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经常用Linux系统的人应该都接触过which和whereis命令，并会用这些命令进行文件查找/定位。但其中的用法和区别一般不会深究，这里我先简单说一下，待之后阅读了which和whereis命令的源码之后再来细说。

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whereis

locate the binary, source, and manual page files for a command

BUGS

Since whereis uses chdir(2V) to run faster, pathnames given with the -M, -S, or -B must be full; that is, they must begin with a ‘/’.
whereis has a hard-coded path, so may not always find what you’re looking for.

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which

shows the full path of (shell) commands.

Which takes one or more arguments. For each of its arguments it prints to stdout the full path of the executables that would have been executed when this argument had been entered at the shell prompt. It does this by searching for an executable or script in the directories listed in the environment variable PATH using the same algorithm as bash(1).

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$ whereis identify    # whereis命令会在特定目录（但特定目录是哪些？）中查找符合条件的文件
$ whereis -B /usr/local identify    # 在特定目录中查找符合条件的文件(没有用...)
$ whereis -B /usr/local/ImageMagick-6.9.3-10/bin identify    # 没有用...
$ which identify      # which命令会在 $PATH 中查找，找到一个就退出

下面是测试which和whereis命令功能区别的脚本：

#!/bin/bash
# set -x

echo -e "\$PATH = $PATH"
echo -e "[whereis identify]\t$(whereis identify)"
echo -e "[which identify]\t$(which identify)"
echo -e "\n"

export PATH=/usr/local/ImageMagick-6.9.2-10/bin:/usr/local/bin:/bin:/usr/bin:/sbin:/usr/sbin/:/usr/local/sbin
echo -e "\$PATH = $PATH"
echo -e "[whereis identify]\t$(whereis identify)"
echo -e "[which identify]\t$(which identify)"
echo -e "\n"

export PATH=/usr/local/ImageMagick-6.9.3-10/bin:/usr/local/bin:/bin:/usr/bin:/sbin:/usr/sbin/:/usr/local/sbin
echo -e "\$PATH = $PATH"
echo -e "[whereis identify]\t$(whereis identify)"
echo -e "[which identify]\t$(which identify)"

然后再放一个用Python实现的「which命令」：

#!/usr/bin/env python
# coding=utf-8

# A minimal version of the UNIX which utility, in Python.
# Author: Vasudev Ram - www.dancingbison.com

from __future__ import print_function
import sys
import os
import os.path
import stat

def usage():
    sys.stderr.write("Usage: python which.py name\n")
    sys.stderr.write("or: which.py name\n")

def which(name):
    found = 0
    for path in os.getenv("PATH").split(os.path.pathsep):
        full_path = path + os.sep + name
        if os.path.exists(full_path):
            """
            if os.stat(full_path).st_mode & stat.S_IXUSR:
                found = 1
                print(full_path)
            """
            found = 1
            print(full_path)
    # Return a UNIX-style exit code so it can be checked by calling scripts.
    # Programming shortcut to toggle the value of found: 1 => 0, 0 => 1.
    sys.exit(1 - found)

def main():
    if len(sys.argv) != 2:
        usage()
        sys.exit(1)
    which(sys.argv[1])

if "__main__" == __name__:
    main()

更多参考链接：

linux下which、whereis、locate、find 命令的区别

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