1.什么是build in命令：

shell内建命令是指bash(或其它版本)工具集中的命令。一般都会有一个与之同名的系统命令，比如bash中的echo命令与/bin/echo是两个不同的命令，尽管他们行为大体相仿。当在bash中键入一个命令时系统会先看他是否是一个内建命令，如果不是才会查看是否是系统命令或第三方工具。所以在bash中键入echo命令实际上执行bash工具集中的bash命令也就是内建命令，而不是/bin/echo这个系统命令。

2.内建命令与系统命令

内建命令要比系统论命令有比较高的执行效率。外部命令执行时往往需要fork出(产生出)一个子进程，而内建命令一般不用。下面(或许以后还会有)这一篇文章将介简bash的内建命令。

3.查看一个命令是系统命令还是内建命令：type

linuxidc@www.linuxidc.com:~/Documents

$ type -a pwd

pwd is a shell builtin

pwd is /bin/pwd

linuxidc@www.linuxidc.com:~/Documents

$ type -a echo

echo is a shell builtin

echo is /bin/echo

可以看出，有些命令，echo和pwd同时是内建命令和系统命令。

4.常见命令的类型

[root@linuxidc ~]# type -a cd

cd is a shell builtin

[root@linuxidc ~]# type -a pwd

pwd is a shell builtin

pwd is /bin/pwd

[root@linuxidc ~]# type -a time

time is a shell keyword

time is /usr/bin/time

[root@linuxidc ~]# type -a date

date is /bin/date

[root@linuxidc ~]# type -a which

which is aliased to `alias | /usr/bin/which --tty-only --read-alias --show-dot --show-tilde'

which is /usr/bin/which

[root@linuxidc ~]# type -a whereis

whereis is /usr/bin/whereis

[root@linuxidc ~]# type -a whatis

whatis is /usr/bin/whatis

[root@linuxidc ~]# type -a function

function is a shell keyword

[root@linuxidc ~]# type -a ls

ls is aliased to `ls --color=tty'

ls is /bin/ls

[root@linuxidc ~]# type -a ll

ll is aliased to `ls -l --color=tty'

[root@linuxidc ~]# type -a echo

echo is a shell builtin

echo is /bin/echo

[root@linuxidc ~]# type -a bulitin

-bash: type: bulitin: not found

[root@linuxidc ~]# type -a builtin

builtin is a shell builtin

[root@linuxidc ~]# type -a keyword

-bash: type: keyword: not found

[root@linuxidc ~]# type -a command

command is a shell builtin

[root@linuxidc ~]# type -a alias

alias is a shell builtin

[root@linuxidc ~]# type -a grep

grep is /bin/grep

[root@linuxidc ~]#