The shell keeps track of commmand you issued.

follow these steps to learn:

1. type this at the shell:

history

a list of the command you have executed will be displayed, with a number next to each command you can tell the shell to rerun an earlier command based on tis number or based on tis name

now try this

!d

or

r d

the last command that began witht he letter d is executed

these could be some great time savers (shorcuts) if you get used to them.