Simple IMAP client (STARTTLS + cram-sha256) with Bash and OpenSSL

#!/bin/bash
set -euo pipefail

################################################################################
## Fill this in!                                                              ##
################################################################################
declare -r server='your.server.name'            # server to connect to
declare -ri port='143'                          # IMAP with STARTTLS == 143
declare -r username='someone@your.server.name'  # some servers use only someone
declare -r password='your.clear.text.password'  # used for authentication
declare -r hash='sha256'                        # md5, sha1, sha256
################################################################################

# This is taken from here: https://www.atmail.com/blog/advanced-imap/
respond_to_challenge() {
  local digest _
  read _ digest < <(openssl base64 -d <<< "$1" |
                    openssl dgst "-${hash}" -hmac "$password")
  printf '%s %s' "$username" "$digest" | openssl base64 -A
  echo
}

tmpdir=$(mktemp -d)
declare -r tmpdir
declare -a pids
trap 'rm -Rf "$tmpdir"; ((${#pids[@]})) && kill -9 "${pids[@]/#/-}" || :;' EXIT

declare -r input="${tmpdir}/imap_server_input"
declare -r output="${tmpdir}/imap_server_output"
declare -r value_fifo="${tmpdir}/value_fifo"
mkfifo "$input" "$output" "$value_fifo"

# Start the OpenSSL client to connect to the IMAP server.
openssl s_client -starttls imap -crlf -connect "${server}:$((port))" \
    <"$input" >"$output" 2>/dev/null&
pids+=("$!")

# Start the IMAP server input writer.
{
  echo "1 authenticate cram-${hash}"
  respond_to_challenge "$(< "$value_fifo")"
  [[ "$(< "$value_fifo")" == 'OK' ]]
  echo '2 list "" "*"'
  [[ "$(< "$value_fifo")" == 'OK' ]]
  echo '3 select INBOX'
  [[ "$(< "$value_fifo")" == 'OK' ]]
  echo '4 status INBOX (MESSAGES)'
  [[ "$(< "$value_fifo")" == 'OK' ]]
  echo '5 logout'
  [[ "$(< "$value_fifo")" == 'OK' ]]
} | tee "$input" &
pids+=("$!")

declare -r color_default=$'\033[00m'
declare -r color_yellow=$'\033[01;33m'

# Start the IMAP server output reader.
{
  expected_prefixes=('+' '1' '2' '3' '4' '5')
  output() { :; }
  for expected in "${expected_prefixes[@]}"; do
    prefix=
    while read prefix value rest; do
      prefix="${prefix%$'\r'}"
      value="${value%$'\r'}"
      rest="${rest%$'\r'}"
      [[ "$prefix" != "$expected" ]] || break
      output "$prefix" "$value" "$rest"
    done
    [[ "$prefix" == "$expected" ]]
    output() {
      printf "${color_yellow}%s %s %s${color_default}\n" "$1" "$2" "$3"
    }
    output "$prefix" "$value" "$rest"
    printf '%s' "$value" > "$value_fifo"
  done
  cat >/dev/null
} <"$output" &
pids+=("$!")

declare -i errors=0 idx
for idx in "${!pids[@]}"; do
  pid="${pids[idx]}"
  unset 'pids[idx]'
  wait -n "$pid" || ((++errors))
done
((! errors))