diff --git a/dir.go b/dir.go
index 31b478a..423f2f6 100644
--- a/dir.go
+++ b/dir.go
@@ -116,7 +116,9 @@ func (d *Dir) _read(srcDir, path string, task *Task, cfg *Site) error {
 
 			if cmd, ok := task.Preprocess[strings.TrimPrefix(ext, ".")]; ok {
 				var buf bytes.Buffer
-				execute(cmd, bytes.NewReader(content), &buf)
+				if err := execute(cmd, bytes.NewReader(content), &buf); err != nil {
+					return err
+				}
 				content = buf.Bytes()
 			}
 			page.Content = string(content)
@@ -266,7 +268,9 @@ func (d *Dir) write(dstDir string, task *Task) error {
 		var content []byte
 		if cmd := task.Postprocess; cmd != "" {
 			var buf bytes.Buffer
-			execute(cmd, strings.NewReader(page.Content), &buf)
+			if err := execute(cmd, strings.NewReader(page.Content), &buf); err != nil {
+				return err
+			}
 			content = buf.Bytes()
 		} else {
 			content = []byte(page.Content)
@@ -329,7 +333,7 @@ func (d *Dir) sort() {
 }
 
 // execute runs a command.
-func execute(command string, input io.Reader, output io.Writer) {
+func execute(command string, input io.Reader, output io.Writer) error {
 	split := strings.Split(command, " ")
 	cmd := exec.Command(split[0], split[1:]...)
 	cmd.Stdin = input
@@ -337,8 +341,9 @@ func execute(command string, input io.Reader, output io.Writer) {
 	cmd.Stdout = output
 	err := cmd.Run()
 	if err != nil {
-		log.Fatal(err)
+		return err
 	}
+	return nil
 }
 
 func (d *Dir) Title() string {
diff --git a/funcs.go b/funcs.go
index 7e40dd9..989f068 100644
--- a/funcs.go
+++ b/funcs.go
@@ -55,10 +55,12 @@ func (_strings) TrimRight(a, b string) string         { return strings.TrimRight
 func (_strings) TrimSpace(s string) string            { return strings.TrimSpace(s) }
 func (_strings) TrimSuffix(a, b string) string        { return strings.TrimSuffix(a, b) }
 
-func executeString(command, input string) string {
+func executeString(command, input string) (string, error) {
 	var b strings.Builder
-	execute(command, strings.NewReader(input), &b)
-	return b.String()
+	if err := execute(command, strings.NewReader(input), &b); err != nil {
+		return "", err
+	}
+	return b.String(), nil
 }
 
 func reverse(s interface{}) interface{} {