Logging globally (across packages)

The proper way is what you think is the ideal way. Just create a package (preferably following Go's conventions https://golang.org/doc/code.html) and make your Log global:

package mylog

// Define your custom logger type.
type logger struct { /* Whatever you want */ }

// Optionally make it a interface.
type Logger interface { /* Your functions */ }

// And just go global.
var defaultLogger *Logger

func init(){
   defaultLogger = new(logger)
}

func Debug(params ...string){
   // Have some fun.
}

// ...

Also I would recommend to describe in documentation that your project uses that logging feature.

I'm posting the solution worked for me!. I just created my own package, and I used the init function.

package logging

import (
    "io"
    logging "log"
    "os"

    "github.com/Sirupsen/logrus"
)

var (
    log *logrus.Logger
)

func init() {
    f, err := os.OpenFile("logs/application.log", os.O_RDWR|os.O_CREATE|os.O_APPEND, 0666)

    if err != nil {
        logging.Fatalf("error opening file: %v", err)
    }

    log = logrus.New()

    //log.Formatter = &logrus.JSONFormatter{}

    log.SetReportCaller(true)

    mw := io.MultiWriter(os.Stdout, f)
    log.SetOutput(mw)
}

// Info ...
func Info(format string, v ...interface{}) {
    log.Infof(format, v...)
}

// Warn ...
func Warn(format string, v ...interface{}) {
    log.Warnf(format, v...)
}

// Error ...
func Error(format string, v ...interface{}) {
    log.Errorf(format, v...)
}

var (

    // ConfigError ...
    ConfigError = "%v type=config.error"

    // HTTPError ...
    HTTPError = "%v type=http.error"

    // HTTPWarn ...
    HTTPWarn = "%v type=http.warn"

    // HTTPInfo ...
    HTTPInfo = "%v type=http.info"
)

And on any package, just import my package and I execute the (Info, Warn, Error) function

package main

import (

    log "logging"
)

func main() {
    log.Error(log.ConfigError, "Testing the error")
}

The log entry will be saved rendered on the screen and it will be saved on a file.