The Go Programming Language

7.4 Parsing Flags with flag.Value

In this section, we’ll see how another standard interface, flag.Value, helps us define new notations for command-line flags. Consider the program below, which sleeps for a specified period of time.

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gopl.io/ch7/sleep

var period = flag.Duration("period", 1*time.Second, "sleep period")

func main() {
    flag.Parse()
    fmt.Printf("Sleeping for %v...", *period)
    time.Sleep(*period)
    fmt.Println()
}

Before it goes to sleep it prints the time period. The fmt package calls the time.Duration’s String method to print the period not as a number of nanoseconds, but in a user-friendly notation:

$ go build gopl.io/ch7/sleep
$ ./sleep
Sleeping for 1s...

By default, the sleep period is one second, ...