envconfig is a library which allows you to parse your configuration from environment variables and fill an arbitrary struct.

See the example to understand how to use it, it's pretty simple.

• Almost all standard types plus time.Duration are supported by default.
• Slices and arrays
• Arbitrary structs
• Custom types via the Unmarshaler interface.

envconfig takes the hierarchy of your configuration struct and the names of the fields to create a environment variable key.

For example:

var conf struct {
    Name string
    Shard struct {
        Host string
        Port int
    }
}

This will check for those 3 keys:

• NAME or name
• SHARD_HOST, or shard_host
• SHARD_PORT, or shard_port

envconfig supports having underscores in the key names where there is a word boundary. Now, that term is not super explicit, so let me show you an example:

var conf struct {
    Cassandra struct {
        SSLCert string
        SslKey string
    }
}

This will check all of the following keys:

• CASSANDRA_SSL_CERT, CASSANDRA_SSLCERT, cassandra_ssl_cert, cassandra_sslcert
• CASSANDRA_SSL_KEY, CASSANDRA_SSLKEY, cassandra_ssl_key, cassandra_sslkey

If that is not good enough, look just below.

envconfig supports custom environment variable names:

var conf struct {
    Name string `envconfig:"myName"`
}

envconfig supports default values:

var conf struct {
    Name string `envconfig:"default=Vincent"`
}

envconfig supports optional values:

var conf struct {
    Name string `envconfig:"optional"`
    Age int     `envconfig:"-"`
}

The two syntax are equivalent.

You can of course combine multiple options:

var conf struct {
    Name string `envconfig:"default=Vincent,myName"`
}

With slices or arrays, the same naming is applied for the slice. To put multiple elements into the slice or array, you need to separate them with a , (will probably be configurable in the future, or at least have a way to escape)

For example:

var conf struct {
    Ports []int
}

This will check for the key PORTS:

• if your variable is 9000 the slice will contain only 9000
• if your variable is 9000,100 the slice will contain 9000 and 100

For slices of structs, it's a little more complicated. The same splitting of slice elements is done with a comma, however, each token must follow a specific format like this: {<first field>,<second field>,...}

For example:

var conf struct {
    Shards []struct {
        Name string
        Port int
    }
}

This will check for the key SHARDS. Example variable content: {foobar,9000},{barbaz,20000}

This will result in two struct defined in the Shards slice.

• support for time.Time values with a layout defined via a field tag
• support for complex types