A tail implementation using inotify (fsnotify)

tail.go

package main

import (
	"bufio"
	"fmt"
	"log"
	"os"

	"github.com/fsnotify/fsnotify"
)

func main() {
	// Create new watcher.
	watcher, err := fsnotify.NewWatcher()
	if err != nil {
		log.Fatal(err)
	}
	defer watcher.Close()
	if err = watcherFunc(watcher, "foo"); err != nil {
		log.Fatal(err)
	}
}

func watcherFunc(watcher *fsnotify.Watcher, paths ...string) error {
	var filemap = make(map[string]*bufio.Reader)
	for _, path := range paths {
		fd, err := os.Open(path)
		if err != nil {
			return fmt.Errorf("error opening file: %w", err)
		}
		filemap[path] = bufio.NewReader(fd)
		err = watcher.Add(path)
		if err != nil {
			return fmt.Errorf("error adding path to watcher: %w", err)
		}
	}
	for {
		select {
		case event, ok := <-watcher.Events:
			if !ok {
				return fmt.Errorf("something went wrong")
			}
			if event.Has(fsnotify.Write) {
				log.Println("modified file:", event.Name)
				data, err := filemap[event.Name].ReadString('\n')
				if err != nil {
					return fmt.Errorf("error reading file: %w", err)
				}
				log.Println("file contents:", data)
			}
		case err := <-watcher.Errors:
			return fmt.Errorf("error reading from watcher: %w", err)
		}

	}
}

A more robust implementation is in this package. This is a really simple demo of how to use inotify to only read a file when it changed.