Node.js如何删除文件中的第一行 [英] Node.js How to delete first line in file

问题描述

我正在制作简单的Node.js应用程序，我需要删除文件中的第一行.请以任何方式怎么做?我认为fs.write可以实现，但是怎么办?

I'm making simple Node.js app and I need to delete first line in file. Please is any way how to do it? I think that It will be possible with fs.write, but how?

推荐答案

以下是从文件中删除第一行的流式版本.
由于它使用流，因此您不需要将整个文件加载到内存中，因此它效率更高，速度更快，并且可以在不占用硬件内存的情况下处理非常大的文件.

Here is streamed version of removing first line from file.
As it uses streams, means you don't need to load whole file in memory, so it is way more efficient and fast, as well can work on very large files without filling memory on your hardware.

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


// Transform sctreamer to remove first line
function RemoveFirstLine(args) {
    if (! (this instanceof RemoveFirstLine)) {
        return new RemoveFirstLine(args);
    }
    Transform.call(this, args);
    this._buff = '';
    this._removed = false;
}
util.inherits(RemoveFirstLine, Transform);

RemoveFirstLine.prototype._transform = function(chunk, encoding, done) {
    if (this._removed) { // if already removed
        this.push(chunk); // just push through buffer
    } else {
        // collect string into buffer
        this._buff += chunk.toString();

        // check if string has newline symbol
        if (this._buff.indexOf('\n') !== -1) {
            // push to stream skipping first line
            this.push(this._buff.slice(this._buff.indexOf('\n') + 2));
            // clear string buffer
            this._buff = null;
            // mark as removed
            this._removed = true;
        }
    }
    done();
};

并像这样使用它:

var fs = require('fs');

var input = fs.createReadStream('test.txt'); // read file
var output = fs.createWriteStream('test_.txt'); // write file

input // take input
.pipe(RemoveFirstLine()) // pipe through line remover
.pipe(output); // save to file

不推荐的另一种方法.
如果文件不是很大，并且您不介意将它们加载到内存中，加载文件，删除行，保存文件，但是它比较慢，并且在大文件上不能很好地工作.

Another way, which is not recommended.
If your files are not large, and you don't mind loading them into memory, load file, remove line, save file, but it is slower and wont work well on large files.

var fs = require('fs');

var filePath = './test.txt'; // path to file

fs.readFile(filePath, function(err, data) { // read file to memory
    if (!err) {
        data = data.toString(); // stringify buffer
        var position = data.toString().indexOf('\n'); // find position of new line element
        if (position != -1) { // if new line element found
            data = data.substr(position + 1); // subtract string based on first line length

            fs.writeFile(filePath, data, function(err) { // write file
                if (err) { // if error, report
                    console.log (err);
                }
            });
        } else {
            console.log('no lines found');
        }
    } else {
        console.log(err);
    }
});

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