anchor.diff:  
       String expected = String.format("" +
               "Usage: <main class> [--option=<option>]%n" +
  -            "      --option=<option>   Some options. Default: some-value%n");
  +            "      --option=<option>   Some option. Default: some-value%n");
       assertEquals(expected, actual);
   }
   ----
   
  -
  -==== JUnit 4 with `EnvironmentVariables`
  -Applications that use JUnit 4 can use Stephan Birkner's https://stefanbirkner.github.io/system-rules[System-Rules] project for https://stefanbirkner.github.io/system-rules/#EnvironmentVariables[testing environment variables].
  -
  -[source,java]
  +.Java/JUnit 4
  +[source,java,role=secondary]
   ----
   import org.junit.contrib.java.lang.system.EnvironmentVariables;
   // ...
  
positive.code: === Testing Environment Variables
  
  Since picocli offers support for using <<Variable Interpolation,environment variables>> in the annotations, you may want to test this.
  
  ==== Java 8+ with `withEnvironmentVariable`
  Applications that use JUnit 5 or Java 8+ can use Stephan Birkner's https://github.com/stefanbirkner/system-lambda[System-Lambda] project for https://github.com/stefanbirkner/system-lambda#environment-variables[testing environment variables].
  
  Example usage:
  
  [source,java]
  ----
  import static com.github.stefanbirkner.systemlambda.SystemLambda.withEnvironmentVariable;
  // ...
  
negative.code:         environmentVariables.set("OTHER_VAR", "second value");
  
          String actual = new CommandLine(new MyApp())
                  .getUsageMessage(CommandLine.Help.Ansi.OFF);
  
          String expected = String.format("" +
                  "Usage: <main class> [--option=<option>]%n" +
                  "      --option=<option>   Some option. Default: some-value%n");
          assertEquals(expected, actual);
      }
  }
  ----
  
  .Kotlin/Kotest
  [source,kotlin,role=secondary]
  ----
  import io.kotest.core.spec.style.StringSpec
  import io.kotest.extensions.system.withEnvironment
  import io.kotest.matchers.shouldBe
  import picocli.CommandLine
  import picocli.CommandLine.Command
  
  class EnvironmentVarTest : StringSpec() {
      init {
          "env var" {
              withEnvironment(mapOf("MYVAR" to "some-value", "OTHER_VAR" to "second value")) {
                  CommandLine(MyApp()).getUsageMessage(CommandLine.Help.Ansi.OFF) shouldBe
                  "Usage: <main class> [--option=<option>]" + System.lineSeparator() +
                  "      --option=<option>   Some option. Default: some-value" + System.lineSeparator()
              }
          }
      }
  }
  
  @Command
  class MyApp {
      @CommandLine.Option(names = ["--option"], defaultValue = "\${env:MYVAR}",
          description = ["Some option. Default: \${DEFAULT-VALUE}"])
      lateinit var option: String
  }
  ----
  
  