Make prompt only display directory when not at $HOME?

shell command-line zsh prompt

setopt PROMPT_SUBSTPROMPT+='$([[ $PWD != $HOME ]] && echo "[%~] ")'

setopt PROMPT_SUBST enables parameter expansion ($name), command substitution ($(command)) and arithmetic expansion ($[exp] or $((exp))) within the prompt
PROMPT+='…' appends to the prompt. Note that you need to use single quotes here so that the contents are not expanded during definiton but only when the prompt is shown.
$(command) runs command and substitutes its output, e.g. $(echo foo) would be replaced with "foo".
[[ exp ]] evaluates a conditional expression. It will return zero only if the expression is true. Note that [[ exp ]] is built-in syntax of zsh, while the similar [ exp ] would run the external [ (aka test) command.
PWD contains the current working directory.
HOME contains the home directory of the current user.
!= tests for inequality.
&& runs the command to the right only if the command to the left was successful (that is, returned zero)
echo "[%~] " prints the (partial) prompt string. Note that double quotes are used so that the surrounding single quotes are not closed prematurely.

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