When using App Engine, docker containers are constantly restarting. Use this to keep a finger on the logs.

docker_log_last.dart

#!/usr/bin/env dart
import 'dart:async';
import 'dart:convert';
import 'dart:io';

const _splitter = const LineSplitter();

const _waitDuration = const Duration(milliseconds: 250);

const _green = '\u001b[32m';
const _red = '\u001b[31m';
const _yellow = '\u001b[33m';
const _noColor = '\u001b[0m';

main() async {
  while (true) {
    var result = Process.runSync('docker', ['ps', '-q']);

    var containers = _splitter.convert(result.stdout);

    if (containers.isEmpty) {
      _log('No Containers. Waiting $_waitDuration', _green);
      await new Future.delayed(_waitDuration);
      continue;
    }
    var first = containers.first;

    _log('Active containers: ${containers.join(', ')}', _green);
    _log('          Tailing: $first', _green);

    await _tailProcess(first);
  }
}

void _log(String value, [String color]) {
  if (color != null) stdout.write(color);
  stdout.write(value);
  if (color != null) stdout.write(_noColor);
  stdout.writeln();
}

Future _tailProcess(String container) async {
  Process proc = await Process.start('docker', ['logs', '-f', container]);

  var values = await Future.wait(
      [proc.exitCode, _drainStd(proc.stdout), _drainStd(proc.stderr, _red)]);

  _log('Container $container closed.', _green);
}

Future _drainStd(Stream<List<int>> std, [String color]) async {
  await std
      .transform(SYSTEM_ENCODING.decoder)
      .transform(_splitter)
      .forEach((line) {
    _log(line, color);
  });
}