diff --git a/cmd/block.go b/cmd/block.go
deleted file mode 100644
index 98f3612..0000000
--- a/cmd/block.go
+++ /dev/null
@@ -1,29 +0,0 @@
-package cmd
-
-import (
-	"fmt"
-	"io"
-	"os"
-
-	"github.com/spf13/cobra"
-)
-
-var blockOutput string
-
-var blockCmd = &cobra.Command{
-	Use:   "block",
-	Short: "Process a block",
-	Run: func(cmd *cobra.Command, args []string) {
-		data, err := io.ReadAll(os.Stdin)
-		if err != nil {
-			fmt.Fprintln(os.Stderr, err)
-			os.Exit(1)
-		}
-		fmt.Print(string(data))
-	},
-}
-
-func init() {
-	blockCmd.Flags().StringVarP(&blockOutput, "output", "o", "", "output format")
-	rootCmd.AddCommand(blockCmd)
-}
diff --git a/cmd/block_test.go b/cmd/block_test.go
deleted file mode 100644
index 001509b..0000000
--- a/cmd/block_test.go
+++ /dev/null
@@ -1,7 +0,0 @@
-package cmd_test
-
-import "testing"
-
-func TestBlockCommand(t *testing.T) {
-	runScript(t, "testdata/script/block.txtar")
-}
diff --git a/cmd/file.go b/cmd/file.go
index d0c78d4..e146666 100644
--- a/cmd/file.go
+++ b/cmd/file.go
@@ -3,26 +3,62 @@ package cmd
 import (
 	"fmt"
 	"os"
+	"path/filepath"
 
 	"litdoc/internal"
 
 	"github.com/spf13/cobra"
 )
 
+var fileWrite bool
+
 var fileCmd = &cobra.Command{
 	Use:   "file <path>",
 	Short: "Process a file",
 	Args:  cobra.ExactArgs(1),
 	Run: func(cmd *cobra.Command, args []string) {
-		data, err := internal.ProcessFile(args[0])
+		path := args[0]
+		data, err := internal.ProcessFile(path)
 		if err != nil {
 			fmt.Fprintln(os.Stderr, err)
 			os.Exit(1)
 		}
+		if fileWrite {
+			if err := writeFileAtomic(path, data); err != nil {
+				fmt.Fprintln(os.Stderr, err)
+				os.Exit(1)
+			}
+			return
+		}
 		fmt.Print(data)
 	},
 }
 
+func writeFileAtomic(path, data string) error {
+	info, err := os.Stat(path)
+	if err != nil {
+		return err
+	}
+	tmp, err := os.CreateTemp(filepath.Dir(path), ".litdoc-*")
+	if err != nil {
+		return err
+	}
+	tmpName := tmp.Name()
+	defer os.Remove(tmpName)
+	if _, err := tmp.WriteString(data); err != nil {
+		tmp.Close()
+		return err
+	}
+	if err := tmp.Close(); err != nil {
+		return err
+	}
+	if err := os.Chmod(tmpName, info.Mode().Perm()); err != nil {
+		return err
+	}
+	return os.Rename(tmpName, path)
+}
+
 func init() {
+	fileCmd.Flags().BoolVarP(&fileWrite, "write", "w", false, "rewrite the file in place")
 	rootCmd.AddCommand(fileCmd)
 }
diff --git a/cmd/file_test.go b/cmd/file_test.go
index 62200d7..4f17d2d 100644
--- a/cmd/file_test.go
+++ b/cmd/file_test.go
@@ -1,34 +1,19 @@
 package cmd_test
 
-import (
-	"fmt"
-	"os"
-	"testing"
-
-	"litdoc/cmd"
-
-	"github.com/rogpeppe/go-internal/testscript"
-)
-
-func TestMain(m *testing.M) {
-	testscript.Main(m, map[string]func(){
-		"litdoc": func() {
-			if err := cmd.Execute(); err != nil {
-				fmt.Fprintln(os.Stderr, err)
-				os.Exit(1)
-			}
-		},
-	})
-}
+import "testing"
 
 func TestFileCommand(t *testing.T) {
 	runScript(t, "testdata/script/file.txtar")
 }
 
-func runScript(t *testing.T, script string) {
-	t.Helper()
+func TestFileWriteFlag(t *testing.T) {
+	runScript(t, "testdata/script/file_write.txtar")
+}
+
+func TestFileCommandNoArgs(t *testing.T) {
+	runScript(t, "testdata/script/file_no_args.txtar")
+}
 
-	testscript.Run(t, testscript.Params{
-		Files: []string{script},
-	})
+func TestFileCommandMissingPath(t *testing.T) {
+	runScript(t, "testdata/script/file_missing_path.txtar")
 }
diff --git a/cmd/testdata/script/block.txtar b/cmd/testdata/script/block.txtar
deleted file mode 100644
index 0e01080..0000000
--- a/cmd/testdata/script/block.txtar
+++ /dev/null
@@ -1,6 +0,0 @@
-stdin input.md
-exec litdoc block -o text
-stdout 'code'
-
--- input.md --
-code
diff --git a/cmd/testdata/script/file_missing_path.txtar b/cmd/testdata/script/file_missing_path.txtar
new file mode 100644
index 0000000..48fda56
--- /dev/null
+++ b/cmd/testdata/script/file_missing_path.txtar
@@ -0,0 +1,2 @@
+! exec litdoc file does_not_exist.md
+stderr 'reading source file'
\ No newline at end of file
diff --git a/cmd/testdata/script/file_no_args.txtar b/cmd/testdata/script/file_no_args.txtar
new file mode 100644
index 0000000..dbebcd8
--- /dev/null
+++ b/cmd/testdata/script/file_no_args.txtar
@@ -0,0 +1,2 @@
+! exec litdoc file
+stderr 'accepts 1 arg'
\ No newline at end of file
diff --git a/cmd/testdata/script/file_write.txtar b/cmd/testdata/script/file_write.txtar
new file mode 100644
index 0000000..615b956
--- /dev/null
+++ b/cmd/testdata/script/file_write.txtar
@@ -0,0 +1,6 @@
+exec litdoc file -w input.md
+! stdout .
+grep '# Hello' input.md
+
+-- input.md --
+# Hello
diff --git a/cmd/testscript_test.go b/cmd/testscript_test.go
new file mode 100644
index 0000000..32680b4
--- /dev/null
+++ b/cmd/testscript_test.go
@@ -0,0 +1,30 @@
+package cmd_test
+
+import (
+	"fmt"
+	"os"
+	"testing"
+
+	"litdoc/cmd"
+
+	"github.com/rogpeppe/go-internal/testscript"
+)
+
+func TestMain(m *testing.M) {
+	testscript.Main(m, map[string]func(){
+		"litdoc": func() {
+			if err := cmd.Execute(); err != nil {
+				fmt.Fprintln(os.Stderr, err)
+				os.Exit(1)
+			}
+		},
+	})
+}
+
+func runScript(t *testing.T, script string) {
+	t.Helper()
+
+	testscript.Run(t, testscript.Params{
+		Files: []string{script},
+	})
+}