Node.js 监控文件的变化并解析它们

Question

我需要监视文件的更改。由于此文件中有大量新条目，我需要“监视”此文件。我需要将新插入的内容添加到此文件中才能解析此内容。

我找到了这个代码：

fs.watchFile('var/log/query.log', function() {
console.log('File Changed ...');
//how to get the new line which is now inserted?
});

Answer 1

下面是一个示例，展示了我如何fs.watchFile监视名为《炉石传说》的游戏的日志文件，并选取新的日志条目来监视玩游戏时发生的游戏事件。https://github.com/chevex-archived/hearthstone-log-watcher/blob/master/index.js

var fs = require('fs');
var options = {
  logFile: '~/Library/Preferences/Blizzard/Hearthstone/log.config',
  endOfLineChar: require('os').EOL
};
// Obtain the initial size of the log file before we begin watching it.
var fileSize = fs.statSync(options.logFile).size;
fs.watchFile(options.logFile, function (current, previous) {
  // Check if file modified time is less than last time.
  // If so, nothing changed so don't bother parsing.
  if (current.mtime <= previous.mtime) { return; }

  // We're only going to read the portion of the file that
  // we have not read so far. Obtain new file size.
  var newFileSize = fs.statSync(options.logFile).size;
  // Calculate size difference.
  var sizeDiff = newFileSize - fileSize;
  // If less than zero then Hearthstone truncated its log file
  // since we last read it in order to save space.
  // Set fileSize to zero and set the size difference to the current
  // size of the file.
  if (sizeDiff < 0) {
    fileSize = 0;
    sizeDiff = newFileSize;
  }
  // Create a buffer to hold only the data we intend to read.
  var buffer = new Buffer(sizeDiff);
  // Obtain reference to the file's descriptor.
  var fileDescriptor = fs.openSync(options.logFile, 'r');
  // Synchronously read from the file starting from where we read
  // to last time and store data in our buffer.
  fs.readSync(fileDescriptor, buffer, 0, sizeDiff, fileSize);
  fs.closeSync(fileDescriptor); // close the file
  // Set old file size to the new size for next read.
  fileSize = newFileSize;

  // Parse the line(s) in the buffer.
  parseBuffer(buffer);
});

function stop () {
  fs.unwatchFile(options.logFile);
};

function parseBuffer (buffer) {
  // Iterate over each line in the buffer.
  buffer.toString().split(options.endOfLineChar).forEach(function (line) {
    // Do stuff with the line :)
  });
};

它首先计算文件的初始大小，因为在这个日志观察器模块中，我只想读取游戏写入的新数据。我不关心现有数据。然后它开始监视文件的更改。当更改处理程序触发时，我们检查修改时间是否确实较新，因为当我们关心的数据没有实际更改时，有关文件的其他一些更改可能会触发处理程序。我们希望这个观察者尽可能地表现出色。

然后我们读取文件的新大小并计算与上次的差异。这准确地告诉我们要从文件中读取多少数据才能仅获取新写入的数据。然后我们将数据存储在缓冲区中并将其解析为字符串。只需用换行符分割字符串即可。使用 core 模块os来获取os.EOL将为您所运行的操作系统提供正确的行结束字符（windows 行结束字符与 linux/unix 不同）。

现在您已将行数组写入文件:)

Answer 2

在 bash 上，你会用tail --follow.

还有一个包tail可用。您可以观看文件，并在事件中获取新行：

const Tail = require('tail').Tail;
var tail = new Tail("var/log/query.log");
tail.watch()
tail.on("line", data => {
  console.log(data);
});