envflag

Helper functions wrapping the *Var functions of "flag" package, but default values are optional

- func DurationVar(p *time.Duration, name string, value time.Duration, usage string)
+ func DurationVar(p *time.Duration, name string, value *time.Duration, usage string)

and most importantly, default values can be provided by env. e.g. a flag named session-duration could have its default value set by env var SESSION_DURATION

Panic

Command-line flags are things that we want to parse upfront and abort if the values are invalid. So, though we don't like Go functions to panic, it's fine in this case.

• if SESSION_DURATION=42m is set, yes 42 * time.Minute is the default value
• But if SESSION_DURATION=42 is set by accident, the program should abort instead of defaulting to the hardcoded default of 5 * time.Hour

Usage

type Flag struct {
	myBool     bool
	myDuration time.Duration
	myFloat64  float64
	myInt64    int64
	myInt      int
	myString   string
	myText     LogLevel // 'debug', 'info', or 'error'
	myUint64   uint64
	myUint     uint
}

func main() {
	f := Flag{}
	envflag.BoolVar(&f.myBool, "my-bool", envflag.Ptr(true), "a bool value")
	envflag.DurationVar(&f.myDuration, "my-duration", envflag.Ptr(time.Minute), "a duration value")
	envflag.Float64Var(&f.myFloat64, "my-float64", envflag.Ptr(1.1), "a float64 value")
	envflag.Int64Var(&f.myInt64, "my-int64", envflag.Ptr(int64(1)), "a int64 value")
	envflag.IntVar(&f.myInt, "my-int", nil, "a int value")
	envflag.StringVar(&f.myString, "my-string", nil, "a string value")
	envflag.TextVar(&f.myText, "my-text", LogLevel("debug"), "a text value of either 'debug', 'info', or 'error'")
	envflag.Uint64Var(&f.myUint64, "my-uint64", nil, "a uint64 value")
	envflag.UintVar(&f.myUint, "my-uint", nil, "a uint value")
	envflag.Parse() //  or the standard `flag.Parse()`

	fmt.Printf("%#v\n", f)
}