Golang: Working with ZIP archives

Golang has several packages that work with different type of archives. In this post I will show you how to use archive/zip package to compress and uncompress zip archives.

Zip is one of the most common file formats. It supports lossless data compression of one ore more files and directories.

Extracting

You can read the content of zip package by using zip reader. Its File property exposes all files and directories of particular zip package. If you want to extract the package, you should recreate all directories and files:

func unzip(archive, target string) error {
	reader, err := zip.OpenReader(archive)
	if err != nil {
		return err
	}

	if err := os.MkdirAll(target, 0755); err != nil {
		return err
	}

	for _, file := range reader.File {
		path := filepath.Join(target, file.Name)
		if file.FileInfo().IsDir() {
			os.MkdirAll(path, file.Mode())
			continue
		}

		fileReader, err := file.Open()
		if err != nil {
			return err
		}
		defer fileReader.Close()

		targetFile, err := os.OpenFile(path, os.O_WRONLY|os.O_CREATE|os.O_TRUNC, file.Mode())
		if err != nil {
			return err
		}
		defer targetFile.Close()

		if _, err := io.Copy(targetFile, fileReader); err != nil {
			return err
		}
	}

	return nil
}

Compressing

This operation is more complicated than extracing. You can compress a single file or an hierarchy of directories. Handling both cases requires to change the file header name depending on its type. If the copied content is directory, the header name should be changed to <directory_name>/. For a regular files, its header name is relative path <directory_name>/<file_name>.

The following function illustrates the algorithm:

func zipit(source, target string) error {
	zipfile, err := os.Create(target)
	if err != nil {
		return err
	}
	defer zipfile.Close()

	archive := zip.NewWriter(zipfile)
	defer archive.Close()

	info, err := os.Stat(source)
	if err != nil {
		return nil
	}

	var baseDir string
	if info.IsDir() {
		baseDir = filepath.Base(source)
	}

	filepath.Walk(source, func(path string, info os.FileInfo, err error) error {
		if err != nil {
			return err
		}

		header, err := zip.FileInfoHeader(info)
		if err != nil {
			return err
		}

		if baseDir != "" {
			header.Name = filepath.Join(baseDir, strings.TrimPrefix(path, source))
		}

		if info.IsDir() {
			header.Name += "/"
		} else {
			header.Method = zip.Deflate
		}

		writer, err := archive.CreateHeader(header)
		if err != nil {
			return err
		}

		if info.IsDir() {
			return nil
		}

		file, err := os.Open(path)
		if err != nil {
			return err
		}
		defer file.Close()
		_, err = io.Copy(writer, file)
		return err
	})

	return err
}

Usage

Lets see both functions in action:

zipit("/tmp/documents", "/tmp/backup.zip")
zipit("/tmp/report.txt", "/tmp/report-2015.zip")
unzip("/tmp/report-2015.zip", "/tmp/reports/")

You can download the samples from here.