可以用type命令查看一个命令是否为内建命令
$ type cd
cd is a shell builtin
$ type ls
ls is an alias for ls --color=tty
$ type echo
echo is a shell builtin
$ type cd
cd is a shell builtin
$ type ls
ls is an alias for ls --color=tty
$ type echo
echo is a shell builtin
2019-04-06