zsh: read -p "Prompt: " VAR fails with "read: -p: no coprocess" (bash prompt syntax in zsh)

TL;DR

read -p "Prompt: " VAR is bash syntax. In zsh, -p means "read from the coprocess", so with no coprocess running zsh prints read: -p: no coprocess, returns 1 and does not assign VAR. Use printf 'Prompt: ' && read -rs VAR && echo. It works in both bash and zsh and does not echo the input.

Context (observed in this deployment)

Setup commands written for bash were pasted into a Google Cloud Shell session whose login shell was zsh. The line read -p "Prompt: " VAR failed with:

read: -p: no coprocess

Cause (documented)

• zsh: the read builtin documents -p as "Input is read from the coprocess." In zsh you set a prompt with a ? in the first argument: "If the first argument contains a ?, the remainder of this word is used as a prompt on standard error when the shell is interactive." Source: zshbuiltins read, docs source https://github.com/zsh-users/zsh/blob/master/Doc/Zsh/builtins.yo (rendered: https://zsh.sourceforge.io/Doc/Release/Shell-Builtin-Commands.html).
• The message comes from bin_read in https://github.com/zsh-users/zsh/blob/master/Src/builtin.c: when -p is set and there is no coprocess, it calls zwarnnam(name, "-p: no coprocess") and then return 1.
• bash: help read (bash 5.2) lists -p prompt as "output the string PROMPT without a trailing newline before attempting to read".

Fix (bash and zsh)

printf 'Prompt: ' && read -rs VAR && echo
export VAR   # only if child processes need it

• printf prints the prompt without a newline, in either shell.
• -r: backslashes are not treated as escapes. Both shells support it.
• -s: input typed at a terminal is not echoed, which suits API keys and other secrets. Both shells support it. zsh documents it as "Don't echo back characters if reading from the terminal."
• echo adds the newline that the hidden Enter key did not print.

zsh-only equivalent: read -rs 'VAR?Prompt: '. read -s is not POSIX sh, so this fix is for bash and zsh, not dash or /bin/sh scripts.

Verify

echo "$0"   # which shell you are in (e.g. -zsh or -bash)
printf 'Prompt: ' && read -rs VAR && echo && echo "read ${#VAR} characters"

In both shells the prompt appears, nothing you type is echoed, and the length is printed. In zsh, read -p "x: " VAR; echo "exit=$?" reproduces the error with exit=1.