How can we keep watching a large log file and output the last 10 lines using node.js without using external package and tail -f?

Question

I am having a large log file on my server and I want to keep watching that log file and get only last 10 lines of that log file without using the tail -f or any other packages which provides the same functionality as tail -f in unix.

We can do this by just traversing the log file from start and get last 10 lines but in case of large log file it will create a problem.

Let me know if there is any other alternatives ?

Thanks!

Answer 1

A (probably naive) attempt to emulate tail -n:

import fs from 'fs';

const path = 'test.txt';
const lastLinesLimit = 5;

const fileSize = fs.statSync(path).size;
const fd = fs.openSync(path);

// Increase the next number appropriately to decrease read calls.
let bufferSize = Math.min(10, fileSize);
let buffer = Buffer.alloc(bufferSize);

let stringTail = '';
let position = fileSize;

while (position !== 0) { // Rapeat from end till start of file.
  position -= bufferSize; // Step back by buffer size.

  if (position < 0) { // In case we reach too far:
    bufferSize += position; // decrease buffer size to read just what remains,
    buffer = buffer.subarray(0, position); // decrease buffer appropriately,
    position = 0; // set position at start of file.
  }

  fs.readSync(fd, buffer, 0, bufferSize, position); // Read a chunk.
  stringTail = buffer.toString() + stringTail; // Prepend to previously read data.
  if (stringTail.match(/\n(?!$)/g)?.length >= lastLinesLimit) break; // Check if we have enough lines.
}

console.log(
  stringTail
    .split(/\n(?!$)/)
    .slice(-lastLinesLimit)
    .join('\n')
);