go-env is a Go library that can populate a struct with environment variable values. A common use of go-env is to load a configuration struct with values set in the environment variables.

Go 1.13 or above.

Run the following command to install the package:

go get github.com/qiangxue/go-env

The easiest way of using go-env is to call env.Load(), like the following:

package main

import (
	"fmt"
	"github.com/qiangxue/go-env"
	"os"
)

type Config struct {
	Host string
	Port int
}

func main() {
	_ = os.Setenv("APP_HOST", "127.0.0.1")
	_ = os.Setenv("APP_PORT", "8080")

	var cfg Config
	if err := env.Load(&cfg); err != nil {
		panic(err)
	}
	fmt.Println(cfg.Host)
	fmt.Println(cfg.Port)
	// Output:
	// 127.0.0.1
	// 8080
}

When go-env populates a struct from environment variables, it uses the following rules to match a struct field with an environment variable:

• Only public struct fields will be populated
• If the field has an env tag, use the tag value as the name, unless the tag value is - in which case it means the field should NOT be populated.
• If the field has no env tag, turn the field name into UPPER_SNAKE_CASE format and use that as the name. For example, a field name HostName will be turned into HOST_NAME, and MyURL becomes MY_URL.
• Names are prefixed with the specified prefix when they are used to look up in the environment variables.

By default, prefix APP_ will be used. You can customize the prefix by using env.New() to create a customized loader. For example,

package main

import (
	"fmt"
	"github.com/qiangxue/go-env"
	"log"
	"os"
)

type Config struct {
	Host     string `env:"ES_HOST"`
	Port     int    `env:"ES_PORT"`
	Password string `env:"ES_PASSWORD,secret"`
}

func main() {
	_ = os.Setenv("API_ES_HOST", "127.0.0.1")
	_ = os.Setenv("API_ES_PORT", "8080")
	_ = os.Setenv("API_ES_PASSWORD", "test")

	var cfg Config
	loader := env.New("API_", log.Printf)
	if err := loader.Load(&cfg); err != nil {
		panic(err)
	}
	fmt.Println(cfg.Host)
	fmt.Println(cfg.Port)
	fmt.Println(cfg.Password)
	// Output:
	// 127.0.0.1
	// 8080
	// test
}

In the above code, the Password field is tagged as secret. The log function respects this flag by masking the field value when logging it in order not to reveal sensitive information.

By setting the prefix to an empty string, you can disable the name prefix completely.

Because the values of environment variables are strings, if the corresponding struct fields are of different types, go-env will convert the string values into appropriate types before assigning them to the struct fields.

• If a struct contains embedded structs, the fields of the embedded structs will be populated like they are directly under the containing struct.

• If a struct field type implements env.Setter, env.TextMarshaler, or env.BinaryMarshaler interface, the corresponding interface method will be used to load a string value into the field.

• If a struct field is of a primary type, such as int, string, bool, etc., a string value will be parsed accordingly and assigned to the field. For example, the string value TRUE can be parsed correctly into a boolean true value, while TrUE will cause a parsing error.

• If a struct field is of a complex type, such as map, slice, struct, the string value will be treated as a JSON string, and json.Unmarshal() will be called to populate the struct field from the JSON string.