golang append file to zip

To append a file to an existing ZIP archive in Golang, you can use the archive/zip package. Here is an example code snippet:

package main

import (
	"archive/zip"
	"fmt"
	"io"
	"os"
)

func main() {
	// Open the existing ZIP file for appending
	zipfile, err := os.OpenFile("existing.zip", os.O_WRONLY|os.O_APPEND, 0644)
	if err != nil {
		fmt.Println(err)
		return
	}
	defer zipfile.Close()

	// Open the new file that you want to add to the ZIP archive
	newfile, err := os.Open("newfile.txt")
	if err != nil {
		fmt.Println(err)
		return
	}
	defer newfile.Close()

	// Create a new ZIP writer
	zipwriter := zip.NewWriter(zipfile)
	defer zipwriter.Close()

	// Create a new file header for the new file
	header := &zip.FileHeader{
		Name:   "newfile.txt",
		Method: zip.Deflate,
	}

	// Add the new file header to the ZIP writer
	writer, err := zipwriter.CreateHeader(header)
	if err != nil {
		fmt.Println(err)
		return
	}

	// Copy the contents of the new file to the ZIP writer
	_, err = io.Copy(writer, newfile)
	if err != nil {
		fmt.Println(err)
		return
	}

	fmt.Println("New file added to ZIP archive")
}

This code opens the existing ZIP file for appending, opens the new file that you want to add to the ZIP archive, creates a new ZIP writer, creates a new file header for the new file, adds the new file header to the ZIP writer, and copies the contents of the new file to the ZIP writer. Finally, it prints a message indicating that the new file has been added to the ZIP archive.