前言
Shell有很多内置在其源代码中的命令。由于命令是内置的，所以Shell不必到磁盘上搜索它们。内置命令执行速度更快，不同的Shell内置命令有所不同。
如何查找内置命令
之前查了好久怎么收索内置命令的方法，今天在看type命令的时候，发现man手册里面就有
执行man type，有一段就告诉你内置命令都有啥了
BASH BUILTIN COMMANDS
Unless otherwise noted, each builtin command documented in this section as
accepting options preceded by - accepts -- to signify the end of the
options. The :, true, false, and test builtins do not accept options and
do not treat -- specially. The exit, logout, break, continue, let, and
shift builtins accept and process arguments beginning with - without
requiring --. Other builtins that accept arguments but are not specified
as accepting options interpret arguments beginning with - as invalid
options and require -- to prevent this interpretation.