Tail

The zero dependency Node.js module for tailing a file

Made with ❤️ by Luca Grulla

1. Tail
   1. Installation
   2. Use
   3. Configuration
      1. Constructor parameters
   4. Emitted events
   5. How to contribute
   6. History
   7. License

Installation

npm install tail

Use

Tail = require("tail").Tail;

tail = new Tail("fileToTail");

tail.on("line", function(data) {
  console.log(data);
});

tail.on("error", function(error) {
  console.log("ERROR: ", error);
});

If you want to stop tail:

tail.unwatch()

To start watching again:

tail.watch()

Configuration

The only mandatory parameter is the path to the file to tail.

var fileToTail = "/path/to/fileToTail.txt";
new Tail(fileToTail)

If the file is missing or invalid Tail constructor will throw an Exception and won’t initialize.

try {
  new Tail("missingFile.txt")
} catch (ex) {
  console.log(ex)
}

Optional parameters can be passed via a hash:

var options= {separator: /[\r]{0,1}\n/, fromBeginning: false, fsWatchOptions: {}, follow: true, logger: console}
new Tail(fileToTail, options)

Constructor parameters

• separator: the line separator token (default: /[\r]{0,1}\n/ to handle linux/mac (9+)/windows). Pass null for binary files with no line separator.
• fsWatchOptions: the full set of options that can be passed to fs.watch as per node documentation (default: {}).
• fromBeginning: tail from the beginning of the file (default: false). If fromBeginning is true nLines will be ignored.
• follow: simulate tail -F option. In the case the file is moved/renamed/logrotated, if set to true will start tailing again after a 1 second delay; if set to false it will emit an error event (default: true).
• logger: a logger object(default: no logger). The passed logger should follow the following signature:
  • info([data][, ...])
  • error([data][, ...])
• nLines: tail from the last n lines. (default: undefined). Ignored if fromBeginning is set to true.
• useWatchFile: if set to true will force the use of fs.watchFile over delegating to the library the choice between fs.watch and fs.watchFile (default: false).
• encoding: the file encoding (default:utf-8).
• flushAtEOF: set to true to force flush of content when end of file is reached. Useful when there’s no separator character at the end of the file (default: false).

Emitted events

Tail emits two events:

• line

tail.on("line", (data) => {
  console.log(data)  
})

• error

tail.on("error", (err) => {
  console.log(err)  
})

The error emitted is either the underlying exception or a descriptive string.

How to contribute

Node Tail code repo is here Tail is written in ES6. Pull Requests are welcome.

History

Tail was born as part of a data firehose. Read more about that project here. Tail originally was written in CoffeeScript. Since December 2020 it’s pure ES6.

License

MIT. Please see License file for more details.