发现/usr/bin是在/usr/local/mysql这个路径之前的，这里做一个假设，系统先读到/usr/bin之后就直接运行了5.0.96的mysql客户端，而不管之后的路径了，所以即便推出session重新连接，始终都是先进入/usr/bin读取mysql。来验证一下，把这2个位置对调一下
[root@bak ~]# export PATH=/usr/kerberos/sbin:/usr/kerberos/bin:/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/local/mysql/bin:/usr/bin:/root/bin
[root@bak ~]# mysql
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 8
Server version: 5.0.96-community-log MySQL Community Edition (GPL)
Copyright (c) 2000, 2014, Oracle and/or its affiliates. All rights reserved.
Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.
Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.
mysql> \s
--------------
mysql Ver 14.14 Distrib 5.5.39, for linux2.6 (i686) using readline 5.1 --又以5.5.39的客户端连接到5.0.96的服务器了
Connection id: 8
Current database:
Current user: root@localhost
SSL: Not in use
Current pager: stdout
Using outfile: ''
Using delimiter: ;
Server version: 5.0.96-community-log MySQL Community Edition (GPL)
Protocol version: 10
Connection: Localhost via UNIX socket
Server characterset: latin1
Db characterset: latin1
Client characterset: utf8
Conn. characterset: utf8
UNIX socket: /var/lib/mysql/mysql.sock
Uptime: 21 min 15 sec
Threads: 1 Questions: 32 Slow queries: 0 Opens: 12 Flush tables: 1 Open tables: 6 Queries per second avg: 0.025
--------------
mysql>
不过这样做并没有固化到环境变量中，退出session以后再连上，PATH又变成之前的值了，就算source .bash_profile也是如此，来看一下root用户的.bash_profile文件
[root@bak ~]# cat .bash_profile
# .bash_profile
# Get the aliases and functions
if [ -f ~/.bashrc ]; then
. ~/.bashrc
fi
# User specific environment and startup programs
PATH=$PATH:$HOME/bin
export PATH
unset USERNAME
[root@bak ~]#
这里面没有显示指定PATH的具体值是哪些，只是用$调用了一下PATH变量，既然这样，我们就把期望的值写进去，然后就可以做到固化PATH路径的绝对值了
[root@bak ~]# vi .bash_profile
把$PATH改为
/usr/kerberos/sbin:/usr/kerberos/bin:/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/local/mysql/bin:/usr/bin:/root/bin
[root@bak ~]# . .bash_profile --使环境变量在当前session生效
这样，即使退出session也不会丢失给PATH设置的值了，当我们需要哪个版本的客户端时，就手动调整一下PATH的路径，让目标路径放在前面供系统查找即可