Flag

Flags are arguments passed to a program. In Go, we parse these flags with the flag package. We call funcs like flag.Int and flag.String.

With the flag package, we must first register flags. Then we call flag.Parse() to parse these in from the command line.

Flag.Int example

Here we register an Int flag by calling the flag.Int function in main(). Note that the "flag" package is imported in the import block at the top.

package main

import (
    "flag"
    "fmt"
)

func main() {
    // Register Int flag.
    count := flag.Int("count", 5, "count of iterations")
    // Parse the flags.
    flag.Parse()

    // Print the argument.
    fmt.Println("Argument", *count)

    // Get int from the Int pointer.
    value := *count
    fmt.Println("Value", value)
}
C:\Go\bin\Go.exe run C:\programs\file.go -count=20
Argument 20
Value 20

Flag.String

Let us consider a String argument. With a String, no special parsing method is invoked internally, but we must still call Parse.

package main

import (
    "flag"
    "fmt"
    "strings"
)

func main() {
    // A string flag.
    greeting := flag.String("greeting", "Hello", "startup message")
    flag.Parse()

    // Get String.
    // ... Uppercase it for emphasis.
    value := *greeting
    fmt.Println("Program says:", strings.ToUpper(value))
}
C:\Go\bin\Go.exe run C:\programs\file.go -greeting=Hey
Program says: HEY

Command line syntax

We can omit the equals sign when using an argument on the command line. But a leading hyphen is required. And case is important.

C:\Go\bin\Go.exe run C:\programs\file.go -greeting Hey
Program says: HEY

C:\Go\bin\Go.exe run C:\programs\file.go greeting Hey
Program says: HELLO

C:\Go\bin\Go.exe run C:\programs\file.go GREETING=Hey
Program says: HELLO

Invalid argument

With a func like flag.Int we must pass an argument that can be parsed correctly. We cannot parse the string "bird" as an Int. An error occurs.

package main

import (
    "flag"
    "fmt"
)

func main() {
    // A string flag.
    size := flag.Int("size", 0, "file size")
    flag.Parse()

    // Print size.
    fmt.Println(*size)
}
C:\Go\bin\Go.exe run C:\programs\file.go -size=bird
invalid value "bird" for flag -size:
strconv.ParseInt: parsing "bird": invalid syntax
Usage of C:\...\go-build032374134\command-line-arguments\_obj\exe\file.exe:
  -size int
        file size
exit status 2

With command lines, programs communicate with other programs. Simple arguments can be passed directly. With "flag," a Go package, we make this communication easier.