diff --git a/rootfs.go b/rootfs.go
index 03f1efd..39bedc2 100644
--- a/rootfs.go
+++ b/rootfs.go
@@ -1,7 +1,13 @@
 package main
 
 import (
+	"archive/tar"
+	"encoding/json"
 	"errors"
+	"fmt"
+	"io"
+	"os"
+	"strings"
 
 	goflags "github.com/jessevdk/go-flags"
 )
@@ -13,9 +19,87 @@ type cmdRootFS struct {
 	} `positional-args:"yes" required:"yes"`
 }
 
+const (
+	_manifestJSON = "manifest.json"
+)
+
 func (r *cmdRootFS) Execute(args []string) error {
 	if len(args) != 0 {
 		return errors.New("too many args")
 	}
-	return nil
+
+	in, err := os.Open(string(r.PositionalArgs.Infile))
+	if err != nil {
+		return err
+	}
+	defer in.Close()
+
+	out, err := os.Create(string(r.PositionalArgs.Outfile))
+	if err != nil {
+		return err
+	}
+	defer out.Close()
+	return r.rootfs(in, out)
+}
+
+type dockerManifestJSON []struct {
+	Config string   `json:"Config"`
+	Layers []string `json:"Layers"`
+}
+
+// rootfs accepts a docker layer tarball and writes it to outfile.
+// 1. create map[string]io.ReadSeeker for each layer.
+// 2. parse manifest.json and get the layer order.
+// 3. go through each layer in order and write:
+//    a) to an ordered slice: the file name.
+//    b) to an FS map: where does the file come from?
+//       I) layer name
+//       II) offset (0 being the first file in the layer)
+// 4. go through
+func (r *cmdRootFS) rootfs(in io.ReadSeeker, out io.Writer) error {
+	tr := tar.NewReader(in)
+	layerOffsets := map[string]int64{}
+	var manifest dockerManifestJSON
+
+	// phase 1: get layer offsets and manifest.json
+	for {
+		hdrIn, err := tr.Next()
+		if err == io.EOF {
+			break
+		}
+
+		if hdrIn.Typeflag != tar.TypeReg {
+			continue
+		}
+
+		switch {
+		case hdrIn.Name == _manifestJSON:
+			dec := json.NewDecoder(tr)
+			if err := dec.Decode(&manifest); err != nil {
+				return fmt.Errorf("decode manifest.json: %w", err)
+			}
+		case strings.HasSuffix(hdrIn.Name, "/layer.tar"):
+			here, err := in.Seek(0, io.SeekCurrent)
+			if err != nil {
+				return fmt.Errorf("seek: %w", err)
+			}
+			layers[hdrIn.Name] = here
+		}
+	}
+
+	// phase 1.5: enumerate layers
+	layers := make([]int64, len(layers))
+	for i, name := range manifest.Layers {
+		layers[i] = layerOffsets[name]
+	}
+
+	// phase 2: iterate through all layers and extract filenames
+	// to files and ordered
+	files := map[string]struct{
+		layer string
+
+	fmt.Printf("layers: %+v\n", layers)
+
+	return nil
+	//return tw.Close()
 }