添加环境变量主要是在/etc/profile中

例如，在/usr/local/下安装了python2.7
因为系统默认是2.4，所以

#python
Python 2.4.3 (#1, Sep 

3 2009, 15:37:37)
[GCC 4.1.2 20080704 (Red Hat 4.1.2-46)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>>

进入python2.7的安装路径，在/bin下找到python文件

./python
Python 2.7.2 (default, Feb  2 2012, 11:45:14)
[GCC 4.1.2 20080704 (Red Hat 4.1.2-51)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>>

先查看下目前的环境变量

#echo $PATH
/usr/local/jdk1.5/bin:/usr/kerberos/sbin:/usr/kerberos/bin:/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

默认是没有最新的python2.7的
我进入新装的python2.7目录查看
/usr/local/python/bin/python2.7
这个就是python2.7的启动文件

# PATH=$PATH:/usr/local/python/bin/
# echo $PATH
/usr/local/jdk1.5/bin:/usr/kerberos/sbin:/usr/kerberos/bin:/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin:/usr/local/python/bin/

将新装的/usr/local/python/bin添加到环境变量中(但这个只是临时的做法，退出python后就无效了)

永久有效的做法是：进入/etc/profile，

PATH=$PATH:/usr/local/python/bin
export PATH

这时当我们输入

# python2.7
Python 2.7.2 (default, Feb  2 2012, 11:45:14)
[GCC 4.1.2 20080704 (Red Hat 4.1.2-51)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>>

因为系统会自动在环境中找寻python2.7这个文件并启动，但会优先找/usr/bin下的启动文件，所以
当#python时，还是会进入2.4的默认系统

这时我们可以利用软连接来做到直接进入python2.7，进入/usr/bin，建立连接
ln -s /usr/local/python/bin/python ./python27
这样我们#python27 的时候，就会直接进入2.7的环境了