How can I use the "compress/gzip" package to gzip a file?

Question

I'm new to Go, and can't figure out how to use the compress/gzip package to my advantage. Basically, I just want to write something to a file, gzip it and read it directly from the zipped format through another script. I would really appreciate if someone could give me an example on how to do this.

Answer 1

All the compress packages implement the same interface. You would use something like this to compress:

var b bytes.Buffer
w := gzip.NewWriter(&b)
w.Write([]byte("hello, world\n"))
w.Close()

And this to unpack:

r, err := gzip.NewReader(&b)
io.Copy(os.Stdout, r)
r.Close()

Answer 2

Pretty much the same answer as Laurent, but with the file io:

import (
  "bytes"
  "compress/gzip"
  "io/ioutil"
)
// ...
var b bytes.Buffer
w := gzip.NewWriter(&b)
w.Write([]byte("hello, world\n"))
w.Close() // You must close this first to flush the bytes to the buffer.
err := ioutil.WriteFile("hello_world.txt.gz", b.Bytes(), 0666)

Answer 3

For the Read part, something like the useful ioutil.ReadFile for .gz files could be :

func ReadGzFile(filename string) ([]byte, error) {
    fi, err := os.Open(filename)
    if err != nil {
        return nil, err
    }
    defer fi.Close()

    fz, err := gzip.NewReader(fi)
    if err != nil {
        return nil, err
    }
    defer fz.Close()

    s, err := ioutil.ReadAll(fz)
    if err != nil {
        return nil, err
    }
    return s, nil   
}

Answer 4

Here the func for unpack gzip file to destination file:

func UnpackGzipFile(gzFilePath, dstFilePath string) (int64, error) {
    gzFile, err := os.Open(gzFilePath)
    if err != nil {
        return 0, fmt.Errorf("open file %q to unpack: %w", gzFilePath, err)
    }
    dstFile, err := os.OpenFile(dstFilePath, os.O_CREATE|os.O_WRONLY|os.O_APPEND, 0660)
    if err != nil {
        return 0, fmt.Errorf("create destination file %q to unpack: %w", dstFilePath, err)
    }
    defer dstFile.Close()

    ioReader, ioWriter := io.Pipe()
    defer ioReader.Close()

    go func() { // goroutine leak is possible here
        gzReader, _ := gzip.NewReader(gzFile)
        // it is important to close the writer or reading from the other end of the
        // pipe or io.copy() will never finish
        defer func(){
            gzFile.Close()
            gzReader.Close()
            ioWriter.Close()
        }()

        io.Copy(ioWriter, gzReader)
    }()

    written, err := io.Copy(dstFile, ioReader)
    if err != nil {
        return 0, err // goroutine leak is possible here
    }

    return written, nil
}

Answer 5

I decided to combine ideas from others answers and just provide a full example program. Obviously there are many different ways to do the same thing. This is just one way:

package main

import (
    "compress/gzip"
    "fmt"
    "io/ioutil"
    "os"
)

var zipFile = "zipfile.gz"

func main() {
    writeZip()
    readZip()
}

func writeZip() {
    handle, err := openFile(zipFile)
    if err != nil {
        fmt.Println("[ERROR] Opening file:", err)
    }

    zipWriter, err := gzip.NewWriterLevel(handle, 9)
    if err != nil {
        fmt.Println("[ERROR] New gzip writer:", err)
    }
    numberOfBytesWritten, err := zipWriter.Write([]byte("Hello, World!\n"))
    if err != nil {
        fmt.Println("[ERROR] Writing:", err)
    }
    err = zipWriter.Close()
    if err != nil {
        fmt.Println("[ERROR] Closing zip writer:", err)
    }
    fmt.Println("[INFO] Number of bytes written:", numberOfBytesWritten)

    closeFile(handle)
}

func readZip() {
    handle, err := openFile(zipFile)
    if err != nil {
        fmt.Println("[ERROR] Opening file:", err)
    }

    zipReader, err := gzip.NewReader(handle)
    if err != nil {
        fmt.Println("[ERROR] New gzip reader:", err)
    }
    defer zipReader.Close()

    fileContents, err := ioutil.ReadAll(zipReader)
    if err != nil {
        fmt.Println("[ERROR] ReadAll:", err)
    }

    fmt.Printf("[INFO] Uncompressed contents: %s\n", fileContents)

    // ** Another way of reading the file **
    //
    // fileInfo, _ := handle.Stat()
    // fileContents := make([]byte, fileInfo.Size())
    // bytesRead, err := zipReader.Read(fileContents)
    // if err != nil {
    //     fmt.Println("[ERROR] Reading gzip file:", err)
    // }
    // fmt.Println("[INFO] Number of bytes read from the file:", bytesRead)

    closeFile(handle)
}

func openFile(fileToOpen string) (*os.File, error) {
    return os.OpenFile(fileToOpen, openFileOptions, openFilePermissions)
}

func closeFile(handle *os.File) {
    if handle == nil {
        return
    }

    err := handle.Close()
    if err != nil {
        fmt.Println("[ERROR] Closing file:", err)
    }
}

const openFileOptions int = os.O_CREATE | os.O_RDWR
const openFilePermissions os.FileMode = 0660

Having a full example like this should be helpful for future reference.

Answer 6

To compress any Go object of interface type as input

func compress(obj interface{}) ([]byte, error) {
    var b bytes.Buffer
    objBytes, err := json.Marshal(obj)
    if err != nil {
        return nil, err
    }
    gz := gzip.NewWriter(&b)
    defer gz.Close() //NOT SUFFICIENT, DON'T DEFER WRITER OBJECTS
    if _, err := gz.Write(objBytes); err != nil {
        return nil, err
    }
    // NEED TO CLOSE EXPLICITLY
    if err := gz.Close(); err != nil {
        return nil, err
    }
    return b.Bytes(), nil
}

To decompress the same,

func decompress(obj []byte) ([]byte, error) {
    r, err := gzip.NewReader(bytes.NewReader(obj))
    if err != nil {
        return nil, err
    }
    defer r.Close()
    res, err := ioutil.ReadAll(r)
    if err != nil {
        return nil, err
    }
    return res, nil
}

Note, ioutil.ReadAll(r) returns io.EOF or io.ErrUnexpectedEOF if you do not close the Writer object after writing. I assumed defer on Close() would close the object properly, but it won't. Don't defer writer objects.