Sometimes after installing MySQL, entering commands like mysql or mysqladmin directly results in a “command not found” error. This happens because the system looks for commands under /usr/bin by default. If the command is not located in that directory, it naturally cannot be found. What we need to do is create a symbolic link to the /usr/bin directory, essentially setting up a linked file.
First, you need to know the full path of the mysql or mysqladmin command. For example, if the mysql path is: /usr/local/mysql/bin/mysql, you can execute the command like this:
ln -s /usr/local/mysql/bin/mysql /usr/bin
Return to the home directory and enter mysql -u username -p to proceed.
Source: http://blog.csdn.net/johnsonvily/article/details/6704002