Probably the easiest choice, but beware, the version is not always the newest one. Just open up terminal and type (depending on your distribution)

in Debian or Ubuntu using apt

$> sudo apt install ruby

in CentOS, openSUSE or Fedora

$> sudo yum install ruby

You can use the -y option so you are not prompted to agree with the installation but in my opinion it is a good practice to always check what is the package manager trying to install.