今天在做centos的内核升级测试，在使用源码升级的时候，在网上开到在升级内核之前先进行yum update、yum upgrade。这个两个到底有什么区别吗？网上搜索了一大把发现全是这样的说法：
yum -y update
升级所有包，改变软件设置和系统设置,系统版本内核都升级
yum -y upgrade
升级所有包，不改变软件设置和系统设置，系统版本升级，内核不改变
而且还信誓旦旦的贴上了测试结果，真的是这样的吗？？？！！！我们还是看下man yum 是怎么说的吧！！！
update
If run without any packages, update will update every currently installed package. If one or more packages or pack-age globs are specified, Yum will only update the listed packages. While updating packages, yum will ensure that all dependencies are satisfied. (See Specifying package names for more information) If the packages or globs specified match to packages which are not currently installed then update will not install them. update operates on groups,files, provides and filelists just like the "install" command.
If the main obsoletes configure option is true (default) or the --obsoletes flag is present yum will include package obsoletes in its calculations - this makes it better for distro-version changes, for example: upgrading from somelinux 8.0 to somelinux 9.
Note that "update" works on installed packages first, and only if there are no matches does it look for available packages. The difference is most noticeable when you do "update foo-1-2" which will act exactly as "update foo" if foo-1-2 is installed. You can use the "update-to" if you’d prefer that nothing happen in the above case.
upgrade
Is the same as the update command with the --obsoletes flag set. See update for more details.