README.md - third_party/spf13/viper - Git at Google

 ![viper logo](https://cloud.githubusercontent.com/assets/173412/10886745/998df88a-8151-11e5-9448-4736db51020d.png)

 Go configuration with fangs!

 Many Go projects are built using Viper including:

 * [Hugo](http://gohugo.io)
 * [EMC RexRay](http://rexray.readthedocs.org/en/stable/)
 * [Imgur's Incus](https://github.com/Imgur/incus)
 * [Nanobox](https://github.com/nanobox-io/nanobox)/[Nanopack](https://github.com/nanopack)
 * [Docker Notary](https://github.com/docker/Notary)
 * [BloomApi](https://www.bloomapi.com/)
 * [doctl(https://github.com/digitalocean/doctl)

  [![Build Status](https://travis-ci.org/spf13/viper.svg)](https://travis-ci.org/spf13/viper) [![Join the chat at https://gitter.im/spf13/viper](https://badges.gitter.im/Join%20Chat.svg)](https://gitter.im/spf13/viper?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)


 ## What is Viper?

 Viper is a complete configuration solution for go applications including 12 factor apps. It is designed
 to work within an application, and can handle all types of configuration needs
 and formats. It supports:

 * setting defaults
 * reading from JSON, TOML, YAML, HCL, and Java properties config files
 * live watching and re-reading of config files (optional)
 * reading from environment variables
 * reading from remote config systems (etcd or Consul), and watching changes
 * reading from command line flags
 * reading from buffer
 * setting explicit values

 Viper can be thought of as a registry for all of your applications
 configuration needs.

 ## Why Viper?

 When building a modern application, you don’t want to worry about
 configuration file formats; you want to focus on building awesome software.
 Viper is here to help with that.

 Viper does the following for you:

 1. Find, load, and unmarshal a configuration file in JSON, TOML, YAML, HCL, or Java properties formats.
 2. Provide a mechanism to set default values for your different
    configuration options.
 3. Provide a mechanism to set override values for options specified through
    command line flags.
 4. Provide an alias system to easily rename parameters without breaking existing
    code.
 5. Make it easy to tell the difference between when a user has provided a
    command line or config file which is the same as the default.

 Viper uses the following precedence order. Each item takes precedence over the
 item below it:

  * explicit call to Set
  * flag
  * env
  * config
  * key/value store
  * default

 Viper configuration keys are case insensitive.

 ## Putting Values into Viper

 ### Establishing Defaults

 A good configuration system will support default values. A default value is not
 required for a key, but it's useful in the event that a key hasn’t been set via
 config file, environment variable, remote configuration or flag.

 Examples:

 ```go
 viper.SetDefault("ContentDir", "content")
 viper.SetDefault("LayoutDir", "layouts")
 viper.SetDefault("Taxonomies", map[string]string{"tag": "tags", "category": "categories"})
 ```

 ### Reading Config Files

 Viper requires minimal configuration so it knows where to look for config files.
 Viper supports JSON, TOML, YAML, HCL, and Java Properties files. Viper can search multiple paths, but
 currently a single Viper instance only supports a single configuration file.
 Viper does not default to any configuration search paths leaving defaults decision
 to an application.

 Here is an example of how to use Viper to search for and read a configuration file.
 None of the specific paths are required, but at least one path should be provided
 where a configuration file is expected.

 ```go
 viper.SetConfigName("config") // name of config file (without extension)
 viper.AddConfigPath("/etc/appname/")   // path to look for the config file in
 viper.AddConfigPath("$HOME/.appname")  // call multiple times to add many search paths
 viper.AddConfigPath(".")               // optionally look for config in the working directory
 err := viper.ReadInConfig() // Find and read the config file
 if err != nil { // Handle errors reading the config file
 	panic(fmt.Errorf("Fatal error config file: %s \n", err))
 }
 ```

 ### Watching and re-reading config files

 Viper supports the ability to have your application live read a config file while running.

 Gone are the days of needing to restart a server to have a config take effect,
 viper powered applications can read an update to a config file while running and
 not miss a beat.

 Simply tell the viper instance to watchConfig.
 Optionally you can provide a function for Viper to run each time a change occurs.

 **Make sure you add all of the configPaths prior to calling `WatchConfig()`**

 ```go
 		viper.WatchConfig()
 		viper.OnConfigChange(func(e fsnotify.Event) {
 			fmt.Println("Config file changed:", e.Name)
 		})
 ```

 ### Reading Config from io.Reader

 Viper predefines many configuration sources such as files, environment
 variables, flags, and remote K/V store, but you are not bound to them. You can
 also implement your own required configuration source and feed it to viper.

 ```go
 viper.SetConfigType("yaml") // or viper.SetConfigType("YAML")

 // any approach to require this configuration into your program.
 var yamlExample = []byte(`
 Hacker: true
 name: steve
 hobbies:
 - skateboarding
 - snowboarding
 - go
 clothing:
   jacket: leather
   trousers: denim
 age: 35
 eyes : brown
 beard: true
 `)

 viper.ReadConfig(bytes.NewBuffer(yamlExample))

 viper.Get("name") // this would be "steve"
 ```

 ### Setting Overrides

 These could be from a command line flag, or from your own application logic.

 ```go
 viper.Set("Verbose", true)
 viper.Set("LogFile", LogFile)
 ```

 ### Registering and Using Aliases

 Aliases permit a single value to be referenced by multiple keys

 ```go
 viper.RegisterAlias("loud", "Verbose")

 viper.Set("verbose", true) // same result as next line
 viper.Set("loud", true)   // same result as prior line

 viper.GetBool("loud") // true
 viper.GetBool("verbose") // true
 ```

 ### Working with Environment Variables

 Viper has full support for environment variables. This enables 12 factor
 applications out of the box. There are four methods that exist to aid working
 with ENV:

  * `AutomaticEnv()`
  * `BindEnv(string...) : error`
  * `SetEnvPrefix(string)`
  * `SetEnvReplacer(string...) *strings.Replacer`

 _When working with ENV variables, it’s important to recognize that Viper
 treats ENV variables as case sensitive._

 Viper provides a mechanism to try to ensure that ENV variables are unique. By
 using `SetEnvPrefix`, you can tell Viper to use add a prefix while reading from
 the environment variables. Both `BindEnv` and `AutomaticEnv` will use this
 prefix.

 `BindEnv` takes one or two parameters. The first parameter is the key name, the
 second is the name of the environment variable. The name of the environment
 variable is case sensitive. If the ENV variable name is not provided, then
 Viper will automatically assume that the key name matches the ENV variable name,
 but the ENV variable is IN ALL CAPS. When you explicitly provide the ENV
 variable name, it **does not** automatically add the prefix.

 One important thing to recognize when working with ENV variables is that the
 value will be read each time it is accessed. Viper does not fix the value when
 the `BindEnv` is called.

 `AutomaticEnv` is a powerful helper especially when combined with
 `SetEnvPrefix`. When called, Viper will check for an environment variable any
 time a `viper.Get` request is made. It will apply the following rules. It will
 check for a environment variable with a name matching the key uppercased and
 prefixed with the `EnvPrefix` if set.

 `SetEnvReplacer` allows you to use a `strings.Replacer` object to rewrite Env
 keys to an extent. This is useful if you want to use `-` or something in your
 `Get()` calls, but want your environmental variables to use `_` delimiters. An
 example of using it can be found in `viper_test.go`.

 #### Env example

 ```go
 SetEnvPrefix("spf") // will be uppercased automatically
 BindEnv("id")

 os.Setenv("SPF_ID", "13") // typically done outside of the app

 id := Get("id") // 13
 ```

 ### Working with Flags

 Viper has the ability to bind to flags. Specifically, Viper supports `Pflags`
 as used in the [Cobra](https://github.com/spf13/cobra) library.

 Like `BindEnv`, the value is not set when the binding method is called, but when
 it is accessed. This means you can bind as early as you want, even in an
 `init()` function.

 The `BindPFlag()` method provides this functionality.

 Example:

 ```go
 serverCmd.Flags().Int("port", 1138, "Port to run Application server on")
 viper.BindPFlag("port", serverCmd.Flags().Lookup("port"))
 ```

 The use of [pflag](https://github.com/spf13/pflag/) in Viper does not preclude
 the use of other packages that use the [flag](https://golang.org/pkg/flag/)
 package from the standard library. The pflag package can handle the flags
 defined for the flag package by importing these flags. This is accomplished
 by a calling a convenience function provided by the pflag package called
 AddGoFlagSet().

 Example:

 ```go
 package main

 import (
 	"flag"
 	"github.com/spf13/pflag"
 )

 func main() {
 	pflag.CommandLine.AddGoFlagSet(flag.CommandLine)
 	pflag.Parse()
     ...
 }
 ```

 #### Flag interfaces

 Viper provides two Go interfaces to bind other flag systems if you don't use `Pflags`.

 `FlagValue` represents a single flag. This is a very simple example on how to implement this interface:

 ```go
 type myFlag struct {}
 func (f myFlag) IsChanged() { return false }
 func (f myFlag) Name() { return "my-flag-name" }
 func (f myFlag) ValueString() { return "my-flag-value" }
 func (f myFlag) ValueType() { return "string" }
 ```

 Once your flag implements this interface, you can simply tell Viper to bind it:

 ```go
 viper.BindFlagValue("my-flag-name", myFlag{})
 ```

 `FlagValueSet` represents a group of flags. This is a very simple example on how to implement this interface:

 ```go
 type myFlagSet struct {
 	flags []myFlag
 }

 func (f myFlagSet) VisitAll(fn func(FlagValue)) {
 	for _, flag := range flags {
 		fn(flag)
 	}
 }
 ```

 Once your flag set implements this interface, you can simply tell Viper to bind it:

 ```go
 fSet := myFlagSet{
 	flags: []myFlag{myFlag{}, myFlag{}},
 }
 viper.BindFlagValues("my-flags", fSet)
 ```

 ### Remote Key/Value Store Support

 To enable remote support in Viper, do a blank import of the `viper/remote`
 package:

 `import _ "github.com/spf13/viper/remote"`

 Viper will read a config string (as JSON, TOML, YAML or HCL) retrieved from a path
 in a Key/Value store such as etcd or Consul.  These values take precedence over
 default values, but are overridden by configuration values retrieved from disk,
 flags, or environment variables.

 Viper uses [crypt](https://github.com/xordataexchange/crypt) to retrieve
 configuration from the K/V store, which means that you can store your
 configuration values encrypted and have them automatically decrypted if you have
 the correct gpg keyring.  Encryption is optional.

 You can use remote configuration in conjunction with local configuration, or
 independently of it.

 `crypt` has a command-line helper that you can use to put configurations in your
 K/V store. `crypt` defaults to etcd on http://127.0.0.1:4001.

 ```bash
 $ go get github.com/xordataexchange/crypt/bin/crypt
 $ crypt set -plaintext /config/hugo.json /Users/hugo/settings/config.json
 ```

 Confirm that your value was set:

 ```bash
 $ crypt get -plaintext /config/hugo.json
 ```

 See the `crypt` documentation for examples of how to set encrypted values, or
 how to use Consul.

 ### Remote Key/Value Store Example - Unencrypted

 ```go
 viper.AddRemoteProvider("etcd", "http://127.0.0.1:4001","/config/hugo.json")
 viper.SetConfigType("json") // because there is no file extension in a stream of bytes, supported extensions are "json", "toml", "yaml", "yml", "properties", "props", "prop"
 err := viper.ReadRemoteConfig()
 ```

 ### Remote Key/Value Store Example - Encrypted

 ```go
 viper.AddSecureRemoteProvider("etcd","http://127.0.0.1:4001","/config/hugo.json","/etc/secrets/mykeyring.gpg")
 viper.SetConfigType("json") // because there is no file extension in a stream of bytes,  supported extensions are "json", "toml", "yaml", "yml", "properties", "props", "prop"
 err := viper.ReadRemoteConfig()
 ```

 ### Watching Changes in etcd - Unencrypted

 ```go
 // alternatively, you can create a new viper instance.
 var runtime_viper = viper.New()

 runtime_viper.AddRemoteProvider("etcd", "http://127.0.0.1:4001", "/config/hugo.yml")
 runtime_viper.SetConfigType("yaml") // because there is no file extension in a stream of bytes, supported extensions are "json", "toml", "yaml", "yml", "properties", "props", "prop"

 // read from remote config the first time.
 err := runtime_viper.ReadRemoteConfig()

 // unmarshal config
 runtime_viper.Unmarshal(&runtime_conf)

 // open a goroutine to watch remote changes forever
 go func(){
 	for {
 	    time.Sleep(time.Second * 5) // delay after each request

 	    // currently, only tested with etcd support
 	    err := runtime_viper.WatchRemoteConfig()
 	    if err != nil {
 	        log.Errorf("unable to read remote config: %v", err)
 	        continue
 	    }

 	    // unmarshal new config into our runtime config struct. you can also use channel
 	    // to implement a signal to notify the system of the changes
 	    runtime_viper.Unmarshal(&runtime_conf)
 	}
 }()
 ```

 ## Getting Values From Viper

 In Viper, there are a few ways to get a value depending on the value's type.
 The following functions and methods exist:

  * `Get(key string) : interface{}`
  * `GetBool(key string) : bool`
  * `GetFloat64(key string) : float64`
  * `GetInt(key string) : int`
  * `GetString(key string) : string`
  * `GetStringMap(key string) : map[string]interface{}`
  * `GetStringMapString(key string) : map[string]string`
  * `GetStringSlice(key string) : []string`
  * `GetTime(key string) : time.Time`
  * `GetDuration(key string) : time.Duration`
  * `IsSet(key string) : bool`

 One important thing to recognize is that each Get function will return a zero
 value if it’s not found. To check if a given key exists, the `IsSet()` method
 has been provided.

 Example:
 ```go
 viper.GetString("logfile") // case-insensitive Setting & Getting
 if viper.GetBool("verbose") {
     fmt.Println("verbose enabled")
 }
 ```
 ### Accessing nested keys

 The accessor methods also accept formatted paths to deeply nested keys. For
 example, if the following JSON file is loaded:

 ```json
 {
     "host": {
         "address": "localhost",
         "port": 5799
     },
     "datastore": {
         "metric": {
             "host": "127.0.0.1",
             "port": 3099
         },
         "warehouse": {
             "host": "198.0.0.1",
             "port": 2112
         }
     }
 }

 ```

 Viper can access a nested field by passing a `.` delimited path of keys:

 ```go
 GetString("datastore.metric.host") // (returns "127.0.0.1")
 ```

 This obeys the precedence rules established above; the search for the root key
 (in this example, `datastore`) will cascade through the remaining configuration
 registries until found. The search for the sub-keys (`metric` and `host`),
 however, will not.

 For example, if the `metric` key was not defined in the configuration loaded
 from file, but was defined in the defaults, Viper would return the zero value.

 On the other hand, if the primary key was not defined, Viper would go through
 the remaining registries looking for it.

 Lastly, if there exists a key that matches the delimited key path, its value
 will be returned instead. E.g.

 ```json
 {
     "datastore.metric.host": "0.0.0.0",
     "host": {
         "address": "localhost",
         "port": 5799
     },
     "datastore": {
         "metric": {
             "host": "127.0.0.1",
             "port": 3099
         },
         "warehouse": {
             "host": "198.0.0.1",
             "port": 2112
         }
     }
 }

 GetString("datastore.metric.host") //returns "0.0.0.0"
 ```

 ### Extract sub-tree

 Extract sub-tree from Viper.

 For example, `viper` represents:

 ```json
 app:
   cache1:
     max-items: 100
     item-size: 64
   cache2:
     max-items: 200
     item-size: 80
 ```

 After executing:

 ```go
 subv := viper.Sub("app.cache1")
 ```

 `subv` represents:

 ```json
 max-items: 100
 item-size: 64
 ```

 Suppose we have:

 ```go
 func NewCache(cfg *Viper) *Cache {...}
 ```

 which creates a cache based on config information formatted as `subv`.
 Now it's easy to create these 2 caches separately as:

 ```go
 cfg1 := viper.Sub("app.cache1")
 cache1 := NewCache(cfg1)

 cfg2 := viper.Sub("app.cache2")
 cache2 := NewCache(cfg2)
 ```

 ### Unmarshaling

 You also have the option of Unmarshaling all or a specific value to a struct, map,
 etc.

 There are two methods to do this:

  * `Unmarshal(rawVal interface{}) : error`
  * `UnmarshalKey(key string, rawVal interface{}) : error`

 Example:

 ```go
 type config struct {
 	Port int
 	Name string
 	PathMap string `mapstructure:"path_map"`
 }

 var C config

 err := Unmarshal(&C)
 if err != nil {
 	t.Fatalf("unable to decode into struct, %v", err)
 }
 ```

 ## Viper or Vipers?

 Viper comes ready to use out of the box. There is no configuration or
 initialization needed to begin using Viper. Since most applications will want
 to use a single central repository for their configuration, the viper package
 provides this. It is similar to a singleton.

 In all of the examples above, they demonstrate using viper in it's singleton
 style approach.

 ### Working with multiple vipers

 You can also create many different vipers for use in your application. Each will
 have it’s own unique set of configurations and values. Each can read from a
 different config file, key value store, etc. All of the functions that viper
 package supports are mirrored as methods on a viper.

 Example:

 ```go
 x := viper.New()
 y := viper.New()

 x.SetDefault("ContentDir", "content")
 y.SetDefault("ContentDir", "foobar")

 //...
 ```

 When working with multiple vipers, it is up to the user to keep track of the
 different vipers.

 ## Q & A

 Q: Why not INI files?

 A: Ini files are pretty awful. There’s no standard format, and they are hard to
 validate. Viper is designed to work with JSON, TOML or YAML files. If someone
 really wants to add this feature, I’d be happy to merge it. It’s easy to specify
 which formats your application will permit.

 Q: Why is it called “Viper”?

 A: Viper is designed to be a [companion](http://en.wikipedia.org/wiki/Viper_(G.I._Joe))
 to [Cobra](https://github.com/spf13/cobra). While both can operate completely
 independently, together they make a powerful pair to handle much of your
 application foundation needs.

 Q: Why is it called “Cobra”?

 A: Is there a better name for a [commander](http://en.wikipedia.org/wiki/Cobra_Commander)?
	![viper logo](https://cloud.githubusercontent.com/assets/173412/10886745/998df88a-8151-11e5-9448-4736db51020d.png)

	Go configuration with fangs!

	Many Go projects are built using Viper including:

	* [Hugo](http://gohugo.io)
	* [EMC RexRay](http://rexray.readthedocs.org/en/stable/)
	* [Imgur's Incus](https://github.com/Imgur/incus)
	* [Nanobox](https://github.com/nanobox-io/nanobox)/[Nanopack](https://github.com/nanopack)
	* [Docker Notary](https://github.com/docker/Notary)
	* [BloomApi](https://www.bloomapi.com/)
	* [doctl(https://github.com/digitalocean/doctl)

	[![Build Status](https://travis-ci.org/spf13/viper.svg)](https://travis-ci.org/spf13/viper) [![Join the chat at https://gitter.im/spf13/viper](https://badges.gitter.im/Join%20Chat.svg)](https://gitter.im/spf13/viper?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)


	## What is Viper?

	Viper is a complete configuration solution for go applications including 12 factor apps. It is designed
	to work within an application, and can handle all types of configuration needs
	and formats. It supports:

	* setting defaults
	* reading from JSON, TOML, YAML, HCL, and Java properties config files
	* live watching and re-reading of config files (optional)
	* reading from environment variables
	* reading from remote config systems (etcd or Consul), and watching changes
	* reading from command line flags
	* reading from buffer
	* setting explicit values

	Viper can be thought of as a registry for all of your applications
	configuration needs.

	## Why Viper?

	When building a modern application, you don’t want to worry about
	configuration file formats; you want to focus on building awesome software.
	Viper is here to help with that.

	Viper does the following for you:

	1. Find, load, and unmarshal a configuration file in JSON, TOML, YAML, HCL, or Java properties formats.
	2. Provide a mechanism to set default values for your different
	configuration options.
	3. Provide a mechanism to set override values for options specified through
	command line flags.
	4. Provide an alias system to easily rename parameters without breaking existing
	code.
	5. Make it easy to tell the difference between when a user has provided a
	command line or config file which is the same as the default.

	Viper uses the following precedence order. Each item takes precedence over the
	item below it:

	* explicit call to Set
	* flag
	* env
	* config
	* key/value store
	* default

	Viper configuration keys are case insensitive.

	## Putting Values into Viper

	### Establishing Defaults

	A good configuration system will support default values. A default value is not
	required for a key, but it's useful in the event that a key hasn’t been set via
	config file, environment variable, remote configuration or flag.

	Examples:

	```go
	viper.SetDefault("ContentDir", "content")
	viper.SetDefault("LayoutDir", "layouts")
	viper.SetDefault("Taxonomies", map[string]string{"tag": "tags", "category": "categories"})
	```

	### Reading Config Files

	Viper requires minimal configuration so it knows where to look for config files.
	Viper supports JSON, TOML, YAML, HCL, and Java Properties files. Viper can search multiple paths, but
	currently a single Viper instance only supports a single configuration file.
	Viper does not default to any configuration search paths leaving defaults decision
	to an application.

	Here is an example of how to use Viper to search for and read a configuration file.
	None of the specific paths are required, but at least one path should be provided
	where a configuration file is expected.

	```go
	viper.SetConfigName("config") // name of config file (without extension)
	viper.AddConfigPath("/etc/appname/") // path to look for the config file in
	viper.AddConfigPath("$HOME/.appname") // call multiple times to add many search paths
	viper.AddConfigPath(".") // optionally look for config in the working directory
	err := viper.ReadInConfig() // Find and read the config file
	if err != nil { // Handle errors reading the config file
	panic(fmt.Errorf("Fatal error config file: %s \n", err))
	}
	```

	### Watching and re-reading config files

	Viper supports the ability to have your application live read a config file while running.

	Gone are the days of needing to restart a server to have a config take effect,
	viper powered applications can read an update to a config file while running and
	not miss a beat.

	Simply tell the viper instance to watchConfig.
	Optionally you can provide a function for Viper to run each time a change occurs.

	Make sure you add all of the configPaths prior to calling `WatchConfig()`

	```go
	viper.WatchConfig()
	viper.OnConfigChange(func(e fsnotify.Event) {
	fmt.Println("Config file changed:", e.Name)
	})
	```

	### Reading Config from io.Reader

	Viper predefines many configuration sources such as files, environment
	variables, flags, and remote K/V store, but you are not bound to them. You can
	also implement your own required configuration source and feed it to viper.

	```go
	viper.SetConfigType("yaml") // or viper.SetConfigType("YAML")

	// any approach to require this configuration into your program.
	var yamlExample = []byte(`
	Hacker: true
	name: steve
	hobbies:
	- skateboarding
	- snowboarding
	- go
	clothing:
	jacket: leather
	trousers: denim
	age: 35
	eyes : brown
	beard: true
	`)

	viper.ReadConfig(bytes.NewBuffer(yamlExample))

	viper.Get("name") // this would be "steve"
	```

	### Setting Overrides

	These could be from a command line flag, or from your own application logic.

	```go
	viper.Set("Verbose", true)
	viper.Set("LogFile", LogFile)
	```

	### Registering and Using Aliases

	Aliases permit a single value to be referenced by multiple keys

	```go
	viper.RegisterAlias("loud", "Verbose")

	viper.Set("verbose", true) // same result as next line
	viper.Set("loud", true) // same result as prior line

	viper.GetBool("loud") // true
	viper.GetBool("verbose") // true
	```

	### Working with Environment Variables

	Viper has full support for environment variables. This enables 12 factor
	applications out of the box. There are four methods that exist to aid working
	with ENV:

	* `AutomaticEnv()`
	* `BindEnv(string...) : error`
	* `SetEnvPrefix(string)`
	* `SetEnvReplacer(string...) *strings.Replacer`

	_When working with ENV variables, it’s important to recognize that Viper
	treats ENV variables as case sensitive._

	Viper provides a mechanism to try to ensure that ENV variables are unique. By
	using `SetEnvPrefix`, you can tell Viper to use add a prefix while reading from
	the environment variables. Both `BindEnv` and `AutomaticEnv` will use this
	prefix.

	`BindEnv` takes one or two parameters. The first parameter is the key name, the
	second is the name of the environment variable. The name of the environment
	variable is case sensitive. If the ENV variable name is not provided, then
	Viper will automatically assume that the key name matches the ENV variable name,
	but the ENV variable is IN ALL CAPS. When you explicitly provide the ENV
	variable name, it does not automatically add the prefix.

	One important thing to recognize when working with ENV variables is that the
	value will be read each time it is accessed. Viper does not fix the value when
	the `BindEnv` is called.

	`AutomaticEnv` is a powerful helper especially when combined with
	`SetEnvPrefix`. When called, Viper will check for an environment variable any
	time a `viper.Get` request is made. It will apply the following rules. It will
	check for a environment variable with a name matching the key uppercased and
	prefixed with the `EnvPrefix` if set.

	`SetEnvReplacer` allows you to use a `strings.Replacer` object to rewrite Env
	keys to an extent. This is useful if you want to use `-` or something in your
	`Get()` calls, but want your environmental variables to use `_` delimiters. An
	example of using it can be found in `viper_test.go`.

	#### Env example

	```go
	SetEnvPrefix("spf") // will be uppercased automatically
	BindEnv("id")

	os.Setenv("SPF_ID", "13") // typically done outside of the app

	id := Get("id") // 13
	```

	### Working with Flags

	Viper has the ability to bind to flags. Specifically, Viper supports `Pflags`
	as used in the [Cobra](https://github.com/spf13/cobra) library.

	Like `BindEnv`, the value is not set when the binding method is called, but when
	it is accessed. This means you can bind as early as you want, even in an
	`init()` function.

	The `BindPFlag()` method provides this functionality.

	Example:

	```go
	serverCmd.Flags().Int("port", 1138, "Port to run Application server on")
	viper.BindPFlag("port", serverCmd.Flags().Lookup("port"))
	```

	The use of [pflag](https://github.com/spf13/pflag/) in Viper does not preclude
	the use of other packages that use the [flag](https://golang.org/pkg/flag/)
	package from the standard library. The pflag package can handle the flags
	defined for the flag package by importing these flags. This is accomplished
	by a calling a convenience function provided by the pflag package called
	AddGoFlagSet().

	Example:

	```go
	package main

	import (
	"flag"
	"github.com/spf13/pflag"
	)

	func main() {
	pflag.CommandLine.AddGoFlagSet(flag.CommandLine)
	pflag.Parse()
	...
	}
	```

	#### Flag interfaces

	Viper provides two Go interfaces to bind other flag systems if you don't use `Pflags`.

	`FlagValue` represents a single flag. This is a very simple example on how to implement this interface:

	```go
	type myFlag struct {}
	func (f myFlag) IsChanged() { return false }
	func (f myFlag) Name() { return "my-flag-name" }
	func (f myFlag) ValueString() { return "my-flag-value" }
	func (f myFlag) ValueType() { return "string" }
	```

	Once your flag implements this interface, you can simply tell Viper to bind it:

	```go
	viper.BindFlagValue("my-flag-name", myFlag{})
	```

	`FlagValueSet` represents a group of flags. This is a very simple example on how to implement this interface:

	```go
	type myFlagSet struct {
	flags []myFlag
	}

	func (f myFlagSet) VisitAll(fn func(FlagValue)) {
	for _, flag := range flags {
	fn(flag)
	}
	}
	```

	Once your flag set implements this interface, you can simply tell Viper to bind it:

	```go
	fSet := myFlagSet{
	flags: []myFlag{myFlag{}, myFlag{}},
	}
	viper.BindFlagValues("my-flags", fSet)
	```

	### Remote Key/Value Store Support

	To enable remote support in Viper, do a blank import of the `viper/remote`
	package:

	`import _ "github.com/spf13/viper/remote"`

	Viper will read a config string (as JSON, TOML, YAML or HCL) retrieved from a path
	in a Key/Value store such as etcd or Consul. These values take precedence over
	default values, but are overridden by configuration values retrieved from disk,
	flags, or environment variables.

	Viper uses [crypt](https://github.com/xordataexchange/crypt) to retrieve
	configuration from the K/V store, which means that you can store your
	configuration values encrypted and have them automatically decrypted if you have
	the correct gpg keyring. Encryption is optional.

	You can use remote configuration in conjunction with local configuration, or
	independently of it.

	`crypt` has a command-line helper that you can use to put configurations in your
	K/V store. `crypt` defaults to etcd on http://127.0.0.1:4001.

	```bash
	$ go get github.com/xordataexchange/crypt/bin/crypt
	$ crypt set -plaintext /config/hugo.json /Users/hugo/settings/config.json
	```

	Confirm that your value was set:

	```bash
	$ crypt get -plaintext /config/hugo.json
	```

	See the `crypt` documentation for examples of how to set encrypted values, or
	how to use Consul.

	### Remote Key/Value Store Example - Unencrypted

	```go
	viper.AddRemoteProvider("etcd", "http://127.0.0.1:4001","/config/hugo.json")
	viper.SetConfigType("json") // because there is no file extension in a stream of bytes, supported extensions are "json", "toml", "yaml", "yml", "properties", "props", "prop"
	err := viper.ReadRemoteConfig()
	```

	### Remote Key/Value Store Example - Encrypted

	```go
	viper.AddSecureRemoteProvider("etcd","http://127.0.0.1:4001","/config/hugo.json","/etc/secrets/mykeyring.gpg")
	viper.SetConfigType("json") // because there is no file extension in a stream of bytes, supported extensions are "json", "toml", "yaml", "yml", "properties", "props", "prop"
	err := viper.ReadRemoteConfig()
	```

	### Watching Changes in etcd - Unencrypted

	```go
	// alternatively, you can create a new viper instance.
	var runtime_viper = viper.New()

	runtime_viper.AddRemoteProvider("etcd", "http://127.0.0.1:4001", "/config/hugo.yml")
	runtime_viper.SetConfigType("yaml") // because there is no file extension in a stream of bytes, supported extensions are "json", "toml", "yaml", "yml", "properties", "props", "prop"

	// read from remote config the first time.
	err := runtime_viper.ReadRemoteConfig()

	// unmarshal config
	runtime_viper.Unmarshal(&runtime_conf)

	// open a goroutine to watch remote changes forever
	go func(){
	for {
	time.Sleep(time.Second * 5) // delay after each request

	// currently, only tested with etcd support
	err := runtime_viper.WatchRemoteConfig()
	if err != nil {
	log.Errorf("unable to read remote config: %v", err)
	continue
	}

	// unmarshal new config into our runtime config struct. you can also use channel
	// to implement a signal to notify the system of the changes
	runtime_viper.Unmarshal(&runtime_conf)
	}
	}()
	```

	## Getting Values From Viper

	In Viper, there are a few ways to get a value depending on the value's type.
	The following functions and methods exist:

	* `Get(key string) : interface{}`
	* `GetBool(key string) : bool`
	* `GetFloat64(key string) : float64`
	* `GetInt(key string) : int`
	* `GetString(key string) : string`
	* `GetStringMap(key string) : map[string]interface{}`
	* `GetStringMapString(key string) : map[string]string`
	* `GetStringSlice(key string) : []string`
	* `GetTime(key string) : time.Time`
	* `GetDuration(key string) : time.Duration`
	* `IsSet(key string) : bool`

	One important thing to recognize is that each Get function will return a zero
	value if it’s not found. To check if a given key exists, the `IsSet()` method
	has been provided.

	Example:
	```go
	viper.GetString("logfile") // case-insensitive Setting & Getting
	if viper.GetBool("verbose") {
	fmt.Println("verbose enabled")
	}
	```
	### Accessing nested keys

	The accessor methods also accept formatted paths to deeply nested keys. For
	example, if the following JSON file is loaded:

	```json
	{
	"host": {
	"address": "localhost",
	"port": 5799
	},
	"datastore": {
	"metric": {
	"host": "127.0.0.1",
	"port": 3099
	},
	"warehouse": {
	"host": "198.0.0.1",
	"port": 2112
	}
	}
	}

	```

	Viper can access a nested field by passing a `.` delimited path of keys:

	```go
	GetString("datastore.metric.host") // (returns "127.0.0.1")
	```

	This obeys the precedence rules established above; the search for the root key
	(in this example, `datastore`) will cascade through the remaining configuration
	registries until found. The search for the sub-keys (`metric` and `host`),
	however, will not.

	For example, if the `metric` key was not defined in the configuration loaded
	from file, but was defined in the defaults, Viper would return the zero value.

	On the other hand, if the primary key was not defined, Viper would go through
	the remaining registries looking for it.

	Lastly, if there exists a key that matches the delimited key path, its value
	will be returned instead. E.g.

	```json
	{
	"datastore.metric.host": "0.0.0.0",
	"host": {
	"address": "localhost",
	"port": 5799
	},
	"datastore": {
	"metric": {
	"host": "127.0.0.1",
	"port": 3099
	},
	"warehouse": {
	"host": "198.0.0.1",
	"port": 2112
	}
	}
	}

	GetString("datastore.metric.host") //returns "0.0.0.0"
	```

	### Extract sub-tree

	Extract sub-tree from Viper.

	For example, `viper` represents:

	```json
	app:
	cache1:
	max-items: 100
	item-size: 64
	cache2:
	max-items: 200
	item-size: 80
	```

	After executing:

	```go
	subv := viper.Sub("app.cache1")
	```

	`subv` represents:

	```json
	max-items: 100
	item-size: 64
	```

	Suppose we have:

	```go
	func NewCache(cfg Viper) Cache {...}
	```

	which creates a cache based on config information formatted as `subv`.
	Now it's easy to create these 2 caches separately as:

	```go
	cfg1 := viper.Sub("app.cache1")
	cache1 := NewCache(cfg1)

	cfg2 := viper.Sub("app.cache2")
	cache2 := NewCache(cfg2)
	```

	### Unmarshaling

	You also have the option of Unmarshaling all or a specific value to a struct, map,
	etc.

	There are two methods to do this:

	* `Unmarshal(rawVal interface{}) : error`
	* `UnmarshalKey(key string, rawVal interface{}) : error`

	Example:

	```go
	type config struct {
	Port int
	Name string
	PathMap string `mapstructure:"path_map"`
	}

	var C config

	err := Unmarshal(&C)
	if err != nil {
	t.Fatalf("unable to decode into struct, %v", err)
	}
	```

	## Viper or Vipers?

	Viper comes ready to use out of the box. There is no configuration or
	initialization needed to begin using Viper. Since most applications will want
	to use a single central repository for their configuration, the viper package
	provides this. It is similar to a singleton.

	In all of the examples above, they demonstrate using viper in it's singleton
	style approach.

	### Working with multiple vipers

	You can also create many different vipers for use in your application. Each will
	have it’s own unique set of configurations and values. Each can read from a
	different config file, key value store, etc. All of the functions that viper
	package supports are mirrored as methods on a viper.

	Example:

	```go
	x := viper.New()
	y := viper.New()

	x.SetDefault("ContentDir", "content")
	y.SetDefault("ContentDir", "foobar")

	//...
	```

	When working with multiple vipers, it is up to the user to keep track of the
	different vipers.

	## Q & A

	Q: Why not INI files?

	A: Ini files are pretty awful. There’s no standard format, and they are hard to
	validate. Viper is designed to work with JSON, TOML or YAML files. If someone
	really wants to add this feature, I’d be happy to merge it. It’s easy to specify
	which formats your application will permit.

	Q: Why is it called “Viper”?

	A: Viper is designed to be a [companion](http://en.wikipedia.org/wiki/Viper_(G.I._Joe))
	to [Cobra](https://github.com/spf13/cobra). While both can operate completely
	independently, together they make a powerful pair to handle much of your
	application foundation needs.

	Q: Why is it called “Cobra”?

	A: Is there a better name for a [commander](http://en.wikipedia.org/wiki/Cobra_Commander)?