Previous command recall
Also known as: the “history”.
↑ and ↓
Most command line interpreters remember the commands you’ve entered. You can get them back by pressing the up-arrow key. Keep pressing the up-arrow to get back previous commands — they aren’t executed, of course, until you press enter. Press down-arrow to come back through the history.
Don’t waste time typing in long commands when you can just tap up-arrow a few times to recall them.
Unix systems also support “history search”, which is another fast way to recall commands you’ve already issued. Press Ctrl+R, then start typing the string to match… then press Ctrl+R again to cycle through the matches. To execute that command (you can edit it first), press enter. If you don’t want to execute it, press Ctrl+C (or down-arrow back out of history) to get back to your prompt.