The flag package

Learn the basics of the flag package.

The flag package

The flag package parses command-line options (-port=8080, -v) and generates help text automatically:

port := flag.Int("port", 8080, "dinlenecek port")
ayrintili := flag.Bool("v", false, "ayrıntılı çıktı")
flag.Parse()
fmt.Println(*port, *ayrintili)

Running ./app -h lists every option with its description and default value.

The bool option trap

Bool options don't take a value after a space: if you write -v false, -v becomes true and false counts as a positional argument. To turn it off write -v=false. Parsing also stops at the first positional argument: in app dosya.txt -v, -v isn't seen as an option.

Exercise Parse with a FlagSet

Create fs := flag.NewFlagSet("app", flag.ContinueOnError); define the options port (int, default 8080) and v (bool). Call fs.Parse([]string{"-port=9000", "-v"}) and print both values.

Show example solution

package main

import (
    "flag"
    "fmt"
)

func main() {
    fs := flag.NewFlagSet("app", flag.ContinueOnError)
    port := fs.Int("port", 8080, "dinlenecek port")
    v := fs.Bool("v", false, "ayrıntılı çıktı")
    if err := fs.Parse([]string{"-port=9000", "-v"}); err != nil {
        fmt.Println(err)
        return
    }
    fmt.Println(*port, *v)
}

Test yourself

What does the flag package do with <code>-port 9000</code>, <code>-port=9000</code> and <code>--port=9000</code>?
- Only -port=9000
- Only --port=9000
- None of them
- It accepts all three
Show answer

It accepts all three — The flag package treats single and double dashes the same; for non-bool options the value can follow a space or =.

Parse

port := flag.Int("port", 8080, "port")
flag.____()
fmt.Println(*port)

Show answer

port := flag.Int("port", 8080, "port")
flag.Parse()
fmt.Println(*port)

Command line

-port=9000

Option with a value
-v

Bool option
-v=false

Turning a bool off
-h

Help text

Next lesson: flag.String and flag.Int

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