Flag

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.

Goals

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

Why?

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.

Usage

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

Parsing Configuration Files

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()

Parsing Environment Variables

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.

License

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

Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:

• Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer.
• Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution.
• Neither the name of Google Inc. nor the names of its contributors may be used to endorse or promote products derived from this software without specific prior written permission.

THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.