Java™ Application Development on Linux - Dator

4.3 The System Class107$ javac AllEnv.java$ java AllEnvand you will get a long list of properties—in no particular order. They are keptin a hashtable and thus not sorted. Of course it would be easier to use this listif they were sorted. Linux to the rescue.$ java AllEnv | sortIt’s often in simple little steps like this that one begins to see the power ofLinux. In Linux, not every desirable feature has to be crammed into every possibleplace where it might be used. Instead, features can be written once andconnected to one another as needed. Here what we need is to have the list ofproperties sorted. We don’t need to worry that our class didn’t sort its output.In Linux we just connect the standard output of the Java program with a sortutility that Linux provides.So what are all these properties? Many of them have to do with Javarelatedinformation (java.version, and so on), but a few are more general.Those that parallel the typical Linux environment variables are:• file.separator is the file separator (“/” on Linux).• path.separator is the path separator (“:” on Linux).• line.separator is the line separator (“\n” on Linux).• user.name is the user’s account name.• user.home is the user’s home directory.• user.dir is the user’s current working directory.But that leaves out so many environment variables, especially the application-specificones (e.g., CVSROOT). How would a Java program get at these?Because of this new, more portable way to describe the environment,there is no easy way to get at other environment variables. There are a fewapproaches, but they are all indirect.First, you can add to the properties list by defining new properties on thecommand line when invoking the program, for example:$ java -Dkey=value AllEnvYou can list several properties on the line by repeating the -D parameter: