How to do it...

1. Open the console and create the folder chapter01/recipe03.
2. Navigate to the directory.
3. Create the main.go file with the following content:

        package main
        import (
          "flag"
          "fmt"
          "log"
          "os"
          "strings"
        )

        // Custom type need to implement
        // flag.Value interface to be able to
        // use it in flag.Var function.
        type ArrayValue []string

        func (s *ArrayValue) String() string {
          return fmt.Sprintf("%v", *s)
        }

        func (a *ArrayValue) Set(s string) error {
          *a = strings.Split(s, ",")
          return nil
        }

        func main() {
 
          // Extracting flag values with methods returning pointers
          retry := flag.Int("retry", -1, "Defines max retry count")

          // Read the flag using the XXXVar function.
          // In this case the variable must be defined
          // prior to the flag.
          var logPrefix string
          flag.StringVar(&logPrefix, "prefix", "", "Logger prefix")

          var arr ArrayValue
          flag.Var(&arr, "array", "Input array to iterate through.")

          // Execute the flag.Parse function, to
          // read the flags to defined variables.
          // Without this call the flag
          // variables remain empty.
          flag.Parse()

          // Sample logic not related to flags
          logger := log.New(os.Stdout, logPrefix, log.Ldate)

          retryCount := 0
          for retryCount < *retry {
            logger.Println("Retrying connection")
            logger.Printf("Sending array %v\n", arr)
            retryCount++
          }
        }

1. Build the binary by executing the go build -o util.
2. From the console, execute ./util -retry 2 -prefix=example -array=1,2.

1. See the output in the Terminal: