什么是Bash Shell的内建(build in)命令

Posted 2020-08-27 一夜飘零

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
 
[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 

[[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] ~]#