envy.go

package envy

import (
	"flag"
	"fmt"
	"os"
	"strings"
)

// Parse takes a string p that is used
// as the environment variable prefix
// for each flag configured.
func Parse(p string) {
	// Build a map of explicitly set flags.
	set := map[string]bool{}
	flag.CommandLine.Visit(func(f *flag.Flag) {
		set[f.Name] = true
	})

	flag.CommandLine.VisitAll(func(f *flag.Flag) {
		// Create an env var name
		// based on the supplied prefix.
		envVar := fmt.Sprintf("%s_%s", p, strings.ToUpper(f.Name))
		envVar = strings.Replace(envVar, "-", "_", -1)

		// Update the Flag.Value if the
		// env var is non "".
		if val := os.Getenv(envVar); val != "" {
			// Update the value if it hasn't
			// already been set.
			if defined := set[f.Name]; !defined {
				flag.CommandLine.Set(f.Name, val)
			}
		}

		// Append the env var to the
		// Flag.Usage field.
		f.Usage = fmt.Sprintf("%s [%s]", f.Usage, envVar)
	})
}

example.go

///////////////
package main

import (
        "flag"
        "fmt"

        "github.com/jamiealquiza/envy"
)

func main() {
        var address = flag.String("address", "127.0.0.1", "Some random address")
        var port = flag.String("port", "8131", "Some random port")

        envy.Parse("MYAPP") // looks for MYAPP_ADDRESS & MYAPP_PORT
        flag.Parse()

        fmt.Println(*address)
        fmt.Println(*port)
}

output.md

Output:

Prints flag defaults

% ./example 127.0.0.1 8131

Flag defaults overridden

% MYAPP_ADDRESS="0.0.0.0" MYAPP_PORT="9080" ./example 0.0.0.0 9080