diff --git a/cmd/testscript/README.md b/cmd/testscript/README.md
index 5207068b..41cc3890 100644
--- a/cmd/testscript/README.md
+++ b/cmd/testscript/README.md
@@ -3,7 +3,7 @@ The testscript command runs github.com/rogpeppe/go-internal/testscript scripts
 in a fresh temporary work directory tree.
 
 Usage:
-    testscript [-v] files...
+    testscript [-v] [-e VAR]... [-u] files...
 
 The testscript command is designed to make it easy to create self-contained
 reproductions of command sequences.
@@ -20,6 +20,17 @@ proxy server. See the documentation for
 github.com/rogpeppe/go-internal/goproxytest for details on the format of these
 files/directories.
 
+Environment variables can be passed through to each script with the -e flag,
+where VAR is the name of the variable. Variables override testscript-defined
+values, with the exception of WORK which cannot be overridden. The -e flag can
+appear multiple times to specify multiple variables.
+
+The -u flag specifies that if a cmp command within a testscript fails and its
+second argument refers to a file inside the testscript file, the command will
+succeed and the testscript file will be updated to reflect the actual content.
+As such, this is the cmd/testcript equivalent of
+testscript.Params.UpdateScripts.
+
 Examples
 ========
 
diff --git a/cmd/testscript/help.go b/cmd/testscript/help.go
index 968e522b..2340668b 100644
--- a/cmd/testscript/help.go
+++ b/cmd/testscript/help.go
@@ -14,7 +14,7 @@ The testscript command runs github.com/rogpeppe/go-internal/testscript scripts
 in a fresh temporary work directory tree.
 
 Usage:
-    testscript [-v] [-e VAR]... files...
+    testscript [-v] [-e VAR]... [-u] files...
 
 The testscript command is designed to make it easy to create self-contained
 reproductions of command sequences.
@@ -36,6 +36,12 @@ where VAR is the name of the variable. Variables override testscript-defined
 values, with the exception of WORK which cannot be overridden. The -e flag can
 appear multiple times to specify multiple variables.
 
+The -u flag specifies that if a cmp command within a testscript fails and its
+second argument refers to a file inside the testscript file, the command will
+succeed and the testscript file will be updated to reflect the actual content.
+As such, this is the cmd/testcript equivalent of
+testscript.Params.UpdateScripts.
+
 Examples
 ========
 
diff --git a/cmd/testscript/main.go b/cmd/testscript/main.go
index 8c3872f4..7d73d484 100644
--- a/cmd/testscript/main.go
+++ b/cmd/testscript/main.go
@@ -62,6 +62,7 @@ func mainerr() (retErr error) {
 		mainUsage(os.Stderr)
 	}
 	var envVars envVarsFlag
+	fUpdate := fs.Bool("u", false, "update archive file if a cmp fails")
 	fWork := fs.Bool("work", false, "print temporary work directory and do not remove when done")
 	fVerbose := fs.Bool("v", false, "run tests verbosely")
 	fs.Var(&envVars, "e", "pass through environment variable to script (can appear multiple times)")
@@ -84,6 +85,20 @@ func mainerr() (retErr error) {
 		files = []string{"-"}
 	}
 
+	// If we are only reading from stdin, -u cannot be specified. It seems a bit
+	// bizarre to invoke testscript with '-' and a regular file, but hey. In
+	// that case the -u flag will only apply to the regular file and we assume
+	// the user knows it.
+	onlyReadFromStdin := true
+	for _, f := range files {
+		if f != "-" {
+			onlyReadFromStdin = false
+		}
+	}
+	if onlyReadFromStdin && *fUpdate {
+		return fmt.Errorf("cannot use -u when reading from stdin")
+	}
+
 	dirNames := make(map[string]int)
 	for _, filename := range files {
 		// TODO make running files concurrent by default? If we do, note we'll need to do
@@ -103,7 +118,7 @@ func mainerr() (retErr error) {
 		if err := os.Mkdir(runDir, 0777); err != nil {
 			return fmt.Errorf("failed to create a run directory within %v for %v: %v", td, renderFilename(filename), err)
 		}
-		if err := run(runDir, filename, *fVerbose, envVars.vals); err != nil {
+		if err := run(runDir, filename, *fUpdate, *fVerbose, envVars.vals); err != nil {
 			return err
 		}
 	}
@@ -162,7 +177,11 @@ func renderFilename(filename string) string {
 	return filename
 }
 
-func run(runDir, filename string, verbose bool, envVars []string) error {
+// run runs the testscript archive in filename within the temporary runDir.
+// verbose causes the output to be verbose (akin to go test -v) and update
+// sets the UpdateScripts parameter passed to testscript.Run such that any
+// updates to the archive get written back to filename
+func run(runDir, filename string, update bool, verbose bool, envVars []string) error {
 	var ar *txtar.Archive
 	var err error
 
@@ -204,12 +223,15 @@ func run(runDir, filename string, verbose bool, envVars []string) error {
 		return fmt.Errorf("failed to write .gomodproxy files: %v", err)
 	}
 
-	if err := ioutil.WriteFile(filepath.Join(runDir, "script.txt"), txtar.Format(&script), 0666); err != nil {
-		return fmt.Errorf("failed to write script for %v: %v", filename, err)
+	scriptFile := filepath.Join(runDir, "script.txt")
+
+	if err := ioutil.WriteFile(scriptFile, txtar.Format(&script), 0666); err != nil {
+		return fmt.Errorf("failed to write script for %v: %v", renderFilename(filename), err)
 	}
 
 	p := testscript.Params{
-		Dir: runDir,
+		Dir:           runDir,
+		UpdateScripts: update,
 	}
 
 	if _, err := exec.LookPath("go"); err == nil {
@@ -282,5 +304,28 @@ func run(runDir, filename string, verbose bool, envVars []string) error {
 		return fmt.Errorf("error running %v in %v\n", renderFilename(filename), runDir)
 	}
 
+	if update && filename != "-" {
+		// Parse the (potentially) updated scriptFile as an archive, then merge
+		// with the original archive, retaining order.  Then write the archive
+		// back to the source file
+		source, err := ioutil.ReadFile(scriptFile)
+		if err != nil {
+			return fmt.Errorf("failed to read from script file %v for -update: %v", scriptFile, err)
+		}
+		updatedAr := txtar.Parse(source)
+		updatedFiles := make(map[string]txtar.File)
+		for _, f := range updatedAr.Files {
+			updatedFiles[f.Name] = f
+		}
+		for i, f := range ar.Files {
+			if newF, ok := updatedFiles[f.Name]; ok {
+				ar.Files[i] = newF
+			}
+		}
+		if err := ioutil.WriteFile(filename, txtar.Format(ar), 0666); err != nil {
+			return fmt.Errorf("failed to write script back to %v for -update: %v", renderFilename(filename), err)
+		}
+	}
+
 	return nil
 }
diff --git a/cmd/testscript/testdata/update.txt b/cmd/testscript/testdata/update.txt
new file mode 100644
index 00000000..e1c15bd1
--- /dev/null
+++ b/cmd/testscript/testdata/update.txt
@@ -0,0 +1,64 @@
+# should support the -update flag
+
+unquote in.txt res.txt
+
+# Should be an error to use -u with only stdin
+stdin in.txt
+! testscript -u
+stderr 'cannot use -u when reading from stdin'
+
+# It is ok to use -u when reading from stdin and
+# a regular file
+testscript -u - in.txt
+cmp in.txt res.txt
+
+-- in.txt --
+>exec printf 'hello\n'
+>cmp stdout stdout.txt
+>
+>-- .gomodproxy/fruit.com_v1.0.0/.mod --
+>module fruit.com
+>
+>-- .gomodproxy/fruit.com_v1.0.0/.info --
+>{"Version":"v1.0.0","Time":"2018-10-22T18:45:39Z"}
+>
+>-- .gomodproxy/fruit.com_v1.0.0/go.mod --
+>module fruit.com
+>
+>-- stdout.txt --
+>goodbye
+>-- .gomodproxy/fruit.com_v1.0.0/fruit/fruit.go --
+>package fruit
+>
+>const Apple = "apple"
+>-- .gomodproxy/fruit.com_v1.0.0/coretest/coretest.go --
+>// package coretest becomes a candidate for the missing
+>// core import in main above
+>package coretest
+>
+>const Mandarin = "mandarin"
+-- res.txt --
+>exec printf 'hello\n'
+>cmp stdout stdout.txt
+>
+>-- .gomodproxy/fruit.com_v1.0.0/.mod --
+>module fruit.com
+>
+>-- .gomodproxy/fruit.com_v1.0.0/.info --
+>{"Version":"v1.0.0","Time":"2018-10-22T18:45:39Z"}
+>
+>-- .gomodproxy/fruit.com_v1.0.0/go.mod --
+>module fruit.com
+>
+>-- stdout.txt --
+>hello
+>-- .gomodproxy/fruit.com_v1.0.0/fruit/fruit.go --
+>package fruit
+>
+>const Apple = "apple"
+>-- .gomodproxy/fruit.com_v1.0.0/coretest/coretest.go --
+>// package coretest becomes a candidate for the missing
+>// core import in main above
+>package coretest
+>
+>const Mandarin = "mandarin"