Sample of monitoring a log file for changes and filtering out lines that describe an error

FileWatching.go

package main

import (
	"bufio"
	"flag"
	"fmt"
	"log"
	"os"
	"regexp"
	"strings"
	"time"

	"gopkg.in/fsnotify.v1"
)

var (
	fileToWatch string
	appName     string
	keyword     string
	interval    int
)

func init() {
	flag.StringVar(&fileToWatch, "file", "", "the path and name of the file to be monitored for changes")
	flag.StringVar(&appName, "appname", "", "the name of the application being monitored")
	flag.StringVar(&keyword, "keyword", "EXCEPTION", "the word to look for that indicates an error line")
	flag.IntVar(&interval, "interval", 20, "time to wait in seconds before parsing file contents")
	flag.Parse()
}

func main() {

	dateRegex := regexp.MustCompile(`^[0-9]{4}-[0-9]{2}-[0-9]{2} [0-9]{2}:[0-9]{2}:[0-9]{2}`)
	keywordRegex := regexp.MustCompile(keyword + ` - (.*)`)

	watcher, err := fsnotify.NewWatcher()
	if err != nil {
		log.Fatal(err)
	}
	defer watcher.Close()

	done := make(chan bool)
	analyze := make(chan bool)

	// monitor for file changes or errors
	go func() {
		for {
			select {
			case event := <-watcher.Events:
				if event.Op&fsnotify.Write == fsnotify.Write {
					analyze <- true
				}
			case err := <-watcher.Errors:
				fmt.Println("error", err)
			}
		}
	}()

	// parse file when change detected every <interval> seconds
	go func() {
		lineNo := 0
		on := true
		for {
			select {
			case <-time.After(time.Duration(interval) * time.Second):
				fmt.Println("checking...")
				select {
				case <-analyze:
					go func() {
						if on {
							f, err := os.Open(fileToWatch)
							if err != nil {
								log.Fatal(err)
							}
							defer f.Close()

							reader := bufio.NewReader(f)
							scanner := bufio.NewScanner(reader)

							current := 0
							for scanner.Scan() {
								current += 1
								if current > lineNo {
									str := scanner.Text()
									if strings.Contains(str, keyword) {
										date := dateRegex.FindString(str)
										groups := keywordRegex.FindStringSubmatch(str)

										if len(groups) == 2 {
											// insert into db
											fmt.Printf("App: %s - Date: %s - Error: %s\n", appName, date, groups[1])
										}
									}
								}
							}
							lineNo = current
						}
						on = !on
					}()
				default:
					// nothing to do here but stop the routine from blocking
				}
			}
		}
	}()

	err = watcher.Add(fileToWatch)
	if err != nil {
		log.Fatal(err)
	}

	fmt.Println("monitoring...")

	<-done
}