普通的alias指令是在terminal中被直接输入的，关闭terminal之后就失效了，是“一次性”的。
alias cls="clear"
如何让cls永久的在terminal永久的代替clear呢，解决办法如下：
You can put such aliases in the ~/.bash_aliases file.
That file is loaded by ~/.bashrc. On Ubuntu 10.04, the following lines need to be uncommented to enable the use of ~/.bash_aliases. On Ubuntu 11.04 and later, it's already enabled:
if [ -f ~/.bash_aliases ]; then
. ~/.bash_aliases
fi
The aliased command will be available on any new terminal. To have the aliased command on any existing terminal one need to source ~/.bashrc from that terminal as,
source ~/.bashrc
from： http://askubuntu.com/questions/1414/how-to-create-a-permanent-alias
