hat 4.8.5-39) (GCC) ) #1 SMP Tue Mar 17 23:49:17 UTC 2020
# cat /etc/redhat-release
CentOS Linux release 7.7.1908 (Core)
完全正确！至此升级成功！
方法二：从7.0到7.5再到7.7版本
如果你已执行了yum update命令并且失败则只能用上面的方法一解决，本方法适用于新装系统。
从7.0升级到7.5版本
yum update命令默认升级到最新版，但我们可修改配置文件让其升级到指定的版本。
检查是否有更新
# yum list centos-release
...
Installed Packages
centos-release.x86_64 7-0.1406.el7.centos.2.5 @updates
Available Packages
centos-release.x86_64 7-7.1908.0.el7.centos base
该命令显示当前版本是7.0，可升级到7.7版本（但我们知道升级会报错）。
修改CentOS-Base.repo文件，指向7.5版本库
把CentOS-Base.repo文件里的mirrorlist注释掉，并增加相应的baseurl，其它不变，如下：
# cat /etc/yum.repos.d/CentOS-Base.repo ... [base]
... #mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=os&infra=$infra #baseurl=http://mirror.centos.org/centos/$releasever/os/$basearch/ baseurl=http://vault.centos.org/7.5.1804/os/$basearch/ ...
[updates]
... #mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=updates&infra=$infra #baseurl=http://mirror.centos.org/centos/$releasever/updates/$basearch/ baseurl=http://vault.centos.org/7.5.1804/updates/$basearch/ ...
[extras] ...
#mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=extras&infra=$infra #baseurl=http://mirror.centos.org/centos/$releasever/extras/$basearch/ baseurl=http://vault.centos.org/7.5.1804/extras/$basearch/ ...
[centosplus]
... #mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=centosplus&infra=$infra #baseurl=http://mirror.centos.org/centos/$releasever/centosplus/$basearch/ baseurl=http://vault.centos.org/7.5.1804/centosplus/$basearch/ ...
清空缓存
修改配置文件后要清理一下缓存：
# yum clean all
再次检查更新
# yum list centos-release Installed Packages centos-release.x86_64 7-0.1406.el7.centos.2.5 @updates Available Packages centos-release.x86_64 7-5.1804.5.el7.centos updates
可见如果升级则将升级到7.5版本。
升级到7.5版本
# yum update
升级成功后请reboot重启系统。
从7.5升级到7.7版本
把上面的CentOS-Base.repo文件还原、清空缓存、然后执行yum update即可。