golang check if cli integer flag exists

Question

I'm able to see if a flag exists when its a bool. I'm trying to figure out how to see if a flag exits if its integer, if it does exist use the value if not ignore.

For example, the code below does something if either Bool flags are used. BUT I want to set a flag for -month ie go run appname.go -month=01 should then save the value of 01 but if -month is not used then it should be ignored like a bool flag. In the code below I got around this by making a default value of 0 so if no -month flag is used the value is 0. Is there a better way to do this?

package main

import (
    "flag"
    "fmt"
    "time"
)

func main() {
    //Adding flags
    useCron := flag.Bool("cron", false, "-cron, uses cron flags and defaults to true")
    useNow := flag.Bool("now", false, "-now, uses cron flags and defaults to true")
    useMonth := flag.Int("month", 0, "-month, specify a month")
    flag.Parse()

    //If -cron flag is used
    if *useCron {
        fmt.Printf("Cron set using cron run as date")
        return
    } else if *useNow {
        fmt.Printf("Use Current date")
        return
    }

    if *useMonth != 0 {
        fmt.Printf("month %v\n", *useMonth)
    }
}

Answer 1

See the documentation and example for the flag.Value interface :

You may define a custom type, which implements flag.Value, for example :

type CliInt struct {
    IsSet bool
    Val   int
}

func (i *CliInt) String() string {
    if !i.IsSet {
        return "<not set>"
    }
    return strconv.Itoa(i.Val)
}

func (i *CliInt) Set(value string) error {
    v, err := strconv.Atoi(value)
    if err != nil {
        return err
    }

    i.IsSet = true
    i.Val = v
    return nil
}

and then use flag.Var() to bind it :

    flag.Var(&i1, "i1", "1st int to parse")
    flag.Var(&i2, "i2", "2nd int to parse")

after calling flag.Parse(), you may check the .IsSet flag to see if that flag was set.

playground

---

Another way is to call flag.Visit() after calling flag.Parse() :

    var isSet1, isSet2 bool

    i1 := flag.Int("i1", 0, "1st int to parse")
    i2 := flag.Int("i2", 0, "2nd int to parse")

    flag.Parse([]string{"-i1=123"})

    // flag.Visit will only visit flags which have been explicitly set :
    flag.Visit(func(fl *flag.Flag) {
        if fl.Name == "i1" {
            isSet1 = true
        }
        if fl.Name == "i2" {
            isSet2 = true
        }
    })

playground