问题 使用Node.js实时读取文件

为什么你认为 tail -f 是一个黑客？

虽然弄清楚我发现了一个很好的例子，但我会做类似的事情。 node.js和WebSocket的实时在线活动监控示例：
http://blog.new-bamboo.co.uk/2009/12/7/real-time-online-activity-monitor-example-with-node-js-and-websocket

只是为了让这个答案完整，我给你写了一个在0.8.0下运行的示例代码 - （http服务器可能是黑客）。

子进程是用tail生成的，因为子进程是一个带有三个流的EventEmitter（在我们的例子中我们使用stdout）你可以添加一个监听器 on

文件名： tailServer.js

用法： node tailServer /var/log/filename.log

var http = require("http");
var filename = process.argv[2];


if (!filename)
    return console.log("Usage: node tailServer filename");

var spawn = require('child_process').spawn;
var tail = spawn('tail', ['-f', filename]);

http.createServer(function (request, response) {
    console.log('request starting...');

    response.writeHead(200, {'Content-Type': 'text/plain' });

    tail.stdout.on('data', function (data) {
      response.write('' + data);                
    });
}).listen(8088);

console.log('Server running at http://127.0.0.1:8088/');

这个模块是@hasanyasin建议的原则的实现：

https://github.com/felixge/node-growing-file

我从@hasanyasin那里得到了答案并将其包装成模块化的承诺。基本思想是传递一个文件和一个处理函数，它对从文件中读取的字符串化缓冲区执行某些操作。如果处理函数返回true，则文件将停止被读取。如果处理程序没有足够快地返回true，您还可以设置一个超时，该超时将终止读取。

如果由于超时调用resolve（），则promiser将返回true，否则返回false。

有关用法示例，请参见底部。

// https://stackoverflow.com/a/11233045

var fs = require('fs');
var Promise = require('promise');

class liveReaderPromiseMe {
    constructor(file, buffStringHandler, opts) {
        /*
            var opts = {
                starting_position: 0,
                byte_size: 256,
                check_for_bytes_every_ms: 3000,
                no_handler_resolution_timeout_ms: null
            };
        */

        if (file == null) {
            throw new Error("file arg must be present");
        } else {
            this.file = file;
        }

        if (buffStringHandler == null) {
            throw new Error("buffStringHandler arg must be present");
        } else {
            this.buffStringHandler = buffStringHandler;
        }

        if (opts == null) {
            opts = {};
        }

        if (opts.starting_position == null) {
            this.current_position = 0;
        } else {
            this.current_position = opts.starting_position;
        }

        if (opts.byte_size == null) {
            this.byte_size = 256;
        } else {
            this.byte_size = opts.byte_size;
        }

        if (opts.check_for_bytes_every_ms == null) {
            this.check_for_bytes_every_ms = 3000;
        } else {
            this.check_for_bytes_every_ms = opts.check_for_bytes_every_ms;
        }

        if (opts.no_handler_resolution_timeout_ms == null) {
            this.no_handler_resolution_timeout_ms = null;
        } else {
            this.no_handler_resolution_timeout_ms = opts.no_handler_resolution_timeout_ms;
        }
    }


    startHandlerTimeout() {
        if (this.no_handler_resolution_timeout_ms && (this._handlerTimer == null)) {
            var that = this;
            this._handlerTimer = setTimeout(
                function() {
                    that._is_handler_timed_out = true;
                },
                this.no_handler_resolution_timeout_ms
            );
        }
    }

    clearHandlerTimeout() {
        if (this._handlerTimer != null) {
            clearTimeout(this._handlerTimer);
            this._handlerTimer = null;
        }
        this._is_handler_timed_out = false;
    }

    isHandlerTimedOut() {
        return !!this._is_handler_timed_out;
    }


    fsReadCallback(err, bytecount, buff) {
        try {
            if (err) {
                throw err;
            } else {
                this.current_position += bytecount;
                var buff_str = buff.toString('utf-8', 0, bytecount);

                var that = this;

                Promise.resolve().then(function() {
                    return that.buffStringHandler(buff_str);
                }).then(function(is_handler_resolved) {
                    if (is_handler_resolved) {
                        that.resolve(false);
                    } else {
                        process.nextTick(that.doReading.bind(that));
                    }
                }).catch(function(err) {
                    that.reject(err);
                });
            }
        } catch(err) {
            this.reject(err);
        }
    }

    fsRead(bytecount) {
        fs.read(
            this.file,
            new Buffer(bytecount),
            0,
            bytecount,
            this.current_position,
            this.fsReadCallback.bind(this)
        );
    }

    doReading() {
        if (this.isHandlerTimedOut()) {
            return this.resolve(true);
        } 

        var max_next_bytes = fs.fstatSync(this.file).size - this.current_position;
        if (max_next_bytes) {
            this.fsRead( (this.byte_size > max_next_bytes) ? max_next_bytes : this.byte_size );
        } else {
            setTimeout(this.doReading.bind(this), this.check_for_bytes_every_ms);
        }
    }


    promiser() {
        var that = this;
        return new Promise(function(resolve, reject) {
            that.resolve = resolve;
            that.reject = reject;
            that.doReading();
            that.startHandlerTimeout();
        }).then(function(was_resolved_by_timeout) {
            that.clearHandlerTimeout();
            return was_resolved_by_timeout;
        });
    }
}


module.exports = function(file, buffStringHandler, opts) {
    try {
        var live_reader = new liveReaderPromiseMe(file, buffStringHandler, opts);
        return live_reader.promiser();
    } catch(err) {
        return Promise.reject(err);
    }
};

然后使用上面这样的代码：

var fs = require('fs');
var path = require('path');
var Promise = require('promise');
var liveReadAppendingFilePromiser = require('./path/to/liveReadAppendingFilePromiser');

var ending_str = '_THIS_IS_THE_END_';
var test_path = path.join('E:/tmp/test.txt');

var s_list = [];
var buffStringHandler = function(s) {
    s_list.push(s);
    var tmp = s_list.join('');
    if (-1 !== tmp.indexOf(ending_str)) {
        // if this return never occurs, then the file will be read until no_handler_resolution_timeout_ms
        // by default, no_handler_resolution_timeout_ms is null, so read will continue forever until this function returns something that evaluates to true
        return true;
        // you can also return a promise:
        //  return Promise.resolve().then(function() { return true; } );
    }
};

var appender = fs.openSync(test_path, 'a');
try {
    var reader = fs.openSync(test_path, 'r');
    try {
        var options = {
            starting_position: 0,
            byte_size: 256,
            check_for_bytes_every_ms: 3000,
            no_handler_resolution_timeout_ms: 10000,
        };

        liveReadAppendingFilePromiser(reader, buffStringHandler, options)
        .then(function(did_reader_time_out) {
            console.log('reader timed out: ', did_reader_time_out);
            console.log(s_list.join(''));
        }).catch(function(err) {
            console.error('bad stuff: ', err);
        }).then(function() {
            fs.closeSync(appender);
            fs.closeSync(reader);
        });

        fs.write(appender, '\ncheck it out, I am a string');
        fs.write(appender, '\nwho killed kenny');
        //fs.write(appender, ending_str);
    } catch(err) {
        fs.closeSync(reader);
        console.log('err1');
        throw err;
    }
} catch(err) {
    fs.closeSync(appender);
        console.log('err2');
    throw err;
}