The command-line history is a mechanism to minimize the number of keystrokes. You can read more about it at http://www.tldp.org/LDP/GNU-Linux-Tools-Summary/html/x1712.htm (retrieved January 2016).

To simply execute the last run command, type the following again:

$ !!

Depending on which shell mode (vi or emacs) you are in, you may prefer other ways to navigate the history. The up and down arrows on your keyboard should also let you navigate history.

A common use case is to search for a long command we executed in the past and run it again. We can search through history, as follows:

$ history|grep <search for something>

You can of course shorten this using the aliasing mechanism or with a desktop text expander. The search gives a list of commands with numbers ranked in chronological order. You can execute, for instance, the command numbered 328, as follows:

$ !328

If, for example, you wish to execute the last command that started with python, type the following:

$ !python