envflag

Minimalist approach to wiring up the standard flag package to the environment.

Usage

func main() {
    listenAddr := flag.String("addr", ":8080", "server listen address")
    flag.Parse()

    envflag.SetPrefix("myapp")
    envflag.Parse()

    fmt.Printf("listenAddr is %q\n", *listenAddr)
}

Rationale

This package connects the standard flag package to the environment with a minimum of ceremony.

12-factor apps store their configuration in the environment. It's also useful, however, to accept command-line flags: they make a program easier to use, and serve as documentation.

Example

package main

import (
	"flag"
	"fmt"
	"os"

	"github.com/dcowgill/envflag"
)

func main() {
	fs := flag.NewFlagSet(os.Args[0], flag.ExitOnError)
	listenAddr := fs.String("addr", ":8888", "server listen address")
	timeout := fs.Int("timeout", 10, "client timeout in seconds")
	fs.Parse(nil) // simulate no command-line args

	// Normally this would happen outside the program.
	os.Setenv("MYAPP_LISTEN_ADDR", ":9999")
	os.Setenv("MYAPP_TIMEOUT", "42")

	vs := envflag.NewVarSet(fs)
	vs.SetPrefix("myapp")
	vs.RenameFlag("addr", "listen-addr")
	vs.Parse()

	fmt.Println(*listenAddr) // prints ":9999"
	fmt.Println(*timeout)    // prints "42"
}