Can command line flags in Go be set to mandatory?

The flag package does not support mandatory or required flags (meaning the flag must be specified explicitly).

What you can do is use sensible default values for (all) flags. And if a flag is something like there is no sensible default, check the value at the start of your application and halt with an error message. You should do flag value validation anyway (not just for required flags), so this shouldn't mean any (big) overhead, and this is a good practice in general.

As already mentioned, the flag package does not provide this feature directly and usually you can (and should) be able to provide a sensible default. For cases where you only need a small number of explicit arguments (e.g. an input and output filename) you could use positional arguments (e.g. after flag.Parse() check that flag.NArg()==2 and then input, output := flag.Arg(0), flag.Arg(1)).

If however, you have a case where this isn't sensible; say a few integer flags you want to accept in any order, where any integer value is reasonable, but no default is. Then you can use the flag.Visit function to check if the flags you care about were explicitly set or not. I think this is the only way to tell if a flag was explicitly set to it's default value (not counting a custom flag.Value type with a Set implementation that kept state).

For example, perhaps something like:

    required := []string{"b", "s"}
    flag.Parse()

    seen := make(map[string]bool)
    flag.Visit(func(f *flag.Flag) { seen[f.Name] = true })
    for _, req := range required {
        if !seen[req] {
            // or possibly use `log.Fatalf` instead of:
            fmt.Fprintf(os.Stderr, "missing required -%s argument/flag\n", req)
            os.Exit(2) // the same exit code flag.Parse uses
        }
    }

Playground

This would produce an error if either the "-b" or "-s" flag was not explicitly set.

go-flags lets you declare both required flags and required positional arguments:

var opts struct {
    Flag string `short:"f" required:"true" name:"a flag"`
    Args struct {
        First   string `positional-arg-name:"first arg"`
        Sencond string `positional-arg-name:"second arg"`
    } `positional-args:"true" required:"2"`
}
args, err := flags.Parse(&opts)