EnvConf

EnvConf is a Go package, for parsing configuration values from different sources.

Installing

go get github.com/antonmashko/envconf

Parse Configs

Usually you need a tag with desire configuration sources and execution of a single function envconf.Parse for getting all configuration values into your golang structure.

Supported Configurations

• command line flags
• environment variables
• default values
• external sources (can be anything that is implementing interface External)

Tags

Use tags for getting values from different configuration sources.

• flag - name of flag;
• env - name of environment variable;
• default - if nothing set this value will be used as field value;
• required - on true checks that configuration exists in flag or env source;
• description - field description in help output.
• envconf - only for structs. override struct name for generating configuration name.

Supported Types

1. Primitives: bool, string, all types of int and unit, float32, float64, complex64, complex128;
2. Collections:
   • Array and Slice - comma-separated string can be converted into slice or array. NOTE: if elements in string more than len of array EnvConf will panic with index out of range.
   • []byte - coverts string config into byte slice
   • Map - comma-separated string with a colon-separated key and value can be converted into map. example input: key1:value1, key2:value2
3. Golang types:
   • interface{}
   • time.Duration
   • Types that are implementing encoding.TextUnmarshaller and encoding.BinaryUnmarshaller. e.g. time.Time, net.ID, url.URL;

Example

Let's take a look at a simple example. Here we're creating struct with 3 tags for different configuration sources: flag, env, and default value. NOTE: It's not necessary to specify tags for each configuration type, add desired only

package main

import (
	"fmt"

	"github.com/antonmashko/envconf"
)

type Example struct {
	Field1 string `flag:"flag-name" env:"ENV_VAR_NAME" default:"default-value"`
}

func main() {
	var cfg Example
	if err := envconf.Parse(&cfg); err != nil {
		panic(err)
	}
	fmt.Printf("%#v\n", cfg)
}

Testing! If you want to get set Field1 from command line flag, use flag name that is set in flag tag.

$ go run main.go -flag-name="variable-from-flag"
main.Example{Field1:"variable-from-flag"}

The same result would be for other configuration types.

-help output

Using envconf will also generate help output with all registered fields and types. Use flag -help or -h for getting it.

$ go run main.go -help

Usage:

Field1 <string> default-value
        flag: flag-name
        environment variable: ENV_VAR_NAME
        required: false
        description: ""

Auto-generating Config Names

EnvConf can generate environment variable name or flag name from golang field path. All you need is to set * in specific tag. For environment variables name envconf will use field path in uppercase and underscore as a delimiter. Example:

type Config struct {
	HTTP struct {
		Addr string `env:"*"`
	}
}

Now we can use HTTP_ADDR environment variable for defining Addr field. The same approach will work for flag. But flag names will be generated in lowercase and the dash will be as a delimiter.

Overriding Parent struct name for Auto-generation

In case if parent struct name doesn't satisfy for configuration variable name, it can be changed with envconf tag. Example:

type Config struct {
	HTTP struct {
		Addr string `env:"*"`
	} `envconf:"httpserver"`
}

Now we'll get HTTPSERVER_ADDR as environment variable name. See: EnvConf example

External

reading json config see: example

Options

Options allow intercept into EnvConf.Parse process

Name	Option	Description
External source	option.WithExternal	Add external configuration source to the parsing process. This option allows you to define field from configuration files, remote servers, etc. Some of Externals already predefined in external folder, e.g. external/json or external/yaml. see: External Doc
Read configuration priority	option.WithPriorityOrder	Change default parsing priority. Default: Flag, Environment variable, External source, Default Value
Log	option.WithLog	Enable logging over parsing process. Prints defined and not defined configuration fields
Custom Usage	option.WithCustomUsage	Generate usage for -help flag from input structure. By default this option is enabled, use option.WithoutCustomUsage option
Flag Parsed Callback	option.WithFlagParsed	This callback allow to use flags after flag.Parse() and before EnvConf.Define process
Read config file	option.WithFlagConfigFile	Read config file from the path specified in the flag. This option working with External option.
External Injection	option.WithExternalInjection	Inject environment variables into external source. Override default injection with option.WithCustomExternalInjection