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

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.

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

package main
import (
	"flag"
	"github.com/csmith/envflag/v2"
)
var (
	myFlag = flag.String("my-flag", "woohoo", "Something or other")
)
func main() {
	envflag.Parse()
	println(*myFlag)
}

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:

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

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

envflag.Parse(envflag.WithFlagSet(someFlagSet))

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

envflag.Parse(envflag.WithShowInUsage(false))

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

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

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

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.