Tail Lines in Deno

Reading lines from a text file is easy with Deno:

import {readLines} from 'https://deno.land/std/io/mod.ts';
const file = await Deno.open('example.log');
for await(const line of readLines(file)) {
  // Do something...

At least, if you’re reading from the first line to the last.

To implement this you basically have to read one byte at a time until you find a newline character. That is 0x0A in hexadecimal and \n escaped in a string. Windows uses the \r\n sequence so trim off the carriage return byte if you care. Reading one byte from disk is very slow so for speed data needs to be buffered.

There is no magic way to know how many lines there are until the entire file is read. If you want the last 10 lines of a ~1 GB log file with 100000-ish lines it takes an eternitely to read every line and throw away the first 99990.

There is a unix command called head:

head -n 1 example.log

And the accompanying tail:

tail -n 1 example.log

Where -n 1 reads N lines from the top or bottom respectively.

The Deno standard library has plenty of utilities to help read text files. Unfortunately they all work reading forwards. readLines is perfect to replicate head. There’s no direct help for tail.

I’ve had a go at implementing this myself.

Checkout Deno Tail Lines on GitHub.

My module provides two functions. The tailLine function returns up to the maxLines from the end of a text file.

To read at most the last 10 lines:

import {tailLine} from 'https://deno.land/x/tail_lines/mod.ts';
const lines = await tailLine('example.log', 10);

The last line of the returned array is the last line of the text file.

The tailLines function is an async generator similar to readLines. It iterates over lines in reverse. So the first yield is the last line of the text file.

import {tailLines} from 'https://deno.land/x/tail_lines/mod.ts';
const file = await Deno.open(path);
for await (const line of tailLines('example.log')) {
  // Do something...

The maxLines parameter is optional for the iterator function.

I’ve written more about performance in the repo README but it’s very fast for reading a small number of lines from the end of a large file.

Find this module on GitHub and Deno.

I accidentally published the module twice on Deno third party modules — whoops! Anyone at Deno care to remove the taillines module? Kinda makes sense that it is immutable because deleting modules could break project dependencies. But Deno uses URL imports, so this could be solved with HTTP redirects? Anyway, use tail_lines.