什麼是bash shell的内建（build in）指令

1.什麼是build in指令：

        shell内建指令是指bash（或其它版本）工具集中的指令。一般都會有一個與之同名的系統指令，比如bash中的echo指令與/bin/echo是兩個不同的指令，盡管他們行為大體相仿。當在bash中鍵入一個指令時系統會先看他是否是一個内建指令，如果不是才會檢視是否是系統指令或第三方工具。是以在bash中鍵入echo指令實際上執行bash工具集中的bash指令也就是内建指令，而不是/bin/echo這個系統指令。

2.内建指令與系統指令

       内建指令要比系統論指令有比較高的執行效率。外部指令執行時往往需要fork出（産生出）一個子程序，而内建指令一般不用。下面（或許以後還會有）這一篇文章将介簡bash的内建指令。

3.檢視一個指令是系統指令還是内建指令：type

[email protected]:~/Documents

$ type -a pwd

pwd is a shell builtin

pwd is /bin/pwd

[email protected]:~/Documents

$ type -a echo

echo is a shell builtin

echo is /bin/echo

可以看出，有些指令，echo和pwd同時是内建指令和系統指令。

4.常見指令的類型

[[email protected] ~]# type -a cd 

cd is a shell builtin

[[email protected] ~]# type -a pwd 

pwd is a shell builtin

pwd is /bin/pwd

[[email protected] ~]# type -a time 

time is a shell keyword

time is /usr/bin/time

[[email protected] ~]# type -a date 

date is /bin/date

[[email protected] ~]# type -a which 

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

which is /usr/bin/which

[[email protected] ~]# type -a whereis 

whereis is /usr/bin/whereis

[[email protected] ~]# type -a whatis 

whatis is /usr/bin/whatis

[[email protected] ~]# type -a function 

function is a shell keyword

[[email protected] ~]# type -a ls 

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

ls is /bin/ls

[[email protected] ~]# type -a ll 

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

[[email protected] ~]# type -a echo 

echo is a shell builtin

echo is /bin/echo

[[email protected] ~]# type -a bulitin 

-bash: type: bulitin: not found

[[email protected] ~]# type -a builtin 

builtin is a shell builtin

[[email protected] ~]# type -a keyword 

-bash: type: keyword: not found

[[email protected] ~]# type -a command 

command is a shell builtin

[[email protected] ~]# type -a alias 

alias is a shell builtin

[[email protected] ~]# type -a grep 

grep is /bin/grep

[[email protected] ~]#