flag.Parse scans command-line parameters after flags are registered and fills the variables or pointers associated with those flags. Define every flag before calling it and access parsed values afterward. section 2.4 section 2.4.1
A typical flag.Parse sequence is:
- Register a flag with flag.StringVar, flag.IntVar, or another typed helper, supplying its destination, name, default, and description. section 2.4.1
- Call flag.Parse after all flags are defined and before reading their values. section 6.4.1
- Use the populated variable, or dereference the pointer returned by flag.String; positional arguments remain available through flag.Arg. section 6.4.1 section 2.4.1
- Validate values such as language codes, because successful parsing does not establish that a value is supported or correctly formatted. section 2.4.1
Language-selectable greeting
var lang string flag.StringVar(&lang, "lang", "en", "The required language, e.g. en, ur...") flag.Parse() greeting := greet(language(lang)) fmt.Println(greeting) Running the program with a language flag changes lang; omitting it leaves the default value "en". section 2.4.1
Listing 2.17 main.go: Using flags
package main
import (
"flag"
"fmt"
)
func main() {
var lang string
flag.StringVar(&lang,
"lang",
"en",
"The required language, e.g. en, ur...") #1
flag.Parse()
greeting := greet(language(lang))
fmt.Println(greeting)
}
// language represents a language
type language string
// phrasebook holds greeting for each supported language
var phrasebook = map[language]string{
...
}
// greet says hello to the world
func greet(l language) string {
...
}
Use the complete listing to see flag registration, flag.Parse, conversion to a domain type, and greeting output together. section 2.4.1
