Go的flag包可輕鬆解析命令行參數，1. 使用flag.Type()定義字符串、整型、布爾等類型標誌；2. 可通過flag.TypeVar()將標誌解析到變量避免指針操作；3. 調用flag.Parse()後，用flag.Args()獲取後續(xù)位置參數；4. 實現(xiàn)flag.Value接口可支持自定義類型，滿足多數簡單CLI需求，複雜場景可用spf13/cobra庫替代。

Parsing command-line flags in Go is straightforward using the built-in flag package. It allows you to define and parse flags of various types (string, int, bool, etc.) with minimal code.

Here's how it works:

1. Define Flags with flag.Type()

You declare flags using functions like flag.String() , flag.Int() , flag.Bool() , etc. Each takes a name, default value, and usage description.

 package main

import (
    "flag"
    "fmt"
)

func main() {
    // Define flags
    name := flag.String("name", "World", "your name")
    age := flag.Int("age", 0, "your age")
    verbose := flag.Bool("verbose", false, "enable verbose mode")

    // Parse the command-line flags
    flag.Parse()

    // Use the values
    fmt.Printf("Hello, %s!\n", *name)
    fmt.Printf("Age: %d\n", *age)
    fmt.Printf("Verbose: %t\n", *verbose)
}

Run it like:

 go run main.go -name Alice -age 30 -verbose

Output:

 Hello, Alice!
Age: 30
Verbose: true

2. Using Struct Tags with flag (via Pointers)

The flag.Type() functions return a pointer to the value. So you must dereference them with * when using.

Alternatively, you can declare a variable first and use flag.TypeVar() :

 var name string
var age int
var verbose bool

flag.StringVar(&name, "name", "World", "your name")
flag.IntVar(&age, "age", 0, "your age")
flag.BoolVar(&verbose, "verbose", false, "enable verbose mode")

flag.Parse()

This avoids pointers in the variable usage later.

3. Positional Arguments After Flags

After flag.Parse() , any remaining arguments are accessible via flag.Args() , and you can get the count with flag.NArg() .

 fmt.Printf("Extra arguments: %v\n", flag.Args())

For:

 go run main.go -name Bob file1.txt file2.txt

flag.Args() returns ["file1.txt", "file2.txt"] .

4. Custom Types and flag.Value Interface

You can define custom flag types by implementing the flag.Value interface:

 type Greets []string

func (g *Greets) String() string {
    return fmt.Sprint(*g)
}

func (g *Greets) Set(value string) error {
    *g = append(*g, value)
    return nil
}

Use it:

 var greets Greets
flag.Var(&greets, "greet", "add a greeting")
flag.Parse()

Now you can pass multiple flags:

 go run main.go -greet Hello -greet Hi -greet "Howdy"

And greets will contain all three strings.

Basically, flag covers most use cases out of the box. For more advanced needs (like subcommands or config files), people often use libraries like spf13/cobra , but for simple CLI tools, flag is clean and sufficient.

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