shell 直译为壳

　　有个著名的油气公司叫做荷兰皇家壳牌（英語：Royal Dutch Shell Plc）是世界第二大石油公司，公司在英國註冊，总部位于荷兰海牙。
外国人什么都叫做壳，蛋壳，地壳，都是shell 有种包裹什么东西在里面的感觉
操作系统分为内核和外壳 这个壳叫做shell, shell 用来和内核交流，shell 有CUI(命令行的), GUI (图形化的).
unix/linux/freebsd 操作系统上最通用的人机交互界面就是命令行，典型 windows上最常用的是 图形化界面(忘记了一个gui系统的几个基本元素mouse ,window, focus,)
下面主要讨论 命令行shell

格式各样的shell

　　常见的bash，sh, zsh,fish (经常听说 bash 4.xx 有什么命令注入漏洞)
ash :
bash(Bourne-Again SHell) :比较流行的一个shell基本各大linux发行版本都默认设置bash为shell
zsh ：号称最后一个shell 因为字母 z是最后一个了。。
fish : 灵活的交互式shell, 自动补全的特性很强大

Bash is the shell, or command language interpreter, for the GNU operating system. The name is an acronym for the ‘ Bourne-Again SHell ‘, a pun on Stephen Bourne, the author of the direct ancestor of the current Unix shell sh , which appeared in the Seventh Edition Bell Labs Research version of Unix.
The C shell (csh or the improved version, tcsh) is a Unix shell created by Bill Joy while he was a graduate student at University of California, Berkeley in the late 1970s.
The Bourne shell (sh) is a shell, or command-line interpreter, for computer operating systems.
The Bourne shell was the default shell for Version 7 Unix. Many Unix-like systems continue to have /bin/sh—which will be the Bourne shell, or a symbolic link or hard link to a compatible shell—even when other shells are used by most users.
KornShell (ksh) is a Unix shell which was developed by David Korn at Bell Labs in the early 1980s and announced at USENIX on July 14, 1983.[1][2] The initial development was based on Bourne shell source code.[7]

dash (Debian Almquist shell)

bash 的奇技淫巧

　　挖坑待填