Find the process id of a java application in a bash script (to see if the target application is already running)

By default, pgrep only matches the command, not the arguements. To match the full command line, you need the -f option.

$ pgrep -f java_app

From the pgrep manpage:

-f The pattern is normally only matched against the process name. When -f is set, the full command line is used