How to put a newline in the zsh prompt without causing terminal redraw issues

command lineosx-snow-leopardpromptzsh

I'm trying to fix a minor (but annoying) issue with my zsh prompt. The problem I'm seeing is that when I'm using a terminal in a GUI OS environment (for example, Terminal.app or iTerm 2 on OS X 10.7.2) and resize the terminal window, the terminal is redrawn with extra prompts.

I've reproduced the issue on a clean .zshrc that contains the following:

export PROMPT=">
"

To be clear, that's a two-line file. There is a literal newline in the string, which I've read in several (admittedly old) places is the way to go. This works until I resize the terminal window, at which point I end up with duplicate prompt strings. For example if I open a new window, I see (* being used a placeholder for my cursor):

Last login: Wed Jan 25 19:05:06 on ttys000
>
*

Then when I resize the window, making it shorter, I end up with:

Last login: Wed Jan 25 19:05:06 on ttys000
>
>
>
>
*

The number of extra prompt printouts seems roughly proportional to how much I change the window size — usually vertically. If I remove the newline from the prompt string, the problem goes away.

I've tried various things, like $(print ">\n"), or trying to wrap the newline in %{%}. I'm assuming this has to do with the fact that newline confuses whatever is drawing w.r.t how long the prompt string actually is, or something. Is this problem fixable or do I just have to live with it?

Best Answer

This problem is due to how ZSH reprints the promt on resize events and has also annoyed me before. The solution is to make $PROMPT single-line and print the first line of the prompt using a precmd.

So in your example that would simply be:

precmd() { print ">" }
export PROMPT=""

or for a more sophisticated example with prompt expansion in the print statement use the -rP parameters:

precmd() { print -rP "%~" }
export PROMPT="%# "

If you have more than one precmd registered you need to use add-zsh-hook precmd (see man zshcontrib).