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flag Module in Go

The flag module in Go is used for parsing command-line arguments. It provides a way to define and use flags in your program.

Basic Example

package main

import (
    "flag"
    "fmt"
)

func main() {
    // Define flags
    name := flag.String("name", "World", "a name to say hello to")
    age := flag.Int("age", 0, "your age")
    isMember := flag.Bool("member", false, "are you a member?")

    // Parse the flags
    flag.Parse()

    // Access the values
    fmt.Printf("Hello, %s! Age: %d, Member: %t\n", *name, *age, *isMember) // we need to dereference pointers

    // Remaining command-line arguments
    fmt.Println("Other arguments:", flag.Args())
}
  • Output
go run main.go -name=Deep -age=22 -member=true extra_arg

You would get: 

Hello, Deep! Age: 23, Member: true
Other arguments: [extra_arg]

  • The one weird thing is that, we need to dereference the pointers to get the actual values.

Better flag usage

  • We can use flag.IntVar instead of flag.Int. But, here we need to pass a pointer to the variable.
package main

import (
    "flag"
    "fmt"
)

func main() {
    var worker int
    flag.IntVar(&worker, "worker", 0, "Number of worker")

    flag.Parse()

    fmt.Println("Number of workers is: ", worker)
}

Help command

  • we can use -h or --help to see auto-generated usage information:
go run . -h 
# or
go run . --help

Flags with Structs (Good example)

package main

import (
    "flag"
    "fmt"
)

type ServerConfig struct {
    port    int
    workers int
    env     string
}

func main() {
    conf := ServerConfig{}

    flag.IntVar(&conf.port, "port", 6969, "which port to run on")
    flag.IntVar(&conf.workers, "worker", 1, "number of workers to run")
    flag.StringVar(&conf.env, "env", "dev", "which mode server is in: dev | prod")

    flag.Parse()

    fmt.Printf("%+v\n", conf)
}
  • Run it:
go run . -port=8080 -worker=2 -env=prod

# Output
{port:8080 workers:2 env:prod}