1.PATH环境变量作用
PATH环境变量是一组字符串。改组字符串的功能是找到linux中指令的位置，执行对应位置的指令功能。
具体说，你在任何当前目录下输入ls，都能显示当前目录下的内容，例如在/home和/usr下输入linux指令ls都能显示该目录下的内容，为什么无论在哪个目录底下输入ls指令，linux都能知道该指令是显示当前目录下的内容的呢。原因就是PATH环境变量。当你输入一个指令时，其实系统会在PATH路径下找该指令。
如果你新安装的一个软件不再现有的path的路径下，那么执行时就找不到路径，不能运行。这时需要向PATH变量添加路径
2.如何向PATH添加路径
export PATH=$PATH:/usr/local/bin/fio
冒号：是PATH中不同路径的分隔符，上述指令将/usr/local/bin/fio添加到已有PATH路径中
查看当前PATH中有哪些内容:echo PATH
运行截图如下：
由上面截图看到，路径成功添加。在添加路径之前，我们运行fio需要指令fio的位置。但添加之后可以直接使用fio ..
3.如何知道向PATH中添加那个目录呢
以上我是如何找到我安装的fio的路径在/usr/local/bin/fio路径下呢。这儿需要使用到which和whereis指令，系统会帮你找到你安装的fio的位置
which fio
whereis fio
which指令原理，在PATH变量中，搜索某个系统命令的位置，利用该指令可以判断系统命令是否存在以及执行的到底是哪个位置的命令
whereis命令原理：得到很多结果，该命令会把包含fio的地点都罗列出来。
综合来说，你可以通过whereis看到你安装同一个指令的不同版本，通过which指令来判断你当前执行的是哪个版本
which,where指令执行如下图：
4.如果某个指令安装了多个版本，如何改变系统默认指令的版本
如你的系统中安装了多个版本的gcc指令，如何切换gcc版本。如下例子说明如何从x版本切换到2.95版本。key 更改软连接
sudo ln -fs /usr/bin/gcc-2.95 /usr/bin/gcc
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