Simple solution (directly installs latest stable version):

rbenv install $(rbenv install -l | grep -v - | tail -1)

Explanation:

rbenv install -l | grep -v - | tail -1

Filters out all versions that contain a hyphen -, which is all non-MRI versions and prerelease MRI versions. Then selects the last one, guaranteed to be the highest because ruby-build output is already sorted by version number ascending.

rbenv install -l | awk -F '.' '   /^[[:space:]]*[0-9]+\.[0-9]+\.[0-9]+[[:space:]]*$/ {      if ( ($1 * 100 + $2) * 100 + $3 > Max ) {          Max = ($1 * 100 + $2) * 100 + $3         Version=$0         }      }   END { print Version }'

• Take the biggest version (sorted order or not)

If list is sorted a simpler sed (posix version) is enough

rbenv install -l | sed -n '/^[[:space:]]*[0-9]\{1,\}\.[0-9]\{1,\}\.[0-9]\{1,\}[[:space:]]*$/ h;${g;p;}'

One should first update the ruby-build to get the latest version of ruby while install using rbenv..Follow the below steps:

• brew reinstall --HEAD ruby-build (if rbenv is already installed brew may through some error then to move ahead and simply
  • brew unlink ruby-build and
  • brew install --HEAD ruby-build)
• brew upgrade
• and then you could use one of the above approaches suggested above to install automatically the latest version or simply rbenv install <required latest version>

working in macOS 10.13.6