This answer comes by way of piecing together bits from other answers – so to the previous contributors…thank you because I would not have figured this out.This example is based on the RHEL 7 AMI (Amazon Managed Image) 3.10.0-229.el7.x86_64.So by default as mentioned above the optional repository is not enabled. Don’t add another repo.d file as it already exists just that it is disabled.To enable first you need the name. I used grep to do this:grep -B1 -i optional /etc/yum.repos.d/*above each name will be the repo id enclosed in [ ] look for the optional not optional-sourceEnable the optional repo:yum-config-manager –enable Refresh the yum cache (not sure if this is necessary but it doesn’t hurt):sudo yum makecacheFinally, you can install ruby-devel:yum install ruby-develDepending on your user’s permissions you may need to use sudo.