Suppress all output in yum except for errors

yum update -q -y

The -q is quiet mode.

The -y assumes yes to everything.

This still prints errors.

The simplest way would probably be to redirect message output.

You can redirect standard message output to send non-error messages to /dev/null (aka not print them). For this, you must automatically accept updates, otherwise it will appear to stall.

yum -y update 1> /dev/null

This still prints error messages. If you want to send the error messages to a file, you can append 2>> [file] to the command to make the total command:

yum -y update 1> /dev/null 2>> [file]

Note: 2>> appends the output to a file, and 2> replaces the file.