Go provides a flag package supporting basic command-line flag parsing. We’ll use this package to implement our example command-line program.
Basic flag declarations are available for string, integer, and boolean options.
package main
import (
"flag"
"fmt"
)
func main() {
// Here we declare a string flag word with a default value "foo" and a short description. This flag.String function returns a string pointer (not a string value); we’ll see how to use this pointer below.
wordPtr := flag.String("word", "foo", "a string")
// This declares numb and fork flags, using a similar approach to the word flag.
numbPtr := flag.Int("numb", 42, "an int")
boolPtr := flag.Bool("fork", false, "a bool")
// It’s also possible to declare an option that uses an existing var declared elsewhere in the program. Note that we need to pass in a pointer to the flag declaration function.
var svar string
flag.StringVar(&svar, "svar", "bar", "a string var")
// Once all flags are declared, call flag.Parse() to execute the command-line parsing.
flag.Parse()
fmt.Println("word:", *wordPtr)
fmt.Println("numb:", *numbPtr)
fmt.Println("fork:", *boolPtr)
fmt.Println("svar:", svar)
fmt.Println("tail:", flag.Args())
}
$ go build command-line-flags.go
$ ./command-line-flags -word=opt -numb=7 -fork -svar=flag
word: opt
numb: 7
fork: true
svar: flag
tail: []
$ ./command-line-flags -word=opt
word: opt
numb: 42
fork: false
svar: bar
tail: []
$ ./command-line-flags -word=opt a1 a2 a3
word: opt
...
tail: [a1 a2 a3]
$ ./command-line-flags -word=opt a1 a2 a3 -numb=7
word: opt
numb: 42
fork: false
svar: bar
tail: [a1 a2 a3 -numb=7]
$ ./command-line-flags -h
Usage of ./command-line-flags:
-fork=false: a bool
-numb=42: an int
-svar="bar": a string var
-word="foo": a string
$ ./command-line-flags -wat
flag provided but not defined: -wat
Usage of ./command-line-flags:
...