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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。which指令会在环境变量$PATH设置的目录里查找符合条件的文件。也就是说，使用which命令，就可以看到某个系统命令是否存在，以及执行的到底是哪一个位置的命令。
语法：
which (选项) (参数)选项：
-n<文件名长度>：制定文件名长度，指定的长度必须大于或等于所有文件中最长的文件名； -p<文件名长度>：与-n参数相同，但此处的<文件名长度>包含了文件的路径； -w：指定输出时栏位的宽度； -V：显示版本信息。参数：
指令名：指令名列表。实例：
查找文件、显示命令路径：
XXXXXX:/ GavinPSU$ which whereis /usr/bin/whereis XXXXXX:/ GavinPSU$ which pwd /bin/pwd XXXXXX:/ GavinPSU$
===BSD General Commands Manual =====
NAME
which -- locate a program file in the user‘s path
SYNOPSIS
which [-as] program ...
DESCRIPTION
The which utility takes a list of command names and searches the path for
each executable file that would be run had these commands actually been
invoked.
The following options are available:
-a List all instances of executables found (instead of just the first one of each).
-s No output, just return 0 if any of the executables are found, or 1 if none are found.
Some shells may provide a builtin which command which is similar or identical to this utility. Consult the builtin(1) manual page.
SEE ALSO
builtin(1), csh(1), find(1), locate(1), whereis(1)
HISTORY
The which command first appeared in FreeBSD 2.1.
AUTHORS
The which utility was originally written in Perl and was contributed by Wolfram Schneider <wosch@FreeBSD.org>. The current version of which was rewritten in C by Daniel Papasian <dpapasia@andrew.cmu.edu>.
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原文地址：http://blog.csdn.net/songzitea/article/details/51339325
