> ## Viper v2 feedback

> Viper is heading towards v2 and we would love to hear what _**you**_ would like to see in it. Share your thoughts here: https://forms.gle/R6faU74qPRPAzchZ9

>

> **Thank you!**

![Viper](.github/logo.png?raw=true)

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**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)

* [Clairctl](https://github.com/jgsqware/clairctl)

* [Mercure](https://mercure.rocks)

## Install

```shell

go get github.com/spf13/viper

```

**Note:** Viper uses [Go Modules](https://github.com/golang/go/wiki/Modules) to manage dependencies.

## What is Viper?

and formats. It supports:

* setting defaults

* reading from JSON, TOML, YAML, HCL, envfile 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, INI, envfile 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

**Important:** Viper configuration keys are case insensitive.

There are ongoing discussions about making that optional.

## 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, INI, envfile 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.SetConfigType("yaml") // REQUIRED if the config file does not have the extension in the name

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: %w", err))

}

```

You can handle the specific case where no config file is found like this:

```go

if err := viper.ReadInConfig(); err != nil {

	if _, ok := err.(viper.ConfigFileNotFoundError); ok {

		// Config file not found; ignore error if desired

	} else {

		// Config file was found but another error was produced

	}

}

// Config file found and successfully parsed

```

*NOTE [since 1.6]:* You can also have a file without an extension and specify the format programmaticaly. For those configuration files that lie in the home of the user without any extension like `.bashrc`

### Writing Config Files

Reading from config files is useful, but at times you want to store all modifications made at run time.

For that, a bunch of commands are available, each with its own purpose:

* WriteConfig - writes the current viper configuration to the predefined path, if exists. Errors if no predefined path. Will overwrite the current config file, if it exists.

* SafeWriteConfig - writes the current viper configuration to the predefined path. Errors if no predefined path. Will not overwrite the current config file, if it exists.

* WriteConfigAs - writes the current viper configuration to the given filepath. Will overwrite the given file, if it exists.

* SafeWriteConfigAs - writes the current viper configuration to the given filepath. Will not overwrite the given file, if it exists.

As a rule of the thumb, everything marked with safe won't overwrite any file, but just create if not existent, whilst the default behavior is to create or truncate.

A small examples section:

```go

viper.WriteConfig() // writes current config to predefined path set by 'viper.AddConfigPath()' and 'viper.SetConfigName'

viper.SafeWriteConfig()

viper.WriteConfigAs("/path/to/my/.config")

viper.SafeWriteConfigAs("/path/to/my/.config") // will error since it has already been written

viper.SafeWriteConfigAs("/path/to/my/.other_config")

```

### 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.OnConfigChange(func(e fsnotify.Event) {

	fmt.Println("Config file changed:", e.Name)

})

viper.WatchConfig()

```

### 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 five methods that exist to aid working

with ENV:

 * `AutomaticEnv()`

 * `BindEnv(string...) : error`

 * `SetEnvPrefix(string)`

 * `SetEnvKeyReplacer(string...) *strings.Replacer`

 * `AllowEmptyEnv(bool)`

_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 a prefix while reading from

the environment variables. Both `BindEnv` and `AutomaticEnv` will use this

prefix.

`BindEnv` takes one or more parameters. The first parameter is the key name, the

rest are the name of the environment variables to bind to this key. If more than

one are provided, they will take precedence in the specified order. The name of

the environment variable is case sensitive. If the ENV variable name is not provided, then

Viper will automatically assume that the ENV variable matches the following format: prefix + "_" + the key name in ALL CAPS. When you explicitly provide the ENV variable name (the second parameter),

it **does not** automatically add the prefix. For example if the second parameter is "id",

Viper will look for the ENV variable "ID".

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 an environment variable with a name matching the key uppercased and

prefixed with the `EnvPrefix` if set.

`SetEnvKeyReplacer` 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`.

Alternatively, you can use `EnvKeyReplacer` with `NewWithOptions` factory function.

Unlike `SetEnvKeyReplacer`, it accepts a `StringReplacer` interface allowing you to write custom string replacing logic.

By default empty environment variables are considered unset and will fall back to

the next configuration source. To treat empty environment variables as set, use

the `AllowEmptyEnv` method.

#### 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.

For individual flags, 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"))

```

You can also bind an existing set of pflags (pflag.FlagSet):

Example:

```go

pflag.Int("flagname", 1234, "help message for flagname")

pflag.Parse()

viper.BindPFlags(pflag.CommandLine)

i := viper.GetInt("flagname") // retrieve values from viper instead of pflag

```

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() {

	// using standard library "flag" package

	flag.Int("flagname", 1234, "help message for flagname")

	pflag.CommandLine.AddGoFlagSet(flag.CommandLine)

	pflag.Parse()

	viper.BindPFlags(pflag.CommandLine)

	i := viper.GetInt("flagname") // retrieve value from viper

	// ...

}

```

#### 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) HasChanged() bool { return false }

func (f myFlag) Name() string { return "my-flag-name" }

func (f myFlag) ValueString() string { return "my-flag-value" }

func (f myFlag) ValueType() string { 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, HCL or envfile) 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/bketelsen/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/bketelsen/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

#### etcd

```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", "env", "dotenv"

err := viper.ReadRemoteConfig()

```

#### etcd3

```go

viper.AddRemoteProvider("etcd3", "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", "env", "dotenv"

err := viper.ReadRemoteConfig()

```

#### Consul

You need to set a key to Consul key/value storage with JSON value containing your desired config.

For example, create a Consul key/value store key `MY_CONSUL_KEY` with value:

```json

{

    "port": 8080,

    "hostname": "myhostname.com"