前言
编写程序的时候，需要读取用户输入的参数来做一些事情。在golang里面有两个库可以来制作这块的功能。
flag
flag包，加载输入信息。官方两个例子：
// Example 1: A single string flag called "species" with default value "gopher".
var species = flag.String("species", "gopher", "the species we are studying")
// Example 2: Two flags sharing a variable, so we can have a shorthand.
// The order of initialization is undefined, so make sure both use the
// same default value. They must be set up with an init function.
var gopherType string
func init() {
const (
defaultGopher = "pocket"
usage = "the variety of gopher"
)
flag.StringVar(&gopherType, "gopher_type", defaultGopher, usage)
flag.StringVar(&gopherType, "g", defaultGopher, usage+" (shorthand)")
}
// PrintDefaults prints, to standard error unless configured otherwise,
// a usage message showing the default settings of all defined
// command-line flags.
// For an integer valued flag x, the default output has the form
// -x int
// usage-message-for-x (default 7)
// The usage message will appear on a separate line for anything but
// a bool flag with a one-byte name. For bool flags, the type is
// omitted and if the flag name is one byte the usage message appears
// on the same line. The parenthetical default is omitted if the
// default is the zero value for the type. The listed type, here int,
// can be changed by placing a back-quoted name in the flag's usage
// string; the first such item in the message is taken to be a parameter
// name to show in the message and the back quotes are stripped from
// the message when displayed. For instance, given
// flag.String("I", "", "search `directory` for include files")
// the output will be
// -I directory
// search directory for include files.
//
// To change the destination for flag messages, call CommandLine.SetOutput.
func PrintDefaults() {
CommandLine.PrintDefaults()
}
