add conditional command execution(35p)

src/cmd.c

@@ -273,14 +273,18 @@ int parse_command(command_t *c, int level, command_t *father)
 		/* TODO: Execute the second command only if the first one
 		 * returns non zero.
 		 */
-		break;
-
+        if (parse_command(c->cmd1, level, c) != 0) {
+            return parse_command(c->cmd2, level, c);
+        }
+        return 0;
 	case OP_CONDITIONAL_ZERO:
 		/* TODO: Execute the second command only if the first one
 		 * returns zero.
 		 */
-		break;
-
+         if (parse_command(c->cmd1, level, c) == 0) {
+            return parse_command(c->cmd2, level, c);
+        }
+        return 0;
 	case OP_PIPE:
 		/* TODO: Redirect the output of the first command to the
 		 * input of the second.