Go by Example: Command-Line Flags

Command-line flags are a common way to specify options for command-line programs. For example, in wc -l the -l is a command-line flag.

 
package main

Go provides a flag package supporting basic command-line flag parsing. We’ll use this package to implement our example command-line program.

 
import "flag"
import "fmt"

func main() {

Basic flag declarations are available for string, integer, and boolean options. 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()

Here we’ll just dump out the parsed options and any trailing positional arguments. Note that we need to dereference the points with e.g. *wordPtr to get the actual option values.

 
	fmt.Println("word:", *wordPtr)
	fmt.Println("numb:", *numbPtr)
	fmt.Println("fork:", *boolPtr)
	fmt.Println("svar:", svar)
	fmt.Println("tail:", flag.Args())
}

To experiment with the command-line flags program it’s best to first compile it and then run the resulting binary directly.

 
$ go build command-line-flags.go

Try out the built program by first giving it values for all flags.

 
$ ./command-line-flags -word=opt -numb=7 -fork -svar=flag
word: opt
numb: 7
fork: true
svar: flag
tail: []

Note that if you omit flags they automatically take their default values.

 
$ ./command-line-flags -word=opt
word: opt
numb: 42
fork: false
svar: bar
tail: []

Trailing positional arguments can be provided after any flags.

 
$ ./command-line-flags -word=opt a1 a2 a3
word: opt
...
tail: [a1 a2 a3]

Note that the flag package requires all flags to appear before positional arguments (otherwise the flags will be interpreted as positional arguments).

 
$ ./command-line-flags -word=opt a1 a2 a3 -numb=7
word: opt
numb: 42
fork: false
svar: bar
trailing: [a1 a2 a3 -numb=7]

Use -h or --help flags to get automatically generated help text for the command-line program.

 
$ ./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

If you provide a flag that wasn’t specified to the flag package, the program will print an error message an show the help text again.

 
$ ./command-line-flags -wat
flag provided but not defined: -wat
Usage of ./command-line-flags:
...

Next we’ll look at environment variables, another common way to parameterize programs.

Previous example: Command-Line Arguments.

Next example: Environment Variables.

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