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官方升级教程：http://wiki.centos.org/TipsAndTricks/CentOSUpgradeTool
# cd /etc/yum.repos.d/ # touch upgradetool.repo
# vi upgradetool.repo
[upg] name=CentOS-$releasever - Upgrade Tool baseurl=http://dev.centos.org/centos/6/upg/x86_64/ gpgcheck=1 enabled=1 gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-6
# yum install redhat-upgrade-tool preupgrade-assistant-contents
# preupg -l
# preupg -s CentOS6_7
Preupg tool doesn‘t do the actual upgrade.
Please ensure you have backed up your system and/or data in the event of a failed upgrade
that would require a full re-install of the system from installation media.
Do you want to continue? y/n
y
# rpm --import http://mirror.centos.org/centos/RPM-GPG-KEY-CentOS-7
# centos-upgrade-tool-cli --network 7 --instrepo=http://mirror.centos.org/centos/7/os/x86_64/
# centos-upgrade-tool-cli --network 7 --instrepo=http://mirrors.163.com/centos/7/os/x86_64/
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原文地址：http://www.cnblogs.com/phpdragon/p/4922526.html
