If the readline package has been compiled in, then when units is used interactively, numerous command line editing features are available. To check if your version of units includes readline, invoke the program with the --version option.
For complete information about readline, consult the documentation for the readline package. Without any configuration, units will allow editing in the style of emacs. Of particular use with units are the completion commands.
If you type a few characters and then hit <ESC> followed by ? then units will display a list of all the units that start with the characters typed. For example, if you type metr and then request completion, you will see something like this:
You have: metr
metre metriccup metrichorsepower metrictenth
metretes metricfifth metricounce metricton
metriccarat metricgrain metricquart metricyarncount
You have: metr
If there is a unique way to complete a unitname, you can hit the <TAB> key and units will provide the rest of the unit name. If units beeps, it means that there is no unique completion. Pressing the <TAB> key a second time will print the list of all completions.