The if statement ACTS as simple errorcontrol, exiting our script if it does not find the file with a somewhat graceful error for our users.
if语句将用作简单的错误控制，如果由于用户的小失误而导致它未找到文件，则退出脚本。
2
Source control allows you to "roll back" to a previous iteration of a file to restore the status quo or to recover from an error.
源代码控制系统允许用户“回滚”到以前的文件版本，从而恢复以前的状态或从错误中恢复。
3
Note that this example demonstrates the basic syntax for this control, but does not demonstrate all the necessary error checking that should be done before saving the file.