Linux升级命令有两个分别是yum upgrade和yum update, 这个两个命令是有区别的:
yum -y update
升级所有包同时也升级软件、系统版本和系统内核
yum -y upgrade
只升级所有包和系统版本，不升级软件和系统内核
这个说法是错误的！！！两个命令是一样的！
yum update：
If run without any packages, update will update every currently installed package. If one or more packages or package 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 speci-
fied 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.
yum upgrade：
Is the same as the update command with the --obsoletes flag set. See update for more details.
