Find all existing JDK or JRE installations on RedHat, Centos or Fedora

jdk_or_jre_installations.sh

# Specifically JDK installations (contain 'javac' command) and not JREs:
yum --disablerepo=* whatprovides '*/bin/javac' | grep ilename | awk -F': ' '{print $2}' | sed 's/\/bin\/javac//'

# All JRE installations (those with bin/java but without bin/javac).  Includes JREs that are included in JDKs.
for candidate in $(yum --disablerepo=* whatprovides '*/bin/java' | grep ilename | awk -F': ' '{print $2}' | sed 's/\/bin\/java//'); do
  if [ ! -x "${candidate}/bin/javac" ]; then
    echo "${candidate}"
  fi
done