make reverse error if no changes were made

Just the exit status code is used. Printing an error would be confusing,
since we're printing the input too. Plus, the error is binary anyway -
either we changed something, or we did not.

One could make a case for only printing if we changed anything, and
showing an error otherwise. But that would mean buffering the entire
input, which could be problematic or slow when e.g. reversing large log
files.

reverse.go

@@ -104,25 +104,37 @@ func commandReverse(args []string) error {
 	}
 	repl := strings.NewReplacer(replaces...)
 
-	// TODO: return a non-zero status code if we could not reverse any string.
 	if len(args) == 0 {
-		return reverseContent(os.Stdout, os.Stdin, repl)
+		modified, err := reverseContent(os.Stdout, os.Stdin, repl)
+		if err != nil {
+			return err
+		}
+		if !modified {
+			return errJustExit
+		}
+		return nil
 	}
+	anyModified := false
 	for _, path := range args {
 		f, err := os.Open(path)
 		if err != nil {
 			return err
 		}
 		defer f.Close()
-		if err := reverseContent(os.Stdout, f, repl); err != nil {
+		any, err := reverseContent(os.Stdout, f, repl)
+		if err != nil {
 			return err
 		}
+		anyModified = anyModified || any
 		f.Close() // since we're in a loop
 	}
+	if !anyModified {
+		return errJustExit
+	}
 	return nil
 }
 
-func reverseContent(w io.Writer, r io.Reader, repl *strings.Replacer) error {
+func reverseContent(w io.Writer, r io.Reader, repl *strings.Replacer) (bool, error) {
 	// Read line by line.
 	// Reading the entire content at once wouldn't be interactive,
 	// nor would it support large files well.
@@ -130,19 +142,25 @@ func reverseContent(w io.Writer, r io.Reader, repl *strings.Replacer) error {
 	// We use bufio.Reader instead of bufio.Scanner,
 	// to also obtain the newline characters themselves.
 	br := bufio.NewReader(r)
+	modified := false
 	for {
 		// Note that ReadString can return a line as well as an error if
 		// we hit EOF without a newline.
 		// In that case, we still want to process the string.
 		line, readErr := br.ReadString('\n')
-		if _, err := repl.WriteString(w, line); err != nil {
-			return err
+
+		newLine := repl.Replace(line)
+		if newLine != line {
+			modified = true
+		}
+		if _, err := io.WriteString(w, newLine); err != nil {
+			return modified, err
 		}
 		if readErr == io.EOF {
-			return nil
+			return modified, nil
 		}
 		if readErr != nil {
-			return readErr
+			return modified, readErr
 		}
 	}
 }

testdata/scripts/reverse.txt

@@ -34,7 +34,7 @@ cmp stdout reverse.stdout
 
 # Reversing a -literals output without the flag should fail.
 stdin main-literals.stderr
-garble reverse
+! garble reverse
 cmp stdout main-literals.stderr
 
 -- go.mod --