今天在做centos的內核升級測試，在使用源碼升級的時候，在網上開到在升級內核之前先進行yum update、yum upgrade。這個兩個到底有什麼區別嗎？網上搜索了一大把發現全是這樣的說法：
yum -y update
升級所有包，改變軟件設置和系統設置,系統版本內核都升級
yum -y upgrade
升級所有包，不改變軟件設置和系統設置，系統版本升級，內核不改變
而且還信誓旦旦的貼上了測試結果，真的是這樣的嗎？？？！！！我們還是看下man yum 是怎麼說的吧！！！
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.