Working with Go

Command-line Args

os.Args

Command-line args are stored as a slice in os.Args. The first argument in list is program itself.

num_args := len(os.Args)

You can check to see if any command-line arguments were passed in

if num_args < 2 {
    fmt.Println(">> No args passed in")
}

Flag Library

Use the flag standard library to parse command-line arguments. The flag library parses parameters passed in and flags for invalid types, invalud flags, and handles most needs for creating a command-line program.

var s string
flag.StringVar(&s, "str", "default value", "text description")

Usage:

$ ./program -str "Hello"

Full Example

Here is a full example program showing how to use command-line arguments and flags in Go

package main
 
import (
    "flag"
    "fmt"
    "os"
)
 
// global vars
var str string
var num int
var help bool
 
func main() {
 
    // define flags
    flag.StringVar(&str, "str", "default value", "text description")
    flag.IntVar(&num, "num", 5, "text description")
    flag.BoolVar(&help, "help", false, "Display Help")
 
    // parse
    flag.Parse()
 
    // check if help was called explicitly
    if help {
        fmt.Println(">> Display help screen")
        os.Exit(1)
    }
 
    // See values assigned
    fmt.Println(">> String:", str)
    fmt.Println(">> Number:", num)
 
    // last command-line argument
    fmt.Println(">> Last Item:", os.Args[num_args-1])
 
    // the os.Args will include flags for example
    // go run command-line-args.go --str=Foo filename
    // os.Args[1]: "--str=Foo"
 
    // If you have flags and want just the arguments
    // then after calling flag.Parse() you can call
    // flag.Args which store only the args
    args := flag.Args()
    if len(args) > 0 {
        fmt.Println(">> Flag Arg:", args[0])
    }
 
}

Run with different parameters

$ go run command-line.go
>> No args passed in
>> String: default value
>> Number: 5
>> Last Item: command-line.go
 
$ go run command-line.go --str=Foo --num=8 filename
>> String: Foo
>> Number: 8
>> Last Item: filename
>> Flag Arg: filename