How to prompt user input from an interactive text editor in POSIX shell

posix-prompt-for-editor-input.sh

#!/bin/sh 
#
# How to prompt user input from an interactive text editor in POSIX shell.
# ======================================================================
#
# Permission to use, copy, modify, and/or distribute this software for any
# purpose with or without fee is hereby granted.
#
# THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES WITH
# REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY
# AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT,
# INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM
# LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR
# OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR
# PERFORMANCE OF THIS SOFTWARE.
#

# Create a temporary file using only POSIX commands
# -------------------------------------------------
umask 0177
tempfile=/tmp/"$0"."$$"."$(awk 'BEGIN {srand();printf "%d\n", rand() * 10^10}')"
trap 'rm -f -- "$tempfile"' INT TERM HUP EXIT
: > "$tempfile"

# Editor to use, set to vi if not specifed in the environment
# -----------------------------------------------------------
: "${EDITOR:=vi}"

# Open the input file for editing
# -------------------------------
$EDITOR "$tempfile"

# Read the file into a variable
# -----------------------------
user_input="$(cat "$tempfile")"

# Use the input
# -------------
printf "User input:\n%s\n" "$user_input"

# Cleanup
# -------
rm "$tempfile"