csmith/envflag

Set flags from environment variables in Go

Yet another envflag library

This is a simple library that allows that enables reading the value of flags from environment variables.

Why?

The Go flags package is well-thought-out, battle-tested, and extensively used, but modern deployment practices tend to favour storing configuration in environment variables rather than passing them in on the command-line.

envflag and its predecessors bridge this gap by mapping environment variables onto flags.

How?

Import this package, and use envflag.Parse instead of flag.Parse:

package main

import (
	"flag"

	"github.com/csmith/envflag"
)

var (
	myFlag = flag.String("my-flag", "woohoo", "Something or other")
)

func main() {
	envflag.Parse()

	println(*myFlag)
}

What?

In its default configuration, envflag will:

• Map all flag names to appropriate environment variable names (my-flag -> MY_FLAG)
• Update the usage information of flags to include the environment variable name
• Show usage and exit with an appropriate status code on error
• Use the default set of flags provided by flag.CommandLine
• Call flag.Parse to also parse any command-line flags

You can customise the behaviour of envflag by passing in options:

Prefix all environment variables

envflag.Parse(envflag.WithPrefix("MYAPP_"))

Changes the mapping of environment variables to always include the given prefix (e.g. my-flag -> MYAPP_MY_FLAG).

Use a different flag set

envflag.Parse(envflag.WithFlagSet(someFlagSet))

Use the given flag.FlagSet instead of flag.CommandLine.

Don't show environment variable names in usage

envflag.Parse(envflag.WithShowInUsage(false))

Suppresses the default behaviour of updating flag usage to include the environment variable names.

Use a different set of arguments

envflag.Parse(envflag.WithArguments([]string{"-option1", "-option2"}))

When parsing the command-line arguments, use the given slice instead of os.Args[1:].

Licence/credits/contributions etc

Released under the MIT licence. See LICENCE for full details.

Heavily inspired by kouhin/envflag which does mostly the same job but can't easily add prefixes and is a bit harder to configure.

Contributions are welcome! Please feel free to open issues or send pull requests.