第358页 Writing Your First Script

The #! character sequence is, in fact, a special construct called a shebang. The shebang is used to tell the system the name of the interpreter that should be used to execute the script that follows. Every shell script should include this as its first line.
文中说每个 shell script 都必须在第一行写上 shebang，然而我不信邪地试了一下没有 shebang 的版本，依然愉快地运行了。不过，猜也知道它使用了一个猜测的解释器，详见 https://unix.stackexchange.com/a/373229。