Allow users to define default values with environment variables

[remkop]

Picocli has a feature, Variable Interpolation, that may be useful: for example, currently users need to always specify their project directory with ami -p some/dir subcommand.

It would be nice if users can set an environment variable, like AMIPROJECT=some/dir, and afterwards they can simply specify ami subcommand.

The Variable Interpolation feature makes this fairly straightforward. We can define the -p option as follows:

@Option(names = {"-p", "--cproject"}, 
        defaultValue = "${AMIPROJECT:-${user.home}/amiprojects/myproject}", 
        paramLabel = "DIR", ...

Then:

• if the user specified a -p some/location option, this specified value is used
• if -p is not specified, then picocli will first try to get a value from the AMIPROJECT environment variables, or system properties
• next, if no value could be found for AMIPROJECT, then picocli will assign the default value ${user.home}/amiprojects/myproject. This contains another variable, "${user.home}", which is guaranteed by the Java language to have a value.

We can do a similar thing for other options. Note that options with a default value are not required options (by definition).

Final note: the default value can be referenced in the description with the ${DEFAULT_VALUE} variable:

@Option(names = {"-p", "--cproject"}, 
    defaultValue = "${AMIPROJECT:-${user.home}/amiprojects/myproject}",
    description = "The CProject (directory) to process."
            + " ..."
            + " The default is: `${DEFAULT-VALUE}`."

[remkop]

For testing environment variables, we can use Stephan Birkner's Environment Variables. (Part of his System Rules toolkit that extends JUnit 4 with some very useful tools.)

[remkop]

I added AMITest to demonstrate how to create tests for environment variables in the option defaults.