Environment Variables in Squirrel

Environment variables are a universal mechanism for conveying configuration information to Unix programs. Let’s look at how to set, get, and list environment variables in Java.

import java.util.Map;

public class EnvironmentVariables {
    public static void main(String[] args) {
        // To set a key/value pair, use System.setProperty
        // To get a value for a key, use System.getenv
        // Note that System.getenv returns null if the key isn't present in the environment
        System.setProperty("FOO", "1");
        System.out.println("FOO: " + System.getenv("FOO"));
        System.out.println("BAR: " + System.getenv("BAR"));

        // Use System.getenv() to get a map of all environment variables
        // Here we print all the keys
        System.out.println();
        Map<String, String> env = System.getenv();
        for (String key : env.keySet()) {
            System.out.println(key);
        }
    }
}

Running the program shows that we don’t pick up the value for FOO that we set in the program using System.setProperty(), because System.getenv() only retrieves environment variables set outside the Java program. BAR is null as it’s not set.

$ java EnvironmentVariables
FOO: null
BAR: null

The list of keys in the environment will depend on your particular machine.

TERM_PROGRAM
PATH
SHELL
...

If we set BAR in the environment first, the running program picks that value up.

$ BAR=2 java EnvironmentVariables
FOO: null
BAR: 2
...

Note that in Java, unlike in some other languages, you can’t modify the environment variables of the current process. The System.setProperty() method sets a system property, which is different from an environment variable. To set an environment variable that’s visible to the Java program, you need to set it before running the Java program, as shown in the last example.