Question

Say, I just ran this command in Vim:

:nmap <CR> <C-]>

And now I want to copy this line and put it into my .vimrc.
How can I select and copy the whole line in command-line mode?

Answer 1

The fastest way is to run the command, switch to the destination
buffer (with .vimrc loaded, in this case) and paste the whole
command from the : register by typing

":p

in Normal mode.

If the command is further back in time, one can first recall it from
history (e.g., by typing the first few letters and pressing the up
arrow key ↑), rerun it, and then use the above method.

When these shortcuts are unhandy, one can resort to the general
approach of using the command-line window (see :help cmdwin).
To open it, either type q: in Normal mode, or press the key
combination set by the cedit option in Command-line mode
(Ctrl+F, by default).