> 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!**
[](https://github.com/avelino/awesome-go#configuration)
[](https://repl.it/@sagikazarmark/Viper-example#main.go)
[](https://gitter.im/spf13/viper?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
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 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.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:
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()
```
### Watching Changes in etcd - Unencrypted
```go
// alternatively, you can create a new viper instance.
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", "env", "dotenv"
// 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.
Viper also supports unmarshaling into embedded structs:
```go
/*
Example config:
module:
enabled: true
token: 89h3f98hbwf987h3f98wenf89ehf
*/
type config struct {
Module struct {
Enabled bool
moduleConfig `mapstructure:",squash"`
}
}
// moduleConfig could be in a module specific package
type moduleConfig struct {
Token string
}
var C config
err := viper.Unmarshal(&C)
if err != nil {
t.Fatalf("unable to decode into struct, %v", err)
}
```
Viper uses [github.com/mitchellh/mapstructure](https://github.com/mitchellh/mapstructure) under the hood for unmarshaling values which uses `mapstructure` tags by default.
### Decoding custom formats
A frequently requested feature for Viper is adding more value formats and decoders.
For example, parsing character (dot, comma, semicolon, etc) separated strings into slices.
This is already available in Viper using mapstructure decode hooks.
Read more about the details in [this blog post](https://sagikazarmark.hu/blog/decoding-custom-formats-with-viper/).
### Marshalling to string
You may need to marshal all the settings held in viper into a string rather than write them to a file.
You can use your favorite format's marshaller with the config returned by `AllSettings()`.
```go
import (
yaml "gopkg.in/yaml.v2"
// ...
)
func yamlStringSettings() string {
c := viper.AllSettings()
bs, err := yaml.Marshal(c)
if err != nil {
log.Fatalf("unable to marshal config to YAML: %v", err)
}
return string(bs)
}
```
## 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 its singleton
style approach.
### Working with multiple vipers
You can also create many different vipers for use in your application. Each will
have its 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
### 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.
### Why is it called “Cobra”?
Is there a better name for a [commander](http://en.wikipedia.org/wiki/Cobra_Commander)?
### Does Viper support case sensitive keys?
**tl;dr:** No.
Viper merges configuration from various sources, many of which are either case insensitive or uses different casing than the rest of the sources (eg. env vars).
In order to provide the best experience when using multiple sources, the decision has been made to make all keys case insensitive.
There has been several attempts to implement case sensitivity, but unfortunately it's not that trivial. We might take a stab at implementing it in [Viper v2](https://github.com/spf13/viper/issues/772), but despite the initial noise, it does not seem to be requested that much.
You can vote for case sensitivity by filling out this feedback form: https://forms.gle/R6faU74qPRPAzchZ9
### Is it safe to concurrently read and write to a viper?
No, you will need to synchronize access to the viper yourself (for example by using the `sync` package). Concurrent reads and writes can cause a panic.
