How can I intermittently show my history command number in my shell prompt?

How can I intermittently show my history command number in my shell prompt? For instance, rather than showing it in EVERY prompt, just do it every 7 times. (I'm using zsh, but I think bash should be virtually identical.) The problem I encounter is that %h is not evaluated until it's in the PROMPT variab开发者_如何学Pythonle, and $HISTCMD is always evaluated as 0 for some reason. So putting a function like this into my prompt fails because $HISTCMD is always 0:

prompt_history() {
CYCLE=$(( $HISTCMD % 7 ))
if [[ "$CYCLE" = "0" ]]; then
echo -ne "$HISTCMD"
fi
}

PROMPT="$(prompt_history) blah-blah >:"

This can be partly fixed by echoing "%h" instead of "$HISTCMD", but only partly.

It is further complicated by the fact that the history command does not (seem to) function within a .zshrc file, so something like this won't work:

CYCLE="$(( $(history 1 | wc -l) % 7 ))"

(If you're using bash, change "history 1" to just "history".)

Also, the history file is not usable as a source of this information since (at least the way I have things configured--and I'd rather not change this configuration) history is not shared between sessions until a zsh session closes and its history is added to my $HISTFILE. Therefore, this won't work:

CYCLE="$(( $(cat $HISTFILE | wc -l) % 7 ))"

I'm on the verge of believing this is currently impossible. I'd love someone to prove me wrong.

You simply need to delay evaluation of the prompt until it's issued. Just change the double quotes to single quotes:

PROMPT='$(prompt_history) blah-blah >:'