Download a file when its ETag changes. Useful for GitHub (which doesn't respect If-Modified-Since).

conditional_get_etag.sh

#!/bin/bash

conditional_get_etag() {
  local _url _file _tmpdir
  local _awk_program='/^ETag:/ {$1=""; printf "If-None-Match: %s", substr($0, 2, length($0)-2); exit;}'

  for _url; do
    if [ '--file=' = "${_url:0:7}" ]; then
      _file="${_url:7}"
      continue
    fi
    if [ -z "$_file" ]; then
      _tmpdir="$(mktemp -d --tmpdir .curl-filename-XXXXXX || { mkdir -p '.tmp' && echo '.tmp'; })"
    fi
    
    if [ -n "$_tmpdir" ] && cd "$_tmpdir"; then
      # This is a guess, but doesn't hurt if we're wrong.
      cp 2>/dev/null ~-/"$(basename "$_url").headers" '.headers'

      curl > '.filename' -JLORSs -w '%{filename_effective}\n' -H "$(awk 2>/dev/null "$_awk_program" ".headers")" -D '.headers' "$_url"

      _file="$( < .filename )"
      [ -n "$_file" ] || _file="$(basename "$_url")"
      if [ -s "$_file" ]; then
        mv "$_file" ~-/"${_file}.tmp" && mv '.headers' ~-/"${_file}.tmp.headers"
      fi

      cd - >/dev/null
      rm -rf "$_tmpdir"
    else
      curl -JLRSs -H "$(awk 2>/dev/null "$_awk_program" "${_file}.headers")" -D "${_file}.tmp.headers" -o "${_file}.tmp" "$_url"
    fi
    unset -v _tmpdir

    if [ -s "${_file}.tmp" ]; then
      [ ! -f "$_file" ] || chown -v --reference "$_file" "${_file}.tmp"
      [ ! -f "$_file" ] || chmod -v --reference "$_file" "${_file}.tmp"
      mv -v "${_file}.tmp" "$_file"
      mv -v "${_file}.tmp.headers" "${_file}.headers"
    fi
    rm -f "${_file}.tmp.headers" "${_file}.tmp"

    unset -v _file
  done; unset -v _url
}

conditional_get_etag "$@"