server

#!/usr/bin/env bash
# Purpose: Provide HTTP server that displays the current server date to
# validate the artifact structure and play with.

declare -a _http_responses=(
  [200]="OK"
  [201]="Created"
  [202]="Accepted"
  [204]="No Content"
  [301]="Moved Permanently"
  [302]="Found"
  [307]="Temporary Redirect"
  [400]="Bad Request"
  [401]="Unauthorized"
  [402]="Payment Required"
  [403]="Forbidden"
  [404]="Not Found"
  [405]="Method Not Allowed"
  [500]="Internal Server Error"
  [501]="Not Implemented Error"
  [502]="Bad Gateway"
  [503]="Temporarily Unavailable"
)

function _recv() {
  echo "< $*" >&2
}

function _send() {
  printf '%s\r\n' "$*"
}

function _send_headers() {
  local -r dt="$(date +"%a, %d %b %Y %H:%M:%S %Z")"
  local -r hst="$(hostname -s)"
  local -a hdrs=(
    "Date: ${dt}"
    "Expires: ${dt}"
    "Content-Type: text/plain"
    "Server: hello-bash@${hst}/0.0.1"
  )
  for h in "${hdrs[@]}"; do
    _send "${h}"
  done
}

function _send_body() {
  _send "Yo yo yo!"
}

function _send_response() {
  _send "${1}"
  _send_headers
  _send ""
  _send_body
}

function _respond_with() {
  if [ "$#" -ne 1 ]; then
    >&2 "Error: Expected one argument for ${FUNCNAME}, received $#."
    >&2 "Usage: ${0} CODE"
    return 1
  fi
  local -r code="${1}"
  _send_response "${code} ${_http_responses[${code}]}"
}

function _parse_request() {
  if [ "$#" -ne 1 ]; then
    >&2 "Error: Expected one argument for ${FUNCNAME}, received $#."
    >&2 "Usage: ${0} REQUEST_LINE"
    return 1
  fi
  local -r req="${1%%$'r'}"
  read -r req_method req_uri req_http_ver <<<"${req}"

  # Validate request (only support GET and / URI so far :)
  if [ "${req_method}" = "GET" -a "${req_uri}" = "/" -a -n "${req_http_ver}" ]; then
    _respond_with 200
  elif [ "${req_uri}" != "/" ]; then
    _respond_with 404
  elif [ -z "${req_http_ver}" ]; then
    _respond_with 400
  else
    _respond_with 500
  fi
}

function _main() {
  read -r line || _respond_with 400
  _parse_request "${line}"
}

if [ "${BASH_SOURCE[0]}" != "${0}" ]; then
  export -f _respond_with
  export -f _parse_request
else
  set -eu
  _main
fi

You can eliminate 2 forks if you want. Bash has both date and hostname built-in.

replace:
local -r dt="$(date +"%a, %d %b %Y %H:%M:%S %Z")"
local -r hst="$(hostname -s)"
with
local -r dt
printf -v dt "%(%a, %d %b %Y %H:%M:%S %Z)T" -1

and replace all instances of hst with HOSTNAME