What is golang flag

There are many ways to handle command line parameters in Golang programs. In simple cases, you can process os.Args directly without using any library; in fact, Golang's standard library provides the flag package to process command line parameters; there are also third-party libraries for processing command line parameters, such as Pflag.

flag package provides a series of functional interfaces for parsing command line parameters                 (Recommended learning: go)

Command Line syntax

Command line syntax mainly has the following forms

-flag //只支持bool类型
-flag=x
-flag x //只支持非bool类型

The above syntax has the same effect for one or two '-' signs, However, it should be noted that for the third case, only non-bool types are supported. The reason is that

cmd -x *

* is 0 when encountering the following situations. False may represent a file name or file, or it may represent the value of the x tag. If it is 0 or false, ambiguity will occur, so it is stipulated that the third type only supports non-bool types. For integer flags, legal values ​​can be 1234, 0664, 0x1234 or negative numbers, etc. For Boolean flag, it can be 1, 0, t, f, T, F, true, false, TRUE, FALSE, True, False, etc.

Command line parameter parsing method

Using flag mainly includes the following steps

There are three ways to define flag parameters

Through flag.String(), Bool (), Int() and other flag.Xxx() methods, this method returns a corresponding pointer

import "flag"
var ip = flag.Int("flagname", 1234, "help message for flagname")

Bind the flag to a variable through the flag.XxxVar() method, this method returns the value type , such as

var flagvar int
func init() {
    flag.IntVar(&flagvar, "flagname", 1234, "help message for flagname")
}

binds a custom type through flag.Var(), and the custom type needs to implement the Value interface (Receives must be a pointer), such as

flag.Var(&flagVal, "name", "help message for flagname")

For this type of flag, The default value is the initial value of the variable type

Call flag.Parse() to parse the command line parameters to the defined flag

flag.Parse()

The parsing function will encounter the first non-flag command line Stop when the parameter is specified. Non-flag command line parameters refer to parameters that do not meet the command line syntax. For example, the command line parameter is cmd --flag=true abc, and the first non-flag command line parameter is "abc"

After calling Parse to parse, you can directly use the flag itself (pointer type) or the bound variable (value type)

fmt.Println("ip has value ", *ip)
fmt.Println("flagvar has value ", flagvar)

You can also obtain non-standard flags through flag.Args(), flag.Arg(i) flag command line parameters

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