The other day I found myself needing to keep a log file for a small Go web application. I'm pretty new to the language so it actually took me longer than I initially thought it would. Mostly for one reason: I couldn't find a clear example of actually using the log package to do what I wanted. I suspect that because this is such a trivial task no one (that I could find) has bothered to document just how to accomplish this. In retrospect this seems so obvious now I can't help but question whether or not this post is even worth writing. I'm going to stick with my original gut instinct.

To demonstrate, here's a small sample program. All it does is open up a file, log a message, and then quit. Let's take a look at the entire program first and then dive a little deeper.

package main

import (
	"fmt"
	"os"
	"log"
)

func main() {
	logFile, err := os.OpenFile("sample.log", os.O_WRONLY | os.O_APPEND | os.O_CREATE, 0666)

	if err != nil {
		fmt.Println(err)

		return
	}

	logger := log.New(logFile, "Sample Log: ", log.Ldate | log.Ltime | log.Lmicroseconds)

	logger.Println("Logger created!")
}

For the sake of clarity: this isn't mean to be a final working example of logging in Go, just a demonstration of the setup.

The first thing we do is open up a file for writing set to append. If the file doesn't exist the os.O_CREATE flag will ensure it is created if possible. After making sure that went well we create a new Logger. Finally we call Println to append our message to the log.

The final output is something like this:

Sample Log: 2013/03/21 20:39:29.348481 Logger created!

For more information on log.New see the documentation. Since the documentation doesn't really tell you what the available options for the flag parameter are you can find them here.