Extracting files from .7z archives in Go

For a project written in Go I needed to access files inside .7z archive.

Unfortunately at this time there is no pure Go implementation for that.

An easy way to achieve this is to sub-launch 7z executable to extract the file to disk and then proceed as usual.

I spent a little more effort and created a way to get an io.ReadCloser stream for a file inside .7z archive.

It still uses 7z executable for the functionality but it doesn’t create temporary file. Instead it streams decompressed data so it’s more efficient.

I’ve packaged this as lzmadec library and this article describes how to use it.

Pre-requisites:

install the library: go get -u github.com/kjk/lzmadec
install 7z binary:
- on mac: brew install p7zip
- on ubuntu: apt-get install p7zip
- on windows: http://www.7-zip.org/

The library offers essentially 2 functions:

get a list of all files and their metadata inside .7z archive
get io.ReadCloser for a given file
and as a convenience a function to extract a file to disk

Here’s a skeleton of the code, error handling skipped for clarity:

var archive *lzmadec.Archive
archive, _ := lzmadec.NewArchive("foo.7z")

// list all files inside archive
for _, e := range archive.Entries {
    fmt.Printf("name: %s, size: %d\n", e.Path, e.Size)
}
firstFile := archive.Entries[0].Path

// extract to a file
archive.ExtractToFile(firstFile + ".extracted", firstFile)

// decompress to in-memory buffer
r, _ := archive.GetFileReader(firstFile)
var buf bytes.Buffer
_, _ = io.Copy(&buf, r)
// if not fully read, calling Close() ensures that sub-launched 7z executable
// is terminated
r.Close()
fmt.Printf("size of file %s after decompression: %d\n", firstFile, len(buf.Bytes()))

To see a more complete example: https://github.com/kjk/lzmadec/blob/master/cmd/test/main.go

If you’re a Go programmer, you can use it as an example of using os.Cmd to capture stdout of a program for progressive reading. 7z has -so options which tells it to send decompressed data to stdout. This is how to capture that as an io.Reader: https://github.com/kjk/lzmadec/blob/master/lzmadec.go#L225

Hopefully some day there will be pure Go library for accessing .7z files and this hack will obsolete. https://github.com/uli-go/xz is quite advanced but not there yet.