When running .java scripts with jbang you can pass the --debug-flag and the script will enable debug, suspend the execution and wait until you connect a debugger to port 4004.

jbang --debug
Listening for transport dt_socket at address: 4004

You can change the debug port and host by passing in a interface pattern and number to the debug argument, e.g., --debug=*:4321.

This will make it use port 4321 and make it listen on all ('*') network interfaces.

Be sure to put a breakpoint in your IDE/debugger before you connect to make the debugger actually stop when you need it.

Debug jbang itself

Java itself will add JAVA_TOOL_OPTIONS which will apply to jbang too.

For finer and more explicit control the scripts, jbang will add JBANG_JAVA_OPTIONS to the call to jbang itself. Thus if you want to enable debug or other details for jbang set that environment variable.