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

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.