Command-line Config Loader

Easily manage program startup arguments with only the definition of a structure with special tags. Marshalling patterns fill in the rest.

go get github.com/dsggregory/config

Usage

Simply:

• define your config structure with optional struct tags
• create a variable of that structure type with defaults if desired
• call ReadConfig() with the address of that variable

The struct type defines the settings needed by your application. Only exported fields will be considered as command-line flag arguments.

Take Care With Nested Structs

Only nested structs that are addressable (pointer) will be traversed. This means, unless you ignore the struct with the flag:"-" tag, then this will try to use command-line flags and environment variables to fill out your http.Client struct in your config (as an example).

Supported Field Types

The common Golang flag types are supported:

• int
• int64
• float64
• string
• bool
• time.Duration

Default Values & Precedence

Structure values at read-time are considered defaults, with corresponding but properly capitalized environment variable settings as a backup default.

Precedence of value choice follows:

• command-line flag
• environment variable
• struct value before call to ReadConfig()

Struct Tags

Struct tags, quoted options following the field declaration, include the following along with their default capitalization style:

Tag	Description	Style
flag	command-line flag name. "-" means ignore. On nested structures, a value overrides the default prefix or an empty string prevents prefixing .	field-name
env	environment variable name. "-" means ignore. Default env name is that of flag.	FIELD_NAME
usage	command-line flag usage

Example

type MyConfig struct {
	FirstName string `flag:"first_name" usage:"first name of user"`
	LastName  string `usage:"last name of user"`
	Age       int    `usage:"User's age in dog years"`
	Addr      Address `flag:""` // no prefix for flags of this struct
	Debug     bool   `env:"Debug"`
}

type Address struct {
	Street string `usage:"the street address"`
	Zip string `usage:"the postcode"`
}

cfg := MyConfig{
    FirstName: "DefaultFN",
    LastName:  "",
    Age:       0,
    Debug:     false,
}
err := ReadConfig(&cfg)
if err != nil {
	flag.PrintDefaults()
	os.exit(1)
}

Calling flag.PrintDefaults() from the above example will produce:

  -age int
    	User's age in dog years
  -debug
    	
  -first_name string
    	first name of user (default "DefaultFN")
  -last-name string
    	last name of user
  -street
        the street address
  -zip
        the postcode