Java Examples: Why does a System.exit(-1) return a 255?

Question:

Why does a System.exit(-1) return a 255?

Answer:

When a linux program terminates, a $? from the command-line gives the exit status of the script.

Because in UNIX/POSIX, the exit code of a program is defined to be an unsigned 8-bit value.

Converting -1 to unsigned 8-bit gives 255.

Code:

public class ExitProgram {
 
   public static void main(String[] args) {
      System.out.println("Before exit");
 
      // Terminates the currently running Java Virtual Machine.
      System.exit(-1);
 
      System.out.println("After exit");
   }
}

Output:

$ java ExitProgram 

$ echo $?
255