首先yum update和yum upgrade的功能是一样的，都是将需要更新的package更新至软件源中的最新版。
唯一不同是yum upgrade会删除旧版本的package，而yum update则会保留。注意！如果你的某些软件依赖旧版本的package，请使用yum update。
yum update originally just did upgrades of packages to new versions.If, for example, foo-awesome obsoleted foo, yum update wouldn't offerto upgrade from foo to foo-awesome. Adding the --obsoletes flag to yumupdate made it do the extra checks to also offer that upgrade path. yum upgrade was added as (essentially) an alias for yum --obsoletesupdate. Since this is the behavior that almost everyone wants all ofthe time, the configuration option obsoletes=1 was added to thedefault /etc/yum.conf, making yum update and yum upgrade equivalent onany recent, stock, Fedora/RHEL/CentOS/etc.If you want to avoid kernel updates when you're running yum update,you can just do yum --exclude=kernel* update. If you want automaticupdates on, but you want to avoid automatic kernel upgrades, thenadding the exclude to yum.conf is probably the right answer.There probably isn't a Right Answer for your question. RHEL andRHEL-based distributions don't have the same philosophy as the Debiandevelopers when it comes to updates, so the tools don't encourage thesame sorts of behavior.