node.js: readSync from stdin?

Have you tried:

fs=require('fs'); console.log(fs.readFileSync('/dev/stdin').toString()); 

However, it will wait for the ENTIRE file to be read in, and won't return on \n like scanf or cin.

After fiddling with this for a bit, I found the answer:

process.stdin.resume(); var fs = require('fs'); var response = fs.readSync(process.stdin.fd, 100, 0, "utf8"); process.stdin.pause(); 

response will be an array with two indexes, the first being the data typed into the console and the second will be the length of the data including the newline character.

It was pretty easy to determine when you console.log(process.stdin) which enumerates all of the properties including one labeled fd which is of course the name of the first parameter for fs.readSync()

Enjoy! :D

An updated version of Marcus Pope's answer that works as of node.js v0.10.4:

Please note:

• In general, node's stream interfaces are still in flux (pun half-intended) and are still classified as 2 - Unstable as of node.js v0.10.4.
• Different platforms behave slightly differently; I've looked at OS X 10.8.3 and Windows 7: the major difference is: synchronously reading interactive stdin input (by typing into the terminal line by line) only works on Windows 7.

Here's the updated code, reading synchronously from stdin in 256-byte chunks until no more input is available:

var fs = require('fs'); var BUFSIZE=256; var buf = new Buffer(BUFSIZE); var bytesRead; while (true) { // Loop as long as stdin input is available. bytesRead = 0; try { bytesRead = fs.readSync(process.stdin.fd, buf, 0, BUFSIZE); } catch (e) { if (e.code === 'EAGAIN') { // 'resource temporarily unavailable' // Happens on OS X 10.8.3 (not Windows 7!), if there's no // stdin input - typically when invoking a script without any // input (for interactive stdin input). // If you were to just continue, you'd create a tight loop. throw 'ERROR: interactive stdin input not supported.'; } else if (e.code === 'EOF') { // Happens on Windows 7, but not OS X 10.8.3: // simply signals the end of *piped* stdin input. break; } throw e; // unexpected exception } if (bytesRead === 0) { // No more stdin input available. // OS X 10.8.3: regardless of input method, this is how the end // of input is signaled. // Windows 7: this is how the end of input is signaled for // *interactive* stdin input. break; } // Process the chunk read. console.log('Bytes read: %s; content:\n%s', bytesRead, buf.toString(null, 0, bytesRead)); } 

I found a library that should be able to accomplish what you need: https://github.com/anseki/readline-sync

Important: I've just been informed by a Node.js contributor that .fd is undocumented and serves as a means for internal debugging purposes. Therefore, one's code should not reference this, and should manually open the file descriptor with fs.open/openSync.

In Node.js 6, it's also worth noting that creating an instance of Buffer via its constructor with new is deprecated, due to its unsafe nature. One should use Buffer.alloc instead:

'use strict'; const fs = require('fs'); // small because I'm only reading a few bytes const BUFFER_LENGTH = 8; const stdin = fs.openSync('/dev/stdin', 'rs'); const buffer = Buffer.alloc(BUFFER_LENGTH); fs.readSync(stdin, buffer, 0, BUFFER_LENGTH); console.log(buffer.toString()); fs.closeSync(stdin); 

Also, one should only open and close the file descriptor when necessary; doing this every time one wishes to read from stdin results in unnecessary overhead.

The following code reads sync from stdin. Input is read up until a newline / enter key. The function returns a string of the input with line feeds (\n) and carriage returns (\r) discarded. This should be compatible with Windows, Linux, and Mac OSX. Added conditional call to Buffer.alloc (new Buffer(size) is currently deprecated, but some older versions lack Buffer.alloc.

function prompt(){ var fs = require("fs"); var rtnval = ""; var buffer = Buffer.alloc ? Buffer.alloc(1) : new Buffer(1); for(;;){ fs.readSync(0, buffer, 0, 1); //0 is fd for stdin if(buffer[0] === 10){ //LF \n return on line feed break; }else if(buffer[0] !== 13){ //CR \r skip carriage return rtnval += new String(buffer); } } return rtnval; } 

function read_stdinSync() { var b = new Buffer(1024) var data = '' while (true) { var n = fs.readSync(process.stdin.fd, b, 0, b.length) if (!n) break data += b.toString(null, 0, n) } return data } 

Here is the implementation with `async await`. In the below code, the input is taken from standard input and after receiving data the standard input is stopped waiting for data by using `process.stdin.pause();`.

process.stdin.setEncoding('utf8'); // This function reads only one line on console synchronously. After pressing `enter` key the console will stop listening for data. function readlineSync() { return new Promise((resolve, reject) => { process.stdin.resume(); process.stdin.on('data', function (data) { process.stdin.pause(); // stops after one line reads resolve(data); }); }); } // entry point async function main() { let inputLine1 = await readlineSync(); console.log('inputLine1 = ', inputLine1); let inputLine2 = await readlineSync(); console.log('inputLine2 = ', inputLine2); console.log('bye'); } main(); 

I used this workaround on node 0.10.24/linux:

var fs = require("fs") var fd = fs.openSync("/dev/stdin", "rs") fs.readSync(fd, new Buffer(1), 0, 1) fs.closeSync(fd) 

This code waits for pressing ENTER. It reads one character from line, if user enters it before pressing ENTER. Other characters will be remained in the console buffer and will be read on subsequent calls to readSync.

I wrote this module to read one line at a time from file or stdin. The module is named as line-reader which exposes an ES6 *Generator function to iterate over one line at a time. here is a code sample(in TypeScript) from readme.md.

import { LineReader } from "line-reader" // FromLine and ToLine are optional arguments const filePathOrStdin = "path-to-file.txt" || process.stdin const FromLine: number = 1 // default is 0 const ToLine: number = 5 // default is Infinity const chunkSizeInBytes = 8 * 1024 // default is 64 * 1024 const list: IterableIterator<string> = LineReader(filePathOrStdin, FromLine, ToLine, chunkSizeInBytes) // Call list.next to iterate over lines in a file list.next() // Iterating using a for..of loop for (const item of list) { console.log(item) } 

Apart from above code, you can also take a look at src > tests folder in the repo.

Note:-
line-reader module doesn't read all stuff into memory instead it uses generator function to generate lines async or sync.