Global variables / Get command line argument and print it

Question

This may sound stupid but how do I define a global variable in Go? const myglobalvariable = "Hi there!" doesn't really work...

I just want to get the command line argument and after this I want to print it. I do this using this code snippet:

package main

import (
    "flag"
    "fmt"
)

func main() {
    gettext();
    fmt.Println(text)
}

func gettext() {
    flag.Parse()
    text := flag.Args()
    if len(text) < 1 {
        fmt.Println("Please give me some text!")
    }
}

The problem is that it just prints an empty line so I thought about declaring a global variable using const myglobalvariable = "Hi there!" but I just get the error cannot use flag.Args() (type []string) as type ideal string in assignment... ...I know this is a noob question so I hope you can help me...

Answer 1

I see at least two questions here, maybe three.

1. How do you declare a global variable?
2. How do you declare a global constant?
3. How do you parse command line options and arguments?

I hope the code below demonstrates this in a helpful way. The flag package was one of the first packages I had to cut my teeth on in Go. At the time it wasn't obvious, though the documentation is improving.

FYI, at the time of this writing I am using http://weekly.golang.org as a reference. The main site is far too out of date.

package main

import (
    "flag"
    "fmt"
    "os"
)

//This is how you declare a global variable
var someOption bool

//This is how you declare a global constant
const usageMsg string = "goprog [-someoption] args\n"

func main() {
    flag.BoolVar(&someOption, "someOption", false, "Run with someOption")
    //Setting Usage will cause usage to be executed if options are provided
    //that were never defined, e.g. "goprog -someOption -foo"
    flag.Usage = usage
    flag.Parse()
    if someOption {
        fmt.Printf("someOption was set\n")
    }
    //If there are other required command line arguments, that are not
    //options, they will now be available to parse manually.  flag does
    //not do this part for you.
    for _, v := range flag.Args() {
        fmt.Printf("%+v\n", v)
    }

    //Calling this program as "./goprog -someOption dog cat goldfish"
    //outputs
    //someOption was set
    //dog
    //cat
    //goldfish
}

func usage() {
    fmt.Printf(usageMsg)
    flag.PrintDefaults()
    os.Exit(1)
}

Answer 2

The closest thing to a global variable in Go is a package variable. You define one like

var text string

Command line arguments though, are already sitting in a package variable, os.Args, waiting for you to access them. You don't even need the flag package.

package main

import (
    "fmt"
    "os"
)

func main() {
    if len(os.Args) < 2 {     // (program name is os.Arg[0])
        fmt.Println("Please give me some text!")
    } else {
        fmt.Println(os.Args[1:])  // print all args
    }
}