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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) [](https://travis-ci.org/spf13/viper) [](https://gitter.im/spf13/viper?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge) [](https://godoc.org/github.com/spf13/viper) ## 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 five methods that exist to aid working with ENV: * `AutomaticEnv()` * `BindEnv(string...) : error` * `SetEnvPrefix(string)` * `SetEnvKeyReplacer(string...) *strings.Replacer` * `AllowEmptyEnvVar(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 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. `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 usi