I am trying to read a large file one line at a time. I found a question on Quora that dealt with the subject but I'm missing some connections to make the whole thing fit together.
var Lazy=require("lazy");
new Lazy(process.stdin)
.lines
.forEach(
function(line) {
console.log(line.toString());
}
);
process.stdin.resume();
The bit that I'd like to figure out is how I might read one line at a time from a file instead of STDIN as in this sample.
I tried:
fs.open('./VeryBigFile.csv', 'r', '0666', Process);
function Process(err, fd) {
if (err) throw err;
// DO lazy read
}
but it's not working. I know that in a pinch I could fall back to using something like PHP, but I would like to figure this out.
I don't think the other answer would work as the file is much larger than the server I'm running it on has memory for.
fs.readSync()
. You can read binary octets into a buffer but there's no easy way to deal with partial UTF-8 or UTF-16 characters without inspecting the buffer before translating it to JavaScript strings and scanning for EOLs. The Buffer()
type doesn't have as rich set of functions to operate on its instances as native strings, but native strings cannot contain binary data. It seems to me that lacking a built-in way to read text lines from arbitrary filehandles is a real gap in node.js.
if (line.length==1 && line[0] == 48) special(line);
node
's API docs github.com/nodejs/node/pull/4609
Since Node.js v0.12 and as of Node.js v4.0.0, there is a stable readline core module. Here's the easiest way to read lines from a file, without any external modules:
const fs = require('fs');
const readline = require('readline');
async function processLineByLine() {
const fileStream = fs.createReadStream('input.txt');
const rl = readline.createInterface({
input: fileStream,
crlfDelay: Infinity
});
// Note: we use the crlfDelay option to recognize all instances of CR LF
// ('\r\n') in input.txt as a single line break.
for await (const line of rl) {
// Each line in input.txt will be successively available here as `line`.
console.log(`Line from file: ${line}`);
}
}
processLineByLine();
Or alternatively:
var lineReader = require('readline').createInterface({
input: require('fs').createReadStream('file.in')
});
lineReader.on('line', function (line) {
console.log('Line from file:', line);
});
The last line is read correctly (as of Node v0.12 or later), even if there is no final \n
.
UPDATE: this example has been added to Node's API official documentation.
For such a simple operation there shouldn't be any dependency on third-party modules. Go easy.
var fs = require('fs'),
readline = require('readline');
var rd = readline.createInterface({
input: fs.createReadStream('/path/to/file'),
output: process.stdout,
console: false
});
rd.on('line', function(line) {
console.log(line);
});
line
events come only after hitting \n
, ie, all the alternatives are missed (see unicode.org/reports/tr18/#Line_Boundaries). #2, data after the last \n
is silently ignored (see stackoverflow.com/questions/18450197/…). i'd call this solution dangerous cause it works for 99% of all files and for 99% of the data but fails silently for the rest. whenever you do fs.writeFileSync( path, lines.join('\n'))
you've written a file that will only be partly read by above solution.
readline
package behaves in truly bizarre ways to an experienced Unix/Linux programmer.
rd.on("close", ..);
can be used as a callback (occurrs when all lines are read)
You don't have to open
the file, but instead, you have to create a ReadStream
.
Then pass that stream to Lazy
new lazy(fs.createReadStream('...')).lines.forEach(function(l) { /* ... */ }).join(function() { /* Done */ })
new lazy(...).lines.forEach(...).on('end', function() {...})
.on('end'...
after .forEach(...)
, when in fact everything behaved as expected when I bound the event first.
Update in 2019
An awesome example is already posted on official Nodejs documentation. here
This requires the latest Nodejs is installed on your machine. >11.4
const fs = require('fs');
const readline = require('readline');
async function processLineByLine() {
const fileStream = fs.createReadStream('input.txt');
const rl = readline.createInterface({
input: fileStream,
crlfDelay: Infinity
});
// Note: we use the crlfDelay option to recognize all instances of CR LF
// ('\r\n') in input.txt as a single line break.
for await (const line of rl) {
// Each line in input.txt will be successively available here as `line`.
console.log(`Line from file: ${line}`);
}
}
processLineByLine();
await
s between the createInterface()
call and the start of the for await
loop, you will mysteriously lose lines from the start of the file. createInterface()
immediately starts emitting lines behind the scenes, and the async iterator implicitly created with const line of rl
can’t start listening for those lines until it is created.
require('fs').readFileSync('file.txt', 'utf-8').split(/\r?\n/).forEach(function(line){
console.log(line);
})
there is a very nice module for reading a file line by line, it's called line-reader
with it you simply just write:
var lineReader = require('line-reader');
lineReader.eachLine('file.txt', function(line, last) {
console.log(line);
// do whatever you want with line...
if(last){
// or check if it's the last one
}
});
you can even iterate the file with a "java-style" interface, if you need more control:
lineReader.open('file.txt', function(reader) {
if (reader.hasNextLine()) {
reader.nextLine(function(line) {
console.log(line);
});
}
});
process/stdin
). At least, if it can, it's certainly not obvious from reading the code and attempting it.
readline
core module.
function(reader)
and function(line)
should be: function(err,reader)
and function(err,line)
.
line-reader
reads the file asynchronously. The synchronous alternative to it is line-reader-sync
Old topic, but this works:
var rl = readline.createInterface({
input : fs.createReadStream('/path/file.txt'),
output: process.stdout,
terminal: false
})
rl.on('line',function(line){
console.log(line) //or parse line
})
Simple. No need for an external module.
readline is not defined
or fs is not defined
, add var readline = require('readline');
and var fs = require('fs');
to get this to work. Otherwise sweet, sweet code. Thanks.
You can always roll your own line reader. I have'nt benchmarked this snippet yet, but it correctly splits the incoming stream of chunks into lines without the trailing '\n'
var last = "";
process.stdin.on('data', function(chunk) {
var lines, i;
lines = (last+chunk).split("\n");
for(i = 0; i < lines.length - 1; i++) {
console.log("line: " + lines[i]);
}
last = lines[i];
});
process.stdin.on('end', function() {
console.log("line: " + last);
});
process.stdin.resume();
I did come up with this when working on a quick log parsing script that needed to accumulate data during the log parsing and I felt that it would nice to try doing this using js and node instead of using perl or bash.
Anyway, I do feel that small nodejs scripts should be self contained and not rely on third party modules so after reading all the answers to this question, each using various modules to handle line parsing, a 13 SLOC native nodejs solution might be of interest .
stdin
... unless I'm missing somethign.
ReadStream
with fs.createReadStream('./myBigFile.csv')
and use it instead of stdin
readline
core module.
With the carrier module:
var carrier = require('carrier');
process.stdin.resume();
carrier.carry(process.stdin, function(line) {
console.log('got one line: ' + line);
});
var inStream = fs.createReadStream('input.txt', {flags:'r'});
But your syntax is cleaner than the documented method of using .on(): carrier.carry(inStream).on('line', function(line) { ...
\r\n
and \n
line endings. If you ever need to deal with MacOS-style test files from before OS X, they used \r
and carrier does not handle this. Surprisingly, there are still such files floating about in the wild. You might also need to handle the Unicode BOM (byte order mark) explicitly, this is used at the beginning of text files in the MS Windows sphere of influence.
readline
core module.
I ended up with a massive, massive memory leak using Lazy to read line by line when trying to then process those lines and write them to another stream due to the way drain/pause/resume in node works (see: http://elegantcode.com/2011/04/06/taking-baby-steps-with-node-js-pumping-data-between-streams/ (i love this guy btw)). I haven't looked closely enough at Lazy to understand exactly why, but I couldn't pause my read stream to allow for a drain without Lazy exiting.
I wrote the code to process massive csv files into xml docs, you can see the code here: https://github.com/j03m/node-csv2xml
If you run the previous revisions with Lazy line it leaks. The latest revision doesn't leak at all and you can probably use it as the basis for a reader/processor. Though I have some custom stuff in there.
Edit: I guess I should also note that my code with Lazy worked fine until I found myself writing large enough xml fragments that drain/pause/resume because a necessity. For smaller chunks it was fine.
readline
core module.
Edit:
Use a transform stream.
With a BufferedReader you can read lines.
new BufferedReader ("lorem ipsum", { encoding: "utf8" })
.on ("error", function (error){
console.log ("error: " + error);
})
.on ("line", function (line){
console.log ("line: " + line);
})
.on ("end", function (){
console.log ("EOF");
})
.read ();
readline
core module.
In most cases this should be enough:
const fs = require("fs")
fs.readFile('./file', 'utf-8', (err, file) => {
const lines = file.split('\n')
for (let line of lines)
console.log(line)
});
I was frustrated by the lack of a comprehensive solution for this, so I put together my own attempt (git / npm). Copy-pasted list of features:
Interactive line processing (callback-based, no loading the entire file into RAM)
Optionally, return all lines in an array (detailed or raw mode)
Interactively interrupt streaming, or perform map/filter like processing
Detect any newline convention (PC/Mac/Linux)
Correct eof / last line treatment
Correct handling of multi-byte UTF-8 characters
Retrieve byte offset and byte length information on per-line basis
Random access, using line-based or byte-based offsets
Automatically map line-offset information, to speed up random access
Zero dependencies
Tests
NIH? You decide :-)
Since posting my original answer, I found that split is a very easy to use node module for line reading in a file; Which also accepts optional parameters.
var split = require('split');
fs.createReadStream(file)
.pipe(split())
.on('data', function (line) {
//each chunk now is a seperate line!
});
Haven't tested on very large files. Let us know if you do.
function createLineReader(fileName){
var EM = require("events").EventEmitter
var ev = new EM()
var stream = require("fs").createReadStream(fileName)
var remainder = null;
stream.on("data",function(data){
if(remainder != null){//append newly received data chunk
var tmp = new Buffer(remainder.length+data.length)
remainder.copy(tmp)
data.copy(tmp,remainder.length)
data = tmp;
}
var start = 0;
for(var i=0; i<data.length; i++){
if(data[i] == 10){ //\n new line
var line = data.slice(start,i)
ev.emit("line", line)
start = i+1;
}
}
if(start<data.length){
remainder = data.slice(start);
}else{
remainder = null;
}
})
stream.on("end",function(){
if(null!=remainder) ev.emit("line",remainder)
})
return ev
}
//---------main---------------
fileName = process.argv[2]
lineReader = createLineReader(fileName)
lineReader.on("line",function(line){
console.log(line.toString())
//console.log("++++++++++++++++++++")
})
data
in the call to stream.on("data")
might ever begin or end with only part of a multibyte UTF-8 character such as ა
which is U+10D0
, made up of the three bytes e1
83
90
readline
core module.
I wanted to tackle this same problem, basically what in Perl would be:
while (<>) {
process_line($_);
}
My use case was just a standalone script, not a server, so synchronous was fine. These were my criteria:
The minimal synchronous code that could reuse in many projects.
No limits on file size or number of lines.
No limits on length of lines.
Able to handle full Unicode in UTF-8, including characters beyond the BMP.
Able to handle *nix and Windows line endings (old-style Mac not needed for me).
Line endings character(s) to be included in lines.
Able to handle last line with or without end-of-line characters.
Not use any external libraries not included in the node.js distribution.
This is a project for me to get a feel for low-level scripting type code in node.js and decide how viable it is as a replacement for other scripting languages like Perl.
After a surprising amount of effort and a couple of false starts this is the code I came up with. It's pretty fast but less trivial than I would've expected: (fork it on GitHub)
var fs = require('fs'),
StringDecoder = require('string_decoder').StringDecoder,
util = require('util');
function lineByLine(fd) {
var blob = '';
var blobStart = 0;
var blobEnd = 0;
var decoder = new StringDecoder('utf8');
var CHUNK_SIZE = 16384;
var chunk = new Buffer(CHUNK_SIZE);
var eolPos = -1;
var lastChunk = false;
var moreLines = true;
var readMore = true;
// each line
while (moreLines) {
readMore = true;
// append more chunks from the file onto the end of our blob of text until we have an EOL or EOF
while (readMore) {
// do we have a whole line? (with LF)
eolPos = blob.indexOf('\n', blobStart);
if (eolPos !== -1) {
blobEnd = eolPos;
readMore = false;
// do we have the last line? (no LF)
} else if (lastChunk) {
blobEnd = blob.length;
readMore = false;
// otherwise read more
} else {
var bytesRead = fs.readSync(fd, chunk, 0, CHUNK_SIZE, null);
lastChunk = bytesRead !== CHUNK_SIZE;
blob += decoder.write(chunk.slice(0, bytesRead));
}
}
if (blobStart < blob.length) {
processLine(blob.substring(blobStart, blobEnd + 1));
blobStart = blobEnd + 1;
if (blobStart >= CHUNK_SIZE) {
// blobStart is in characters, CHUNK_SIZE is in octets
var freeable = blobStart / CHUNK_SIZE;
// keep blob from growing indefinitely, not as deterministic as I'd like
blob = blob.substring(CHUNK_SIZE);
blobStart -= CHUNK_SIZE;
blobEnd -= CHUNK_SIZE;
}
} else {
moreLines = false;
}
}
}
It could probably be cleaned up further, it was the result of trial and error.
Generator based line reader: https://github.com/neurosnap/gen-readlines
var fs = require('fs');
var readlines = require('gen-readlines');
fs.open('./file.txt', 'r', function(err, fd) {
if (err) throw err;
fs.fstat(fd, function(err, stats) {
if (err) throw err;
for (var line of readlines(fd, stats.size)) {
console.log(line.toString());
}
});
});
If you want to read a file line by line and writing this in another:
var fs = require('fs');
var readline = require('readline');
var Stream = require('stream');
function readFileLineByLine(inputFile, outputFile) {
var instream = fs.createReadStream(inputFile);
var outstream = new Stream();
outstream.readable = true;
outstream.writable = true;
var rl = readline.createInterface({
input: instream,
output: outstream,
terminal: false
});
rl.on('line', function (line) {
fs.appendFileSync(outputFile, line + '\n');
});
};
var fs = require('fs');
function readfile(name,online,onend,encoding) {
var bufsize = 1024;
var buffer = new Buffer(bufsize);
var bufread = 0;
var fd = fs.openSync(name,'r');
var position = 0;
var eof = false;
var data = "";
var lines = 0;
encoding = encoding || "utf8";
function readbuf() {
bufread = fs.readSync(fd,buffer,0,bufsize,position);
position += bufread;
eof = bufread ? false : true;
data += buffer.toString(encoding,0,bufread);
}
function getLine() {
var nl = data.indexOf("\r"), hasnl = nl !== -1;
if (!hasnl && eof) return fs.closeSync(fd), online(data,++lines), onend(lines);
if (!hasnl && !eof) readbuf(), nl = data.indexOf("\r"), hasnl = nl !== -1;
if (!hasnl) return process.nextTick(getLine);
var line = data.substr(0,nl);
data = data.substr(nl+1);
if (data[0] === "\n") data = data.substr(1);
online(line,++lines);
process.nextTick(getLine);
}
getLine();
}
I had the same problem and came up with above solution looks simular to others but is aSync and can read large files very quickly
Hopes this helps
Two questions we must ask ourselves while doing such operations are:
What's the amount of memory used to perform it? Is the memory consumption increasing drastically with the file size?
Solutions like require('fs').readFileSync()
loads the whole file into memory. That means that the amount of memory required to perform operations will be almost equivalent to the file size. We should avoid these for anything larger than 50mbs
We can easily track the amount of memory used by a function by placing these lines of code after the function invocation :
const used = process.memoryUsage().heapUsed / 1024 / 1024;
console.log(
`The script uses approximately ${Math.round(used * 100) / 100} MB`
);
Right now the best way to read particular lines from a large file is using node's readline. The documentation has amazing examples.
I have a little module which does this well and is used by quite a few other projects npm readline Note thay in node v10 there is a native readline module so I republished my module as linebyline https://www.npmjs.com/package/linebyline
if you dont want to use the module the function is very simple:
var fs = require('fs'),
EventEmitter = require('events').EventEmitter,
util = require('util'),
newlines = [
13, // \r
10 // \n
];
var readLine = module.exports = function(file, opts) {
if (!(this instanceof readLine)) return new readLine(file);
EventEmitter.call(this);
opts = opts || {};
var self = this,
line = [],
lineCount = 0,
emit = function(line, count) {
self.emit('line', new Buffer(line).toString(), count);
};
this.input = fs.createReadStream(file);
this.input.on('open', function(fd) {
self.emit('open', fd);
})
.on('data', function(data) {
for (var i = 0; i < data.length; i++) {
if (0 <= newlines.indexOf(data[i])) { // Newline char was found.
lineCount++;
if (line.length) emit(line, lineCount);
line = []; // Empty buffer.
} else {
line.push(data[i]); // Buffer new line data.
}
}
}).on('error', function(err) {
self.emit('error', err);
}).on('end', function() {
// Emit last line if anything left over since EOF won't trigger it.
if (line.length){
lineCount++;
emit(line, lineCount);
}
self.emit('end');
}).on('close', function() {
self.emit('close');
});
};
util.inherits(readLine, EventEmitter);
Another solution is to run logic via sequential executor nsynjs. It reads file line-by-line using node readline module, and it doesn't use promises or recursion, therefore not going to fail on large files. Here is how the code will looks like:
var nsynjs = require('nsynjs');
var textFile = require('./wrappers/nodeReadline').textFile; // this file is part of nsynjs
function process(textFile) {
var fh = new textFile();
fh.open('path/to/file');
var s;
while (typeof(s = fh.readLine(nsynjsCtx).data) != 'undefined')
console.log(s);
fh.close();
}
var ctx = nsynjs.run(process,{},textFile,function () {
console.log('done');
});
Code above is based on this exampe: https://github.com/amaksr/nsynjs/blob/master/examples/node-readline/index.js
This is my favorite way of going through a file, a simple native solution for a progressive (as in not a "slurp" or all-in-memory way) file read with modern async/await
. It's a solution that I find "natural" when processing large text files without having to resort to the readline
package or any non-core dependency.
let buf = '';
for await ( const chunk of fs.createReadStream('myfile') ) {
const lines = buf.concat(chunk).split(/\r?\n/);
buf = lines.pop();
for( const line of lines ) {
console.log(line);
}
}
if(buf.length) console.log(buf); // last line, if file does not end with newline
You can adjust encoding in the fs.createReadStream
or use chunk.toString(<arg>)
. Also this let's you better fine-tune the line splitting to your taste, ie. use .split(/\n+/)
to skip empty lines and control the chunk size with { highWaterMark: <chunkSize> }
.
Don't forget to create a function like processLine(line)
to avoid repeating the line processing code twice due to the ending buf
leftover. Unfortunately, the ReadStream
instance does not update its end-of-file flags in this setup, so there's no way, afaik, to detect within the loop that we're in the last iteration without some more verbose tricks like comparing the file size from a fs.Stats()
with .bytesRead
. Hence the final buf
processing solution, unless you're absolutely sure your file ends with a newline \n
, in which case the for await
loop should suffice.
★ If you prefer the evented asynchronous version, this would be it:
let buf = '';
fs.createReadStream('myfile')
.on('data', chunk => {
const lines = buf.concat(chunk).split(/\r?\n/);
buf = lines.pop();
for( const line of lines ) {
console.log(line);
}
})
.on('end', () => buf.length && console.log(buf) );
★ Now if you don't mind importing the stream
core package, then this is the equivalent piped stream version, which allows for chaining transforms like gzip decompression:
const { Writable } = require('stream');
let buf = '';
fs.createReadStream('myfile').pipe(
new Writable({
write: (chunk, enc, next) => {
const lines = buf.concat(chunk).split(/\r?\n/);
buf = lines.pop();
for (const line of lines) {
console.log(line);
}
next();
}
})
).on('finish', () => buf.length && console.log(buf) );
i use this:
function emitLines(stream, re){
re = re && /\n/;
var buffer = '';
stream.on('data', stream_data);
stream.on('end', stream_end);
function stream_data(data){
buffer += data;
flush();
}//stream_data
function stream_end(){
if(buffer) stream.emmit('line', buffer);
}//stream_end
function flush(){
var re = /\n/;
var match;
while(match = re.exec(buffer)){
var index = match.index + match[0].length;
stream.emit('line', buffer.substring(0, index));
buffer = buffer.substring(index);
re.lastIndex = 0;
}
}//flush
}//emitLines
use this function on a stream and listen to the line events that is will emit.
gr-
While you should probably use the readline
module as the top answer suggests, readline
appears to be oriented toward command line interfaces rather than line reading. It's also a little bit more opaque regarding buffering. (Anyone who needs a streaming line oriented reader probably will want to tweak buffer sizes). The readline module is ~1000 lines while this, with stats and tests, is 34.
const EventEmitter = require('events').EventEmitter;
class LineReader extends EventEmitter{
constructor(f, delim='\n'){
super();
this.totalChars = 0;
this.totalLines = 0;
this.leftover = '';
f.on('data', (chunk)=>{
this.totalChars += chunk.length;
let lines = chunk.split(delim);
if (lines.length === 1){
this.leftover += chunk;
return;
}
lines[0] = this.leftover + lines[0];
this.leftover = lines[lines.length-1];
if (this.leftover) lines.pop();
this.totalLines += lines.length;
for (let l of lines) this.onLine(l);
});
// f.on('error', ()=>{});
f.on('end', ()=>{console.log('chars', this.totalChars, 'lines', this.totalLines)});
}
onLine(l){
this.emit('line', l);
}
}
//Command line test
const f = require('fs').createReadStream(process.argv[2], 'utf8');
const delim = process.argv[3];
const lineReader = new LineReader(f, delim);
lineReader.on('line', (line)=> console.log(line));
Here's an even shorter version, without the stats, at 19 lines:
class LineReader extends require('events').EventEmitter{
constructor(f, delim='\n'){
super();
this.leftover = '';
f.on('data', (chunk)=>{
let lines = chunk.split(delim);
if (lines.length === 1){
this.leftover += chunk;
return;
}
lines[0] = this.leftover + lines[0];
this.leftover = lines[lines.length-1];
if (this.leftover)
lines.pop();
for (let l of lines)
this.emit('line', l);
});
}
}
const fs = require("fs")
fs.readFile('./file', 'utf-8', (err, data) => {
var innerContent;
console.log("Asynchronous read: " + data.toString());
const lines = data.toString().split('\n')
for (let line of lines)
innerContent += line + '<br>';
});
I wrap the whole logic of daily line processing as a npm module: line-kit https://www.npmjs.com/package/line-kit
// example var count = 0 require('line-kit')(require('fs').createReadStream('/etc/issue'), (line) => { count++; }, () => {console.log(`seen ${count} lines`)})
I use below code the read lines after verify that its not a directory and its not included in the list of files need not to be check.
(function () {
var fs = require('fs');
var glob = require('glob-fs')();
var path = require('path');
var result = 0;
var exclude = ['LICENSE',
path.join('e2e', 'util', 'db-ca', 'someother-file'),
path.join('src', 'favicon.ico')];
var files = [];
files = glob.readdirSync('**');
var allFiles = [];
var patternString = [
'trade',
'order',
'market',
'securities'
];
files.map((file) => {
try {
if (!fs.lstatSync(file).isDirectory() && exclude.indexOf(file) === -1) {
fs.readFileSync(file).toString().split(/\r?\n/).forEach(function(line){
patternString.map((pattern) => {
if (line.indexOf(pattern) !== -1) {
console.log(file + ' contain `' + pattern + '` in in line "' + line +'";');
result = 1;
}
});
});
}
} catch (e) {
console.log('Error:', e.stack);
}
});
process.exit(result);
})();
I have looked through all above answers, all of them use third-party library to solve it. It's have a simple solution in Node's API. e.g
const fs= require('fs')
let stream = fs.createReadStream('<filename>', { autoClose: true })
stream.on('data', chunk => {
let row = chunk.toString('ascii')
}))
Success story sharing
rl.on('close', cb)