Program Environment

1. import std.stdio;
   import std.conv;
   
   int main(string[] args) {
       if (args.length != 4) {
           stderr.writeln(
               "ERROR! Usage: \n    ", args[0],
               " a_number operator another_number");
           return 1;
       }
   
       immutable first = to!double(args[1]);
       string op = args[2];
       immutable second = to!double(args[3]);
   
       switch (op) {
   
       case "+":
           writeln(first + second);
           break;
   
       case "-":
           writeln(first - second);
           break;
   
       case "x":
           writeln(first * second);
           break;
   
       case "/":
           writeln(first / second);
           break;
   
       default:
           throw new Exception("Invalid operator: " ~ op);
       }
   
       return 0;
   }
   

2. import std.stdio;
   import std.process;
   
   void main() {
       write("Please enter the command line to execute: ");
       string commandLine = readln();
   
       writeln("The output: ", executeShell(commandLine));
   }