Go program to download and overwrite itself with a new copy

main.go

package main

import (
	"fmt"
	"io"
	"net/http"
	"os"
	"syscall"
)

func main() {
	// Run `python3 -m http.server 8080` wherever there's a separate copy of
	// the `main` binary for quick and dirty testing
	req, err := http.Get("http://127.0.0.1:8080/main")
	if err != nil {
		fmt.Printf("error making http request: %v\n", err)
		os.Exit(1)
	}

	body, err := io.ReadAll(req.Body)
	if err != nil {
		fmt.Printf("error reading http response body: %v\n", err)
		os.Exit(1)
	}
	defer req.Body.Close()

	// more complete version of this functionality would make a backup first
	err = syscall.Unlink("main")
	if err != nil {
		fmt.Printf("error unlinking file: %v\n", err)
		os.Exit(1)
	}

	f, err := os.Create("main")
	if err != nil {
		fmt.Printf("error creating file: %v\n", err)
		os.Exit(1)
	}
	defer f.Close()

	_, err = f.Write(body)
	if err != nil {
		fmt.Printf("error writing file, badness: %v\n", err)
	}
}