diff --git a/cmd/txtar/txtar.go b/cmd/txtar/txtar.go
new file mode 100644
index 0000000..e4171dd
--- /dev/null
+++ b/cmd/txtar/txtar.go
@@ -0,0 +1,168 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// The txtar command writes or extracts a text-based file archive in the format
+// provided by the golang.org/x/tools/txtar package.
+//
+// The default behavior is to read a comment from stdin and write the archive
+// file containing the recursive contents of the named files and directories,
+// including hidden files, to stdout. Any non-flag arguments to the command name
+// the files and/or directories to include, with the contents of directories
+// included recursively. An empty argument list is equivalent to ".".
+//
+// The --extract (or -x) flag instructs txtar to instead read the archive file
+// from stdin and extract all of its files to corresponding locations relative
+// to the current, writing the archive's comment to stdout.
+//
+// Shell variables in paths are expanded (using os.Expand) if the corresponding
+// variable is set in the process environment. When writing an archive, the
+// variables (before expansion) are preserved in the archived paths.
+//
+// Example usage:
+//
+// 	txtar *.go <README >testdata/example.txt
+//
+// 	txtar --extract <playground_example.txt >main.go
+//
+package main
+
+import (
+	"bytes"
+	"flag"
+	"fmt"
+	"io/ioutil"
+	"os"
+	"path"
+	"path/filepath"
+	"regexp"
+	"strings"
+	"time"
+
+	"golang.org/x/tools/txtar"
+)
+
+var (
+	extractFlag = flag.Bool("extract", false, "if true, extract files from the archive instead of writing to it")
+)
+
+func init() {
+	flag.BoolVar(extractFlag, "x", *extractFlag, "short alias for --extract")
+}
+
+func main() {
+	flag.Parse()
+
+	var err error
+	if *extractFlag {
+		if len(flag.Args()) > 0 {
+			fmt.Fprintln(os.Stderr, "Usage: txtar --extract <archive.txt")
+			os.Exit(2)
+		}
+		err = extract()
+	} else {
+		paths := flag.Args()
+		if len(paths) == 0 {
+			paths = []string{"."}
+		}
+		err = archive(paths)
+	}
+
+	if err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		os.Exit(1)
+	}
+}
+
+func extract() (err error) {
+	b, err := ioutil.ReadAll(os.Stdin)
+	if err != nil {
+		return err
+	}
+
+	ar := txtar.Parse(b)
+	for _, f := range ar.Files {
+		fileName := filepath.FromSlash(path.Clean(expand(f.Name)))
+		if err := os.MkdirAll(filepath.Dir(fileName), 0777); err != nil {
+			return err
+		}
+		if err := ioutil.WriteFile(fileName, f.Data, 0666); err != nil {
+			return err
+		}
+	}
+
+	if len(ar.Comment) > 0 {
+		os.Stdout.Write(ar.Comment)
+	}
+	return nil
+}
+
+func archive(paths []string) (err error) {
+	txtarHeader := regexp.MustCompile(`(?m)^-- .* --$`)
+
+	ar := new(txtar.Archive)
+	for _, p := range paths {
+		root := filepath.Clean(expand(p))
+		prefix := root + string(filepath.Separator)
+		err := filepath.Walk(root, func(fileName string, info os.FileInfo, err error) error {
+			if err != nil || info.IsDir() {
+				return err
+			}
+
+			suffix := ""
+			if fileName != root {
+				suffix = strings.TrimPrefix(fileName, prefix)
+			}
+			name := filepath.ToSlash(filepath.Join(p, suffix))
+
+			data, err := ioutil.ReadFile(fileName)
+			if err != nil {
+				return err
+			}
+			if txtarHeader.Match(data) {
+				return fmt.Errorf("cannot archive %s: file contains a txtar header", name)
+			}
+
+			ar.Files = append(ar.Files, txtar.File{Name: name, Data: data})
+			return nil
+		})
+		if err != nil {
+			return err
+		}
+	}
+
+	// After we have read all of the source files, read the comment from stdin.
+	//
+	// Wait until the read has been blocked for a while before prompting the user
+	// to enter it: if they are piping the comment in from some other file, the
+	// read should complete very quickly and there is no need for a prompt.
+	// (200ms is typically long enough to read a reasonable comment from the local
+	// machine, but short enough that humans don't notice it.)
+	//
+	// Don't prompt until we have successfully read the other files:
+	// if we encountered an error, we don't need to ask for a comment.
+	timer := time.AfterFunc(200*time.Millisecond, func() {
+		fmt.Fprintln(os.Stderr, "Enter comment:")
+	})
+	comment, err := ioutil.ReadAll(os.Stdin)
+	timer.Stop()
+	if err != nil {
+		return fmt.Errorf("reading comment from %s: %v", os.Stdin.Name(), err)
+	}
+	ar.Comment = bytes.TrimSpace(comment)
+
+	_, err = os.Stdout.Write(txtar.Format(ar))
+	return err
+}
+
+// expand is like os.ExpandEnv, but preserves unescaped variables (instead
+// of escaping them to the empty string) if the variable is not set.
+func expand(p string) string {
+	return os.Expand(p, func(key string) string {
+		v, ok := os.LookupEnv(key)
+		if !ok {
+			return "$" + key
+		}
+		return v
+	})
+}
diff --git a/cmd/txtar/txtar_test.go b/cmd/txtar/txtar_test.go
new file mode 100644
index 0000000..4130336
--- /dev/null
+++ b/cmd/txtar/txtar_test.go
@@ -0,0 +1,120 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main_test
+
+import (
+	"fmt"
+	"io/ioutil"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"runtime"
+	"strings"
+	"sync"
+	"testing"
+)
+
+const comment = "This is a txtar archive.\n"
+
+const testdata = `This is a txtar archive.
+-- one.txt --
+one
+-- dir/two.txt --
+two
+-- $SPECIAL_LOCATION/three.txt --
+three
+`
+
+func TestMain(m *testing.M) {
+	code := m.Run()
+	txtarBin.once.Do(func() {})
+	if txtarBin.name != "" {
+		os.Remove(txtarBin.name)
+	}
+	os.Exit(code)
+}
+
+func TestRoundTrip(t *testing.T) {
+	os.Setenv("SPECIAL_LOCATION", "special")
+	defer os.Unsetenv("SPECIAL_LOCATION")
+
+	// Expand the testdata archive into a temporary directory.
+	parentDir, err := ioutil.TempDir("", "txtar")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(parentDir)
+	dir := filepath.Join(parentDir, "dir")
+	if err := os.Mkdir(dir, 0755); err != nil {
+		t.Fatal(err)
+	}
+	if out := txtar(t, dir, testdata, "--extract"); out != comment {
+		t.Fatalf("txtar --extract: stdout:\n%s\nwant:\n%s", out, comment)
+	}
+
+	// Now, re-archive its contents explicitly and ensure that the result matches
+	// the original.
+	args := []string{"one.txt", "dir", "$SPECIAL_LOCATION"}
+	if out := txtar(t, dir, comment, args...); out != testdata {
+		t.Fatalf("txtar %s: archive:\n%s\n\nwant:\n%s", strings.Join(args, " "), out, testdata)
+	}
+}
+
+// txtar runs the txtar command in the given directory with the given input and
+// arguments.
+func txtar(t *testing.T, dir, input string, args ...string) string {
+	t.Helper()
+	cmd := exec.Command(txtarName(t), args...)
+	cmd.Dir = dir
+	cmd.Stdin = strings.NewReader(input)
+	stderr := new(strings.Builder)
+	cmd.Stderr = stderr
+	out, err := cmd.Output()
+	if err != nil {
+		t.Fatalf("%s: %v\n%s", strings.Join(cmd.Args, " "), err, stderr)
+	}
+	if stderr.String() != "" {
+		t.Logf("OK: %s\n%s", strings.Join(cmd.Args, " "), stderr)
+	}
+	return string(out)
+}
+
+var txtarBin struct {
+	once sync.Once
+	name string
+	err  error
+}
+
+// txtarName returns the name of the txtar executable, building it if needed.
+func txtarName(t *testing.T) string {
+	t.Helper()
+	if _, err := exec.LookPath("go"); err != nil {
+		t.Skipf("cannot build txtar binary: %v", err)
+	}
+
+	txtarBin.once.Do(func() {
+		exe, err := ioutil.TempFile("", "txtar-*.exe")
+		if err != nil {
+			txtarBin.err = err
+			return
+		}
+		exe.Close()
+		txtarBin.name = exe.Name()
+
+		cmd := exec.Command("go", "build", "-o", txtarBin.name, ".")
+		out, err := cmd.CombinedOutput()
+		if err != nil {
+			txtarBin.err = fmt.Errorf("%s: %v\n%s", strings.Join(cmd.Args, " "), err, out)
+		}
+	})
+
+	if txtarBin.err != nil {
+		if runtime.GOOS == "android" {
+			t.Skipf("skipping test after failing to build txtar binary: go_android_exec may have failed to copy needed dependencies (see https://golang.org/issue/37088)")
+		}
+		t.Fatal(txtarBin.err)
+	}
+	return txtarBin.name
+}
diff --git a/go.mod b/go.mod
index 84df2a7..bc31db7 100644
--- a/go.mod
+++ b/go.mod
@@ -8,7 +8,7 @@
 	github.com/go-gl/glfw/v3.3/glfw v0.0.0-20191125211704-12ad95a8df72
 	golang.org/x/image v0.0.0-20190802002840-cff245a6509b
 	golang.org/x/mobile v0.0.0-20190719004257-d2bd2a29d028
-	golang.org/x/mod v0.1.0
+	golang.org/x/mod v0.1.1-0.20191105210325-c90efee705ee
 	golang.org/x/sys v0.0.0-20190412213103-97732733099d
-	golang.org/x/tools v0.0.0-20191012152004-8de300cfc20a
+	golang.org/x/tools v0.0.0-20200207183749-b753a1ba74fa
 )
diff --git a/go.sum b/go.sum
index 67aebad..9a50bee 100644
--- a/go.sum
+++ b/go.sum
@@ -5,7 +5,7 @@
 github.com/go-gl/glfw/v3.3/glfw v0.0.0-20191125211704-12ad95a8df72 h1:b+9H1GAsx5RsjvDFLoS5zkNBzIQMuVKUYQDmxU3N5XE=
 github.com/go-gl/glfw/v3.3/glfw v0.0.0-20191125211704-12ad95a8df72/go.mod h1:tQ2UAYgL5IevRw8kRxooKSPJfGvJ9fJQFa0TUsXzTg8=
 golang.org/x/crypto v0.0.0-20190308221718-c2843e01d9a2/go.mod h1:djNgcEr1/C05ACkg1iLfiJU5Ep61QUkGW8qpdssI0+w=
-golang.org/x/crypto v0.0.0-20190510104115-cbcb75029529/go.mod h1:yigFU9vqHzYiE8UmvKecakEJjdnWj3jj499lnFckfCI=
+golang.org/x/crypto v0.0.0-20191011191535-87dc89f01550/go.mod h1:yigFU9vqHzYiE8UmvKecakEJjdnWj3jj499lnFckfCI=
 golang.org/x/exp v0.0.0-20190306152737-a1d7652674e8/go.mod h1:CJ0aWSM057203Lf6IL+f9T1iT9GByDxfZKAQTCR3kQA=
 golang.org/x/image v0.0.0-20190227222117-0694c2d4d067 h1:KYGJGHOQy8oSi1fDlSpcZF0+juKwk/hEMv5SiwHogR0=
 golang.org/x/image v0.0.0-20190227222117-0694c2d4d067/go.mod h1:kZ7UVZpmo3dzQBMxlp+ypCbDeSB+sBbTgSJuh5dn5js=
@@ -13,8 +13,8 @@
 golang.org/x/image v0.0.0-20190802002840-cff245a6509b/go.mod h1:FeLwcggjj3mMvU+oOTbSwawSJRM1uh48EjtB4UJZlP0=
 golang.org/x/mobile v0.0.0-20190719004257-d2bd2a29d028 h1:4+4C/Iv2U4fMZBiMCc98MG1In4gJY5YRhtpDNeDeHWs=
 golang.org/x/mobile v0.0.0-20190719004257-d2bd2a29d028/go.mod h1:E/iHnbuqvinMTCcRqshq8CkpyQDoeVncDDYHnLhea+o=
-golang.org/x/mod v0.1.0 h1:sfUMP1Gu8qASkorDVjnMuvgJzwFbTZSeXFiGBYAVdl4=
-golang.org/x/mod v0.1.0/go.mod h1:0QHyrYULN0/3qlju5TqG8bIK38QM8yzMo5ekMj3DlcY=
+golang.org/x/mod v0.1.1-0.20191105210325-c90efee705ee h1:WG0RUwxtNT4qqaXX3DPA8zHFNm/D9xaBpxzHt1WcA/E=
+golang.org/x/mod v0.1.1-0.20191105210325-c90efee705ee/go.mod h1:QqPTAvyqsEbceGzBzNggFXnrqF1CaUcvgkdR5Ot7KZg=
 golang.org/x/net v0.0.0-20190404232315-eb5bcb51f2a3/go.mod h1:t9HGtf8HONx5eT2rtn7q6eTqICYqUVnKs3thJo3Qplg=
 golang.org/x/net v0.0.0-20190620200207-3b0461eec859/go.mod h1:z5CRVTTTmAJ677TzLLGU+0bjPO0LkuOLi4/5GtJWs/s=
 golang.org/x/sync v0.0.0-20190423024810-112230192c58/go.mod h1:RxMgew5VJxzue5/jJTE5uejpjVlOe/izrB70Jof72aM=
@@ -23,6 +23,7 @@
 golang.org/x/sys v0.0.0-20190412213103-97732733099d h1:+R4KGOnez64A81RvjARKc4UT5/tI9ujCIVX+P5KiHuI=
 golang.org/x/sys v0.0.0-20190412213103-97732733099d/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
 golang.org/x/text v0.3.0/go.mod h1:NqM8EUOU14njkJ3fqMW+pc6Ldnwhi/IjpwHt7yyuwOQ=
-golang.org/x/tools v0.0.0-20191012152004-8de300cfc20a h1:TwMENskLwU2NnWBzrJGEWHqSiGUkO/B4rfyhwqDxDYQ=
-golang.org/x/tools v0.0.0-20191012152004-8de300cfc20a/go.mod h1:b+2E5dAYhXwXZwtnZ6UAqBI28+e2cm9otk0dWdXHAEo=
-golang.org/x/xerrors v0.0.0-20190717185122-a985d3407aa7/go.mod h1:I/5z698sn9Ka8TeJc9MKroUUfqBBauWjQqLJ2OPfmY0=
+golang.org/x/tools v0.0.0-20200207183749-b753a1ba74fa h1:5E4dL8+NgFOgjwbTKz+OOEGGhP+ectTmF842l6KjupQ=
+golang.org/x/tools v0.0.0-20200207183749-b753a1ba74fa/go.mod h1:TB2adYChydJhpapKDTa4BR/hXlZSLoq2Wpct/0txZ28=
+golang.org/x/xerrors v0.0.0-20191011141410-1b5146add898 h1:/atklqdjdhuosWIl6AIbOeHJjicWYPqR9bpxqxYG2pA=
+golang.org/x/xerrors v0.0.0-20191011141410-1b5146add898/go.mod h1:I/5z698sn9Ka8TeJc9MKroUUfqBBauWjQqLJ2OPfmY0=