Line-by-line manipulation of text files (eg. error log files) using node.js' new Stream.Transform functionality and pipe(). You can call this directly from your shell, eg. $ node parse.js error.log

parse.js

var fs = require('fs');
var TransformLine = require('./TransformLine');

// syntax example:
// $ node parse.js error.log

var inFile = process.argv[2];
var outFile = process.argv[3];

if (!inFile) {
	console.log('no input file specified');
	process.exit();
}

var inStream = fs.createReadStream(inFile);
var outStream = (!outFile) ? process.stdout : fs.createWriteStream(outFile, {flags: 'w'});

var createLineNumberTransformer = function() {
	var i = 1;
	return new TransformLine(function (line) {
		return (i++)+': ' + line;
	}, true);
};

var regexpTransformer = function(regexp, replaceString) {
	return new TransformLine(function (line) {
		var line = line.replace(regexp, replaceString);
		return line ? line + '\n' : '';
	});
};

// example how to remove lines containing '[debug]' from error.log files
var removeDebugMessageTransformer = regexpTransformer(/^.*\[debug\].*$/, '');

inStream
	.pipe(removeDebugMessageTransformer)
	.pipe(createLineNumberTransformer())
	.pipe(outStream);

transformLine.js

var util = require('util');
var Stream = require('stream');
var Transform = Stream.Transform;

function TransformLine(callback, addLineBreak) {
	this._lineBreak = addLineBreak ? '\n' : '';

	// allow use without new
	if (!(this instanceof TransformLine)) {
		return new TransformLine(callback);
	}
	Transform.call(this, {objectMode: true});
	this._callback = callback;
}

util.inherits(TransformLine, Transform);

TransformLine.prototype._transform = function (chunk, encoding, done) {
	var self = this;
	var data = chunk.toString();
	if (this._dataRemaining) {
		data = this._dataRemaining + data;
	}

	var lines = data.split('\n'); 
	this._dataRemaining = lines.splice(lines.length-1,1)[0];
	lines.forEach(function(line) {
		line = self._callback(line);
		self.push(line + self._lineBreak);
	});
	done();
}

TransformLine.prototype._flush = function (done) {
	var self = this;
	if (this._dataRemaining) {
		line = self._callback(_dataRemaining);
		this.push(line + self._lineBreak);
	}
	this._dataRemaining = null;
	done();
}

module.exports = TransformLine;