How to get the jdk install path using shell programmatically when dual jdk are installed in RHEL?

I have a list of servers in which I need to find the jdk-8 install location programmatically. A few of the servers are having single jdk installation which makes it easier to find the install location using the shell command readlink -f $(which java) from /usr/java. But certain RHEL servers have multiple jdk installed both 11 and 8 and the alternative --config java would be set as jdk 11 , in these cases it is difficult to get the install location of the jdk 8 using the above command. Seeking shell script to find the jdk install location when there are multiple jdk version installation in RHEL server and works for both single and multiple jdk installation ?

> Seeking shell script to find the jdk install location when there are multiple jdk version installation in RHEL server and works for both single and multiple jdk installation ?

We are not a code-writing service, but we'll give you information and advice that may help you to serve yourself.

On a machine where the alternatives system is used to select between multiple Java installations, you can get information about all configured Java alternatives via the command alternatives --display java. You can furthermore filter that through grep to select just the master links. Example:

[jbolling@random ~]$ alternatives --display java | grep '^/'
/usr/lib/jvm/java-11-openjdk-11.0.18.0.9-0.3.ea.el8.x86_64/bin/java - family java-11-openjdk.x86_64 priority 1
/usr/lib/jvm/java-1.8.0-openjdk-1.8.0.362.b08-3.el8.x86_64/jre/bin/java - family java-1.8.0-openjdk.x86_64 priority 1800362

That already assumes, however, that the Java 8 JDK is available via the alternatives system, and that there are no newlines in the paths registered with alternatives. That is how the RHEL packages do things, but if Java is installed other than via the official packages, then it might not fulfill those assumptions. Also, it is possible to manually de-register an alternatives entry after the package is installed.

To go beyond that, you need to make additional assumptions -- specifically, that you can recognize the Java installation you are looking for from its path. If all the available Java installations were installed via official RHEL packages then you can just look for "1.8.0" in the path. Thus, we might revise the previous command to:

[jbolling@random ~]$ alternatives --display java | grep '^/.*1[.]8[.]0'
/usr/lib/jvm/java-1.8.0-openjdk-1.8.0.362.b08-3.el8.x86_64/jre/bin/java - family java-1.8.0-openjdk.x86_64 priority 1800362

And if you want grep to parse out just the path to the java command then you could go a bit further:

[jbolling@random ~]$ alternatives --display java | grep -o '^/.*1[.]8[.]0.*bin/java\>'
/usr/lib/jvm/java-1.8.0-openjdk-1.8.0.362.b08-3.el8.x86_64/jre/bin/java

The -o option tells grep to print only the part of the line that it matched to the regex. Adding bin/java to the pattern (probably) prevents the match from extending into the comment field, and the \> is just extra insurance that the match terminates at a word boundary.

That does, however, make the additional assumptions that the java binary is in a subdirectory whose name ends with bin. That is a fairly safe assumption, especially if there are no Java installations other than from official packages, but it is not absolutely safe.