builtins: Added an if command

src/fsh.nim

@@ -18,11 +18,12 @@ for k, v in envPairs():
   envVariables[k]=v
 
 # list of builtin commands
-let builtins: array[4, string] = [
+let builtins: array[5, string] = [
 "set",
 "setenv",
 "echo",
 "exit",
+"if",
 ]
 
 # Exception types
@@ -63,6 +64,7 @@ proc newShellCommandException(msg: string, command: string, line=0, col=0, while
 
 # forward declarations
 proc eval(cmd: string): string
+proc execute(cmd: string): int
 proc substitute(strm: Stream, delim = ";\p"): string
 
 # ctrlc is the ctrl-c handler
@@ -281,6 +283,28 @@ proc runBuiltin(builtin: string, args: openArray[string]): (string, int) =
         except ValueError:
           stdout.write(args.join(" "), "\p")
           quit(0)
+    of "if":
+      var i=0
+      while true:
+        # if i is greater than args.high, we exit out. user probs doesn't want an else branch, we aren't going to stop him
+        if i > args.high:
+          break
+    
+        # Check if this is the last item, if so just execute it
+        # It's an else branch basically
+        if i == args.high:
+          return ("", execute(args[i]))
+        # Get the exitcode of the current arg (the condition)
+        let exitcode = args[i].execute
+        if exitcode == 0:
+          # if the condition is true
+          return ("", args[i+1].execute)
+        else:
+          # if the condition is false
+          i.inc
+          i.inc
+          continue
+    
     else:
       raise newShellCommandException("No such builtin implemented", builtin)