I stumbled into this by accident. Somehow I pressed Ctrl+r and gotten this prompt in my Bash shell:
That pushed me to research on what this Ctrl+r really does. Perhaps it was mentioned in some manual, but I just skipped as unimportant.
Ctrl+r allows one to search backwards in the recent history of the current shell. If the history is long it can save a lot of time going through the old commands. It is also a lot less cumbersome than history | grep “blah” mechanism. Just press this key combination and start typing. Multiple entries can be scrolled through by repeated Ctrl+r. Once you spot the target command, just press [enter] to execute it. Alternatively, using the right or left arrow keys will place the command on an actual command-line so you can edit it. Ctrl+g will abort the search and restore the original line.
Ctrl+r is supported by the GNU readline library. So any shell program, including your own creation, can have this history search functionality if this library is used.