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.

Package notes

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.

Then The flag.Int func returns an Int pointer. We access its Int value with the star character.

Info We can copy the value from the Int pointer into an Int with an assignment statement. We do this with the "value" local.

Here We invoke the Go executable and pass the "run" argument to it. We pass the program file name and specify the count argument.

Result The second argument is the default value for that flag. If Parse() is called and no value is found, 5 will be used.

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=20Argument 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.

Here We specify a special "greeting" string, and then uppercase it as the program executes.

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

A summary

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