How to do `tail -f logfile.txt`-like processing in node.js?

Question

tail -f logfile.txt outputs the last 10 lines of logfile.txt, and then continues to output appended data as the file grows.

What's the recommended way of doing the -f part in node.js?

The following outputs the entire file (ignoring the "show the last 10 lines") and then exits.

var fs = require('fs'); var rs = fs.createReadStream('logfile.txt', { flags: 'r', encoding: 'utf8'}); rs.on('data', function(data) {   console.log(data); }); 

I understand the event-loop is exiting because after the stream end & close event there are no more events -- I'm curious about the best way of continuing to monitor the stream.

Answer 1

The canonical way to do this is with fs.watchFile.

Alternatively, you could just use the node-tail module, which uses fs.watchFile internally and has already done the work for you. Here is an example of using it straight from the documentation:

Tail = require('tail').Tail;  tail = new Tail("fileToTail");  tail.on("line", function(data) {   console.log(data); }); 

Answer 2

node.js APi documentation on fs.watchFile states:

Stability: 2 - Unstable. Use fs.watch instead, if available.

Funny though that it says almost the exact same thing for fs.watch:

Stability: 2 - Unstable. Not available on all platforms.

In any case, I went ahead and did yet another small webapp, TailGate, that will tail your files using the fs.watch variant.

Feel free to check it out here: TailGate on github.