mjhjmtu

In bash, when I use Ctrl-R to retrieve a previous command, why does it not work when the command starts with a whitespace? Can I make it match such a previous command?

$ date
Fri Nov 23 ... 2018
(failed reverse-i-search)` date': cd database/

Check the value of your HISTCONTROL environment variable. If the value contains ignorespace or ignoreboth, any command starting with a space will not be added to command history.

From man bash:

HISTCONTROL:
A colon-separated list of values controlling how commands are saved on the history list. If the list of
values includes ignorespace, lines which begin with a space character are not saved in the history list.
A value of ignoredups causes lines matching the previous history entry to not be saved. A value of
ignoreboth is shorthand for ignorespace and ignoredups. A value of erasedups causes all previous lines
matching the current line to be removed from the history list before that line is saved. Any value not
in the above list is ignored. If HISTCONTROL is unset, or does not include a valid value, all lines
read by the shell parser are saved on the history list, subject to the value of HISTIGNORE. The second
and subsequent lines of a multi-line compound command are not tested, and are added to the history
regardless of the value of HISTCONTROL.

That is intended. White space makes no change to the interpretation of the command. History ignores command starting with a space, so that you can enter commands that you don't want logged. Now very secure, as someone on same machine, can spy when you do it.

I think it can be re-configured. See the bash-manual, under history.