. / represents the current directory.
/ represents the root directory of the entire system.
shutdown.sh file is definitely not in the system root directory.

because the current path will not be added to the PATH by default in the linux/unix environment, so you can only follow the path where the command is located, for the current directory is naturally . / . In fact, the powershell of windows is the same now. By default, the current path will not be added to PATH . It will prompt you that you must use .\ command to execute command programs under the current path