Flag is a drop in replacement for Go's flag package with the addition to parse files and environment variables. If you support the twelve-factor app methodology, Flag complies with the third factor; "Store config in the environment".

An example using a gopher:

$ cat > gopher.go
    package main

    import (
        "fmt"
    	"github.com/namsral/flag"
	)
    
    func main() {
    	var age int
	flag.IntVar(&age, "age", 0, "age of gopher")
	flag.Parse()
	fmt.Print("age:", age)
    }
$ go run gopher.go -age 1
age: 1

Same code but using an environment variable:

$ export AGE=2
$ go run gopher.go
age: 2

Same code but using a configuration file:

$ cat > gopher.conf
age 3

$ go run gopher.go -config gopher.conf
age: 3

The following table shows how flags are translated to environment variables and configuration files:

This package is a port of Go's flag package from the standard library with the addition of two functions ParseEnv and ParseFile.

• Compatability with the original flag package
• Support the twelve-factor app methodology
• Uniform user experience between the three input methods

Why not use one of the many INI, JSON or YAML parsers?

I find it best practice to have simple configuration options to control the behaviour of an applications when it starts up. Use basic types like ints, floats and strings for configuration options and store more complex data structures in the "datastore" layer.

It's intended for projects which require a simple configuration made available through command-line flags, configuration files and shell environments. It's similar to the original flag package.

Example:

import "github.com/namsral/flag"

flag.String(flag.DefaultConfigFlagname, "", "path to config file")
flag.Int("age", 24, "help message for age")

flag.Parse()

Order of precedence:

1. Command line options
2. Environment variables
3. Configuration file
4. Default values

Create a configuration file:

$ cat > ./gopher.conf
# empty newlines and lines beginning with a "#" character are ignored.
name bob

# keys and values can also be separated by the "=" character
age=20

# booleans can be empty, set with 0, 1, true, false, etc
hacker

Add a "config" flag:

flag.String(flag.DefaultConfigFlagname, "", "path to config file")

Run the command:

$ go run ./gopher.go -config ./gopher.conf

The default flag name for the configuration file is "config" and can be changed by setting flag.DefaultConfigFlagname:

flag.DefaultConfigFlagname = "conf"
flag.Parse()

Environment variables are parsed 1-on-1 with defined flags:

$ export AGE=44
$ go run ./gopher.go
age=44

You can also parse prefixed environment variables by setting a prefix name when creating a new empty flag set:

fs := flag.NewFlagSetWithEnvPrefix(os.Args[0], "GO", 0)
fs.Int("age", 24, "help message for age")
fs.Parse(os.Args[1:])
...
$ go export GO_AGE=33
$ go run ./gopher.go
age=33

For more examples see the examples directory in the project repository.

That's it.

Copyright (c) 2012 The Go Authors. All rights reserved.

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