BEL in Bash / Zsh PROMPT

Question

I do have the following config in my .zshrc:

BEL=$(tput bel)
PROMPT+='%(?::$BEL)'

That works/worked in some environment (always Cygwin on Windows), but currently does not anymore.

(Same behaviour: not working, in Bash on Ubuntu on Windows.)

What is wrong?

Answer 1

In zsh, unlike bash, parameter expansions embedded in a prompt are only expanded when the prompt is displayed if the PROMPT_SUBST option is set.

%(...) is a zsh-specific escape sequence, and PROMPT itself is a zsh-specific name, so this will not work in bash, which would require something like

PS1+='$(if test $? -ne 0; then tput bel; fi)'

(which, incidentally, would work in zsh as well, assuming PROMPT_SUBST were enabled. A command that would conditionally enable it if the current shell is, in fact, zsh, might be

if [ -z "$ZSH_VERSION" ]; then
  setopt PROMPT_SUBST
fi