yum supports no command line option to be "quiet". This, coupled with the
inability to redirect output from yum makes it impractical to be used on slow
networks, or on remote machines:

[root@gatekeeper rpm]# yum --help
Loading "installonlyn" plugin
usage: yum [options] < update | install | info | remove | list |
    clean | provides | search | check-update | groupinstall | 
    groupupdate | grouplist | groupinfo | groupremove |
    makecache | localinstall | erase | upgrade | whatprovides |
    localupdate | resolvedep | shell | deplist >

options:
  -h, --help            show this help message and exit
  -t, --tolerant        be tolerant of errors
  -C                    run entirely from cache, don't update cache
  -c  [config file]     config file location
  -R  [minutes]         maximum command wait time
  -d  [debug level]     debugging output level
  -e  [error level]     error output level
  -y                    answer yes for all questions
  --version             show Yum version and exit
  --installroot=[path]  set install root
  --enablerepo=[repo]   enable one or more repositories (wildcards allowed)
  --disablerepo=[repo]  disable one or more repositories (wildcards allowed)
  --exclude=[package]   exclude package(s) by name or glob
  --obsoletes           enable obsoletes processing during updates
  --noplugins           disable Yum plugins