Various additions and improvements

src/builtin/mod.rs

@@ -1,5 +1,15 @@
 pub mod echo;
+pub mod cd;
+pub mod export;
+pub mod pwd;
+pub mod source;
+pub mod shift;
 
-pub const BUILTINS: [&str;1] = [
-	"echo"
+pub const BUILTINS: [&str;6] = [
+	"echo",
+	"cd",
+	"export",
+	"pwd",
+	"source",
+	"shift"
 ];