在系统里执行命令：man which
which命令用于查找并显示给定命令的绝对路径，环境变量PATH中保存了查找命令时需要遍历的目录。在环境变量PATH设置的目录里查找符合条件的文件。
语法：
which [选项][参数]
选项（常用）：
-a，显示所有匹配的可执行的路径
--skip-alias,忽略别名
-V：显示版本信息
参数：
指令名
实例：
[root@localhost ~]# which cp
alias cp='cp -i'
/bin/cp
[root@localhost ~]# which --skip-alias cp
/bin/cp
[root@localhost ~]# which ls
alias ls='ls --color=auto'
/bin/ls
[root@localhost ~]# which -a ls
alias ls='ls --color=auto'
/bin/ls
/usr/bin/ls
[root@localhost ~]# which -V
GNU which v2.20, Copyright (C) 1999 - 2008 Carlo Wood.
GNU which comes with ABSOLUTELY NO WARRANTY;
This program is free software; your freedom to use, change
and distribute this program is protected by the GPL.
