How to use the 'default' struct tag with Go's envconfig library?

user_995862 0
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I'm trying to use the envconfig library, https://github.com/kelseyhightower/envconfig, in a program where most of the configuration variables have default values which I would only like to override if specific environment variables are set.

I've tried to run the following example program:

package main

import (
    "fmt"
    "log"

    "github.com/kelseyhightower/envconfig"
)

type config struct {
    foo string `default:"bar"`
}

func main() {
    var cfg config
    if err := envconfig.Process("app", &cfg); err != nil {
        log.Fatalln(err)
    }
    fmt.Println(cfg.foo)
}

without the APP_FOO environment variable set:

> echo $APP_FOO

However, this does not print bar as I would expect it to:

> go run use_envconfig.go

I don't see how my implementation is different from the documented example at https://github.com/kelseyhightower/envconfig#struct-tag-support to which the following comment pertains:

If envconfig can't find an environment variable value for MYAPP_DEFAULTVAR, it will populate it with "foobar" as a default value.

How can I make this program so that the cfg's value of foo is bar unless I set the APP_FOO environment variable?

go environment-variables
2 Answers

In your answer you accidentally fixed the problem. It's not config that needs to be capitalized. It is Foo. The following works: Demo. 

type config struct {
    Foo string `default:"bar"`
}

In Go, only the capitalized fields are accessible outside the current package. Envconfig can only access capitalized fields. Think of those as private and public properties.

Lowercase is private, Uppercase is public.

user_5602990

A little experimentation showed that both config and foo should be capitalized (to Config and Foo, respectively). The following program,

package main

import (
    "fmt"
    "log"

    "github.com/kelseyhightower/envconfig"
)

type Config struct {
    Foo string `default:"bar"`
}

func main() {
    var cfg Config
    if err := envconfig.Process("app", &cfg); err != nil {
        log.Fatalln(err)
    }
    fmt.Println(cfg.Foo)
}

produces bar as expected:

> go run use_envconfig.go
bar
user_9958620
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