tested in real word and made appropriate corrections to make it work.

Author: Oppodelldog

hook.go

@@ -35,7 +35,7 @@ func ModifyGitCommitMessage(gitCommitMessage string) (modifiedCommitMessage stri
 
 	if !featureBranchDetectFunc(branchName) {
 		if !featureBranchDetectFunc(gitCommitMessage) {
-			err = errors.New("feature reference is required")
+			err = errors.Errorf("feature reference is required in '%s'", gitCommitMessage)
 			return
 		}
 	}

rewrite.go

@@ -0,0 +1,43 @@
+package gitcommithook
+
+import (
+	"io/ioutil"
+
+	"os"
+
+	"github.com/pkg/errors"
+)
+
+type (
+	readFileFuncDef  func(filename string) ([]byte, error)
+	modifyFuncDef    func(string) (string, error)
+	writeFileFuncDef func(string, []byte, os.FileMode) error
+)
+
+var (
+	readFileFunc  = readFileFuncDef(ioutil.ReadFile)
+	modifyFunc    = modifyFuncDef(ModifyGitCommitMessage)
+	writeFileFunc = writeFileFuncDef(ioutil.WriteFile)
+)
+
+//RewriteCommitMessage rewrites the commit message in the given commit message file
+func RewriteCommitMessage(commitMessageFile string) error {
+
+	var outputMessage string
+	fileContent, err := readFileFunc(commitMessageFile)
+	if err != nil {
+		return errors.Errorf("error reading commit message from '%s': %v", commitMessageFile, err.Error())
+	}
+
+	outputMessage, err = modifyFunc(string(fileContent))
+	if err != nil {
+		return errors.Errorf("error modifying commit message: %s", err.Error())
+	}
+
+	err = writeFileFunc(commitMessageFile, []byte(outputMessage), 0777)
+	if err != nil {
+		return errors.Errorf("error writing commit message to '%s': %s", commitMessageFile, err.Error())
+	}
+
+	return nil
+}

rewrite_test.go

@@ -0,0 +1,121 @@
+package gitcommithook
+
+import (
+	"os"
+	"testing"
+
+	"github.com/pkg/errors"
+	"github.com/stretchr/testify/assert"
+)
+
+const commitMessageFile = ".git/COMMIT_MSG_EDIT"
+const commitMessage = "commitMessage"
+const modifiedCommitMessage = "modified commitMessage"
+
+var rewriteOriginals = struct {
+	readFileFunc  readFileFuncDef
+	modifyFunc    modifyFuncDef
+	writeFileFunc writeFileFuncDef
+}{
+	readFileFunc:  readFileFunc,
+	modifyFunc:    modifyFunc,
+	writeFileFunc: writeFileFunc,
+}
+
+func restoreRewriteOriginals() {
+	readFileFunc = rewriteOriginals.readFileFunc
+	modifyFunc = rewriteOriginals.modifyFunc
+	writeFileFunc = rewriteOriginals.writeFileFunc
+}
+
+func TestRewriteCommitMessage_ErrorCase_CannotFindCommitMessageFile(t *testing.T) {
+	defer restoreRewriteOriginals()
+
+	commitMessageFileCannotBeFound(t)
+
+	err := RewriteCommitMessage(commitMessageFile)
+
+	assert.Contains(t, err.Error(), "error reading commit message")
+}
+
+func TestRewriteCommitMessage_ErrorCase_ErrorModifyingMessage(t *testing.T) {
+	defer restoreRewriteOriginals()
+	commitMessageFileCanBeRead(t)
+	errorModifyingCommitMessage(t)
+
+	err := RewriteCommitMessage(commitMessageFile)
+
+	assert.Contains(t, err.Error(), "error modifying commit message")
+}
+
+func TestRewriteCommitMessage_ErrorCase_CannotWriteMessageToFile(t *testing.T) {
+	defer restoreRewriteOriginals()
+
+	commitMessageFileCanBeRead(t)
+	commitMessageIsModified(t)
+	cannotWriteToFile(t)
+
+	err := RewriteCommitMessage(commitMessageFile)
+
+	assert.Contains(t, err.Error(), "error writing commit message to")
+}
+
+func TestRewriteCommitMessage_HappyPath(t *testing.T) {
+	defer restoreRewriteOriginals()
+
+	commitMessageFileCanBeRead(t)
+	commitMessageIsModified(t)
+	commitMessageIsWrittenToFile(t)
+
+	err := RewriteCommitMessage(commitMessageFile)
+
+	assert.NoError(t, err)
+}
+
+func cannotWriteToFile(t *testing.T) {
+	writeFileFunc = func(fileName string, data []byte, perm os.FileMode) error {
+		assert.Exactly(t, commitMessageFile, fileName)
+		assert.Exactly(t, modifiedCommitMessage, string(data))
+		assert.Exactly(t, perm, os.FileMode(0777))
+
+		return errors.New("cannot write to file")
+	}
+}
+
+func commitMessageIsWrittenToFile(t *testing.T) {
+	writeFileFunc = func(fileName string, data []byte, perm os.FileMode) error {
+		assert.Exactly(t, commitMessageFile, fileName)
+		assert.Exactly(t, modifiedCommitMessage, string(data))
+		assert.Exactly(t, perm, os.FileMode(0777))
+
+		return nil
+	}
+}
+
+func commitMessageFileCanBeRead(t *testing.T) {
+	readFileFunc = func(fileName string) ([]byte, error) {
+		assert.Exactly(t, commitMessageFile, fileName)
+		return []byte(commitMessage), nil
+	}
+}
+
+func commitMessageFileCannotBeFound(t *testing.T) {
+	readFileFunc = func(fileName string) ([]byte, error) {
+		assert.Exactly(t, commitMessageFile, fileName)
+		return []byte{}, errors.New("Could not find file")
+	}
+}
+
+func errorModifyingCommitMessage(t *testing.T) {
+	modifyFunc = func(message string) (string, error) {
+		assert.Exactly(t, commitMessage, message)
+		return "", errors.New("error modifying")
+	}
+}
+
+func commitMessageIsModified(t *testing.T) {
+	modifyFunc = func(message string) (string, error) {
+		assert.Exactly(t, commitMessage, message)
+		return modifiedCommitMessage, nil
+	}
+}