How to assign default value if env var is empty?

How do you assign a default value if an environment variable isn't set in Go?

In Python I could do mongo_password = os.getenv('MONGO_PASS', 'pass') where pass is the default value if MONGO_PASS env var isn't set.

I tried an if statement based on os.Getenv being empty, but that doesn't seem to work due to the scope of variable assignment within an if statement. And I'm checking for multiple env var's, so I can't act on this information within the if statement.

There's no built-in to fall back to a default value, so you have to do a good old-fashioned if-else.

But you can always create a helper function to make that easier:

func getenv(key, fallback string) string {
	value := os.Getenv(key)
	if len(value) == 0 {
		return fallback
	}
	return value
}

Note that as @michael-hausenblas pointed out in a comment, keep in mind that if the value of the environment variable is really empty, you will get the fallback value instead.

Even better as @ŁukaszWojciechowski pointed out, using os.LookupEnv:

func getEnv(key, fallback string) string {
    if value, ok := os.LookupEnv(key); ok {
        return value
    }
    return fallback
}

What you're looking for is os.LookupEnv combined with an if statement.

Here is janos's answer updated to use LookupEnv:

func getEnv(key, fallback string) string {
	value, exists := os.LookupEnv(key)
	if !exists {
		value = fallback
	}
	return value
}

Go doesn't have the exact same functionality as Python here; the most idiomatic way to do it though, I can think of, is:

mongo_password := "pass"
if mp := os.Getenv("MONGO_PASS"); mp != "" {
	mongo_password = mp
}

To have a clean code I do this:

myVar := getEnv("MONGO_PASS", "default-pass")

I defined a function that is used in the whole app

// getEnv get key environment variable if exist otherwise return defalutValue
func getEnv(key, defaultValue string) string {
	value := os.Getenv(key)
	if len(value) == 0 {
		return defaultValue
	}
	return value
}

Had the same question as the OP and found someone encapsulated the answers from this thread into a nifty library that is fairly simple to use, hope this help others!

https://github.com/caarlos0/env

For more complex application you can use tooling such as viper, which allows you to set global custom default values, parse configuration files, set a prefix for your app's env var keys (to ensure consistency and name spacing of env var configurations) and many other cool features.

Sample code:

package main

import (
	"fmt"

	"github.com/spf13/viper"
)

func main() {
	viper.AutomaticEnv() // read value ENV variable
	// Set default value
	viper.SetEnvPrefix("app")
	viper.SetDefault("linetoken", "DefaultLineTokenValue")

	// Declare var
	linetoken := viper.GetString("linetoken")

	fmt.Println("---------- Example ----------")
	fmt.Println("linetoken :", linetoken)
}

I also had the same problem and I just created a small package called getenvs exactly to answer this problem.

Getenvs supports string, bool, int and float and it can be used like below:

package main

import (
	"fmt"

	"gitlab.com/avarf/getenvs"
)

func main() {
	value := getenvs.GetEnvString("STRING_GETENV", "default-string-value")
	bvalue, _ := getenvs.GetEnvBool("BOOL_GETENV", false)
	ivalue, _ := getenvs.GetEnvInt("INT_GETENV", 10)

	fmt.Println(value)
	fmt.Println(bvalue)
	fmt.Println(ivalue)
}

In case you are OK with adding little dependency you can use something like https://github.com/urfave/cli

package main

import (
  "os"

  "github.com/urfave/cli"
)

func main() {
  app := cli.NewApp()

  app.Flags = []cli.Flag {
    cli.StringFlag{
      Name: "lang, l",
      Value: "english",
      Usage: "language for the greeting",
      EnvVar: "APP_LANG",
    },
  }

  app.Run(os.Args)
}