How to define and parse command-line flags with custom usage messages in Go?

In Go, you can define and parse command-line flags with custom usage messages using the flag package. Here's how you can do it:

Import the flag package:

import "flag"

Define the flags you want to parse. You can use the flag package's StringVar, IntVar, BoolVar, etc. functions to define the flag variables.

var (
    name string
    age  int
    flagUsage = "Usage: your_command -name=<name> -age=<age>"
)

func init() {
    flag.StringVar(&name, "name", "", "Your name")
    flag.IntVar(&age, "age", 0, "Your age")
}

Set a custom usage message by assigning your desired message to flag.Usage. You can set this in the init() function.

func init() {
    flag.StringVar(&name, "name", "", "Your name")
    flag.IntVar(&age, "age", 0, "Your age")

    flag.Usage = func() {
        fmt.Fprint(os.Stderr, flagUsage)
    }
}

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

func main() {
    flag.Parse()

    // Your command-line flag processing logic here
}

In case of an error or if the required flags are not provided, you can print the custom usage message using flag.Usage().

func main() {
    flag.Parse()

    if name == "" || age == 0 {
        flag.Usage()
        os.Exit(1)
    }

    // Your command-line flag processing logic here
}

Now, when you run your program without the required flags or with the -h or --help flag, it will show your custom usage message.

Here's an example of how you can run the program:

go run your_program.go -name=John -age=25

If you run the program without the required flags or with the -h or --help flag, the custom usage message will be displayed.

Note: It's a good practice to call flag.Parse() in the main function rather than inside the init() function, so you have more control over when the flags are parsed.