README
¶
Envconfig
Envconfig populates struct field values based on environment variables or arbitrary lookup functions. It supports pre-setting mutations, which is useful for things like converting values to uppercase, trimming whitespace, or looking up secrets.
Note: Versions prior to v0.2 used a different import path. This README and examples are for v0.2+.
Usage
Define a struct with fields using the env tag:
type MyConfig struct {
Port int `env:"PORT"`
Username string `env:"USERNAME"`
}
Set some environment variables:
export PORT=5555
export USERNAME=yoyo
Process it using envconfig:
package main
import (
"context"
"log"
"github.com/sethvargo/go-envconfig"
)
func main() {
ctx := context.Background()
var c MyConfig
if err := envconfig.Process(ctx, &c); err != nil {
log.Fatal(err)
}
// c.Port = 5555
// c.Username = "yoyo"
}
You can also use nested structs, just remember that any fields you want to process must be public:
type MyConfig struct {
Database *DatabaseConfig
}
type DatabaseConfig struct {
Port int `env:"PORT"`
Username string `env:"USERNAME"`
}
Configuration
Use the env struct tag to define configuration.
Required
If a field is required, processing will error if the environment variable is unset.
type MyStruct struct {
Port int `env:"PORT,required"`
}
It is invalid to have a field as both required and default.
Default
If an environment variable is not set, the field will be set to the default
value. Note that the environment variable must not be set (e.g. unset PORT).
If the environment variable is the empty string, that counts as a "value" and
the default will not be used.
type MyStruct struct {
Port int `env:"PORT,default=5555"`
}
You can also set the default value to another field or value from the environment, for example:
type MyStruct struct {
DefaultPort int `env:"DEFAULT_PORT,default=5555"`
Port int `env:"OVERRIDE_PORT,default=$DEFAULT_PORT"`
}
The value for Port will default to the value of DEFAULT_PORT.
It is invalid to have a field as both required and default.
Prefix
For shared, embedded structs, you can define a prefix to use when processing struct values for that embed.
type SharedConfig struct {
Port int `env:"PORT,default=5555"`
}
type Server1 struct {
// This processes Port from $FOO_PORT.
*SharedConfig `env:",prefix=FOO_"`
}
type Server2 struct {
// This processes Port from $BAR_PORT.
*SharedConfig `env:",prefix=BAR_"`
}
It is invalid to specify a prefix on non-struct fields.
Overwrite
If overwrite is set, the value will be overwritten if there is an environment variable match regardless if the value is non-zero.
type MyStruct struct {
Port int `env:"PORT,overwrite"`
}
The rules for overwrite + default are:
-
If the struct field has the zero value and a default is set:
-
If no environment variable is specified, the struct field will be populated with the default value.
-
If an environment variable is specified, the struct field will be populate with the environment variable value.
-
-
If the struct field has a non-zero value and a default is set:
-
If no environment variable is specified, the struct field's existing value will be used (the default is ignored).
-
If an environment variable is specified, the struct field's existing value will be overwritten with the environment variable value.
-
Complex Types
Note: Complex types are only decoded or unmarshalled when the environment variable is defined or a default is specified. The decoding/unmarshalling functions are not invoked when a value is not defined.
Durations
In the environment, time.Duration values are specified as a parsable Go
duration:
type MyStruct struct {
MyVar time.Duration `env:"MYVAR"`
}
export MYVAR="10m" # 10 * time.Minute
TextUnmarshaler / BinaryUnmarshaler
Types that implement TextUnmarshaler or BinaryUnmarshaler are processed as such.
json.Unmarshaler
Types that implement json.Unmarshaler are processed as such.
gob.Decoder
Types that implement gob.Decoder are processed as such.
Slices
Slices are specified as comma-separated values:
type MyStruct struct {
MyVar []string `env:"MYVAR"`
}
export MYVAR="a,b,c,d" # []string{"a", "b", "c", "d"}
Define a custom delimiter with delimiter:
type MyStruct struct {
MyVar []string `env:"MYVAR,delimiter=;"`
export MYVAR="a;b;c;d" # []string{"a", "b", "c", "d"}
Note that byte slices are special cased and interpreted as strings from the environment.
Maps
Maps are specified as comma-separated key:value pairs:
type MyStruct struct {
MyVar map[string]string `env:"MYVAR"`
}
export MYVAR="a:b,c:d" # map[string]string{"a":"b", "c":"d"}
Define a custom delimiter with delimiter:
type MyStruct struct {
MyVar map[string]string `env:"MYVAR,delimiter=;"`
export MYVAR="a:b;c:d" # map[string]string{"a":"b", "c":"d"}
Define a separator with separator:
type MyStruct struct {
MyVar map[string]string `env:"MYVAR,separator=|"`
}
export MYVAR="a|b,c|d" # map[string]string{"a":"b", "c":"d"}
Structs
Envconfig walks the entire struct, including nested structs, so deeply-nested fields are also supported.
If a nested struct is a pointer type, it will automatically be instantianted to the non-nil value. To change this behavior, see Initialization.
Custom
You can also define your own decoder.
Prefixing
You can define a custom prefix using the PrefixLookuper. This will lookup
values in the environment by prefixing the keys with the provided value:
type MyStruct struct {
MyVar string `env:"MYVAR"`
}
// Process variables, but look for the "APP_" prefix.
l := envconfig.PrefixLookuper("APP_", envconfig.OsLookuper())
if err := envconfig.ProcessWith(ctx, &c, l); err != nil {
panic(err)
}
export APP_MYVAR="foo"
Initialization
By default, all pointer fields are initialized (allocated) so they are not
nil. To disable this behavior, use the tag the field as noinit:
type MyStruct struct {
// Without `noinit`, DeleteUser would be initialized to the default boolean
// value. With `noinit`, if the environment variable is not given, the value
// is kept as uninitialized (nil).
DeleteUser *bool `env:"DELETE_USER, noinit"`
}
This also applies to nested fields in a struct:
type ParentConfig struct {
// Without `noinit` tag, `Child` would be set to `&ChildConfig{}` whether
// or not `FIELD` is set in the env var.
// With `noinit`, `Child` would stay nil if `FIELD` is not set in the env var.
Child *ChildConfig `env:",noinit"`
}
type ChildConfig struct {
Field string `env:"FIELD"`
}
The noinit tag is only applicable for pointer fields. Putting the tag on a
non-struct-pointer will return an error.
Extension
All built-in types are supported except Func and Chan. If you need to define
a custom decoder, implement the Decoder interface:
type MyStruct struct {
field string
}
func (v *MyStruct) EnvDecode(val string) error {
v.field = fmt.Sprintf("PREFIX-%s", val)
return nil
}
If you need to modify environment variable values before processing, you can
specify a custom Mutator:
type Config struct {
Password `env:"PASSWORD"`
}
func resolveSecretFunc(ctx context.Context, key, value string) (string, error) {
if strings.HasPrefix(value, "secret://") {
return secretmanager.Resolve(ctx, value) // example
}
return value, nil
}
var config Config
envconfig.ProcessWith(ctx, &config, envconfig.OsLookuper(), resolveSecretFunc)
Testing
Relying on the environment in tests can be troublesome because environment variables are global, which makes it difficult to parallelize the tests. Envconfig supports extracting data from anything that returns a value:
lookuper := envconfig.MapLookuper(map[string]string{
"FOO": "bar",
"ZIP": "zap",
})
var config Config
envconfig.ProcessWith(ctx, &config, lookuper)
Now you can parallelize all your tests by providing a map for the lookup function. In fact, that's how the tests in this repo work, so check there for an example.
You can also combine multiple lookupers with MultiLookuper. See the GoDoc for
more information and examples.
Inspiration
This library is conceptually similar to kelseyhightower/envconfig, with the following major behavioral differences:
-
Adds support for specifying a custom lookup function (such as a map), which is useful for testing.
-
Only populates fields if they contain zero or nil values if
overwriteis unset. This means you can pre-initialize a struct and any pre-populated fields will not be overwritten during processing. -
Support for interpolation. The default value for a field can be the value of another field.
-
Support for arbitrary mutators that change/resolve data before type conversion.
Documentation
¶
Overview ¶
Package envconfig populates struct fields based on environment variable values (or anything that responds to "Lookup"). Structs declare their environment dependencies using the "env" tag with the key being the name of the environment variable, case sensitive.
type MyStruct struct {
A string `env:"A"` // resolves A to $A
B string `env:"B,required"` // resolves B to $B, errors if $B is unset
C string `env:"C,default=foo"` // resolves C to $C, defaults to "foo"
D string `env:"D,required,default=foo"` // error, cannot be required and default
E string `env:""` // error, must specify key
}
All built-in types are supported except Func and Chan. If you need to define a custom decoder, implement Decoder:
type MyStruct struct {
field string
}
func (v *MyStruct) EnvDecode(val string) error {
v.field = fmt.Sprintf("PREFIX-%s", val)
return nil
}
In the environment, slices are specified as comma-separated values:
export MYVAR="a,b,c,d" // []string{"a", "b", "c", "d"}
In the environment, maps are specified as comma-separated key:value pairs:
export MYVAR="a:b,c:d" // map[string]string{"a":"b", "c":"d"}
If you need to modify environment variable values before processing, you can specify a custom mutator:
type Config struct {
Password `env:"PASSWORD_SECRET"`
}
func resolveSecretFunc(ctx context.Context, key, value string) (string, error) {
if strings.HasPrefix(value, "secret://") {
return secretmanager.Resolve(ctx, value) // example
}
return value, nil
}
var config Config
ProcessWith(&config, OsLookuper(), resolveSecretFunc)
Index ¶
- Constants
- func ExtractDefaults(ctx context.Context, i interface{}, fns ...MutatorFunc) error
- func Process(ctx context.Context, i interface{}) error
- func ProcessWith(ctx context.Context, i interface{}, l Lookuper, fns ...MutatorFunc) error
- type Base64Bytes
- type Decoder
- type Error
- type HexBytes
- type Lookuper
- type MutatorFunc
Constants ¶
const ( ErrInvalidEnvvarName = Error("invalid environment variable name") ErrInvalidMapItem = Error("invalid map item") ErrLookuperNil = Error("lookuper cannot be nil") ErrMissingKey = Error("missing key") ErrMissingRequired = Error("missing required value") ErrNoInitNotPtr = Error("field must be a pointer to have noinit") ErrNotPtr = Error("input must be a pointer") ErrNotStruct = Error("input must be a struct") ErrPrefixNotStruct = Error("prefix is only valid on struct types") ErrPrivateField = Error("cannot parse private fields") ErrRequiredAndDefault = Error("field cannot be required and have a default value") ErrUnknownOption = Error("unknown option") )
Variables ¶
This section is empty.
Functions ¶
func ExtractDefaults ¶
func ExtractDefaults(ctx context.Context, i interface{}, fns ...MutatorFunc) error
ExtractDefaults is a helper that returns a fully-populated struct with the default values resolved. This is helpful when you want to return a constant "default" configuration that is not affected by the user's environment.
type Config struct {
Port string `env:"PORT,default=8080"`
}
func DefaultConfig() *Config {
var cfg Config
if err := envconfig.ExtractDefaults(ctx, &cfg); err != nil {
panic("failed to extract default config: %s" + err.Error())
}
return &cfg
}
This is effectively the same as calling ProcessWith with an empty MapLookuper.
func Process ¶
Process processes the struct using the environment. See ProcessWith for a more customizable version.
func ProcessWith ¶
func ProcessWith(ctx context.Context, i interface{}, l Lookuper, fns ...MutatorFunc) error
ProcessWith processes the given interface with the given lookuper. See the package-level documentation for specific examples and behaviors.
Types ¶
type Base64Bytes ¶
type Base64Bytes []byte
Base64Bytes is a slice of bytes where the information is base64-encoded in the environment variable.
func (*Base64Bytes) EnvDecode ¶
func (b *Base64Bytes) EnvDecode(val string) error
EnvDecode implements env.Decoder.
type Decoder ¶
Decoder is an interface that custom types/fields can implement to control how decoding takes place. For example:
type MyType string
func (mt MyType) EnvDecode(val string) error {
return "CUSTOM-"+val
}
type HexBytes ¶
type HexBytes []byte
HexBytes is a slice of bytes where the information is hex-encoded in the environment variable.
type Lookuper ¶
type Lookuper interface {
// Lookup searches for the given key and returns the corresponding string
// value. If a value is found, it returns the value and true. If a value is
// not found, it returns the empty string and false.
Lookup(key string) (string, bool)
}
Lookuper is an interface that provides a lookup for a string-based key.
func MapLookuper ¶
MapLookuper looks up environment configuration from a provided map. This is useful for testing, especially in parallel, since it does not require you to mutate the parent environment (which is stateful).
func MultiLookuper ¶
MultiLookuper wraps a collection of lookupers. It does not combine them, and lookups appear in the order in which they are provided to the initializer.
func OsLookuper ¶
func OsLookuper() Lookuper
OsLookuper returns a lookuper that uses the environment (os.LookupEnv) to resolve values.
func PrefixLookuper ¶
PrefixLookuper looks up environment configuration using the specified prefix. This is useful if you want all your variables to start with a particular prefix like "MY_APP_".
Source Files