envy

Automatically exposes environment variables for all of your flags. It supports the standard flags package along with limited support for Cobra commands.

Envy takes a namespace prefix that will be used for environment variable lookups. Each flag registered in your app will be prefixed, uppercased, and hyphens exchanged for underscores; if a matching environment variable is found, it will set the respective flag value as long as the value is not otherwise explicitly set (see usage for precedence).

Example: flag

Code:

package main

import (
        "flag"
        "fmt"

        "github.com/jamiealquiza/envy"
)

func main() {
        var address = flag.String("address", "127.0.0.1", "Some random address")
        var port = flag.String("port", "8131", "Some random port")

        envy.Parse("MYAPP") // Expose environment variables.
        flag.Parse()

        fmt.Println(*address)
        fmt.Println(*port)
}

Output:

# Prints flag defaults
% ./example
127.0.0.1
8131

# Setting flags via env vars.
% MYAPP_ADDRESS="0.0.0.0" MYAPP_PORT="9080" ./example
0.0.0.0
9080

Example: Cobra

Code:

// Where to execute envy depends on the structure
// of your Cobra implementation. A common pattern
// is to define a root command and an 'Execute' function
// that's called from the application main. We can call
// envy ParseCobra here and configure it to recursively
// update all child commands. Alternatively, it can be
// scoped to child commands at some point in their
// initialization.

var rootCmd = &cobra.Command{
 Use: "myapp",
}

func Execute() {
  // Configure envy.
  cfg := CobraConfig{
    // The env var prefix.
    Prefix: "MYAPP",
    // Whether to parse persistent flags.
    Persistent: true,
    // Whether to recursively update child command FlagSets.
    Recursive: true,
  }

  // Apply.
  envy.ParseCobra(rootCmd, cfg)

  if err := rootCmd.Execute(); err != nil {
    fmt.Println(err)
    os.Exit(1)
  }
}

Output:

# Root command flags.
% myapp
Usage:
  myapp [command]

Available Commands:
  help        Help about any command
  doathing    This is a subcommand

Flags:
  -h, --help               help for myapp
      --some-config        A global config [MYAPP_SOME_CONFIG]

Use "myapp [command] --help" for more information about a command.

# Child command flags. Notice that the prefix
# has the subcommand name automatically appended
# while preserving global/parent env vars.
% myapp doathing
Usage:
  myapp doathing [flags]

Flags:
  -h, --help               help for myapp
      --subcmd-config      Another config [MYAPP_DOATHING_SUBCMD_CONFIG]

Global Flags:
      --some-flag          A global flag [MYAPP_SOME_FLAG]

Usage

Variable precedence:

Envy results in the following order of precedence, each item overwriting the previous: flag default -> Envy generated env var -> flag set at the CLI.

Results referencing the stdlib flag example code:

• ./example will result in port being set to 8131
• MYAPP_PORT=5678 ./example will result in port being set to 5678
• MYAPP_PORT=5678 ./example -port=1234 will result in port being set to 1234

Env vars in help output:

Envy can update your app help output so that it includes the environment variable generated/referenced for each flag. This is done by calling envy.Parse() before flag.Parse().

The above example:

Usage of ./example:
  -address string
        Some random address [MYAPP_ADDRESS] (default "127.0.0.1")
  -port string
        Some random port [MYAPP_PORT] (default "8131")

If this isn't desired, simply call envy.Parse() after flag.Parse():

// ...
	flag.Parse()
        envy.Parse("MYAPP") // looks for MYAPP_ADDRESS & MYAPP_PORT
// ...

Usage of ./example:
  -address string
        Some random address (default "127.0.0.1")
  -port string
        Some random port (default "8131")

Satisfying types:

Environment variables should be defined using a type that satisfies the respective type in your Go application's flag. For example:

• string -> APP_ASTRINGVAR="someString"
• int -> APP_ANINTVAR=42
• bool -> APP_ABOOLVAR=true

Side effects:

Setting a flag through an Envy generated environment variable will have the same effects on the default flag.CommandLine as if the flag were set via the command line. This only affect users that may rely on flag.CommandLine methods that make distinctions between set and to-be set flags (such as the Visit method).

Cobra compatibility:

The extensive types in Cobra's underlying pflag have not been tested, hence the "limited support" reference.

Also, keep in mind that Cobra can change in a way that breaks support with envy. Functionality was tested as of 2018-11-19.