Java™ Application Development on Linux - Dator

108Chapter 4Where Am I? Execution Context$ java -DHOME=/home/mydir -DALT=other -DETC="so forth" AllEnvInstead of typing those values, you’d probably want to let the Linux shellput in the values from its environment. So you’d use shell variables, forexample:$ java -DHOME="${HOME}" -DALT="${ALT}" -DETC="${ETC}" AllEnvassuming that HOME, ALT, and ETC have already been defined in the shell’senvironment. 2If there are only a few variables that you need to pass to Java, put them onthe command line as shown above. Put that command line into a shell scriptand use the script to invoke the program so that the parameters are suppliedevery time.But if you want to access many or all of the environment variables thenyou may want to do something a little more complex. Notice the syntax of theoutput of the env command. It is in the same format (name=value) as areproperties. So if we use a shell script to invoke our program, we can have itplace all these values into a file by redirecting output, then open this file as aJava properties file and thus make all the name/value pairs accessible.The following commands in a shell script attempt to do just that:env > /tmp/$$.envjava -DENVFILE=/tmp/$$.env MyClassrm /tmp/$$.envwhere MyClass is the Java program that you wish to run.TIPThe shell variable $$ is the numeric process ID of the running process. Thisprovides a unique ID during each invocation of the program. Each run of thescript will have its own process and thus its own process ID. Thus a single usercould execute this script multiple times concurrently without fear of collisionwith himself or others.2. The quotations around the shell variables keep any embedded spaces as part of the variable’svalue. The curly braces are not strictly necessary in this use.