I use a shell every day. Almost always, I want to repeat a previous command, or repeat it after a slight modification. A very convenient way is to use arrow-up to get the most recent command back. Another common trick is to type ctrl-R and incrementally search for a previously used command. However, there are two other tricks for repeating previous commands that I use all the time, which are not as well known.
Often, you want to repeat only the last argument of the previous command. For example, suppose you want to run git diff path/to/tests, and then git add path/to/tests. For the second command, you can type git add escape-dot (escape followed by a period), and it gets expanded to path/to/tests (the last argument of the previous command).