duet-install-hook displays a helpful message when failing

git-duet-install-hook/main.go

@@ -111,7 +111,12 @@ func main() {
 		if hook == preCommitHook && !strings.Contains(contents, preCommitHook) ||
 			hook == prepareCommitMsgHook && !strings.Contains(contents, prepareCommitMsgHook) ||
 			hook == postCommitHook && !strings.Contains(contents, postCommitHook) {
-			fmt.Printf("can't install hook: file %s already exists\n", hookPath)
+			fmt.Printf(`It seems you already have a "%s" hook.
+To enable the git-duet hook, please append:
+
+  %s
+
+to your %s file.`, hookFileName, hook, hookPath)
 			os.Exit(1)
 		}
 		os.Exit(0) // hook file with the desired content already exists

test/git-duet-install-hooks.bats

@@ -25,15 +25,51 @@ load test_helper
   [ -x .git/hooks/pre-commit ]
 }
 
+@test "does not overwrite existing pre-commit hook file with other contents" {
+  echo "Some content" > .git/hooks/pre-commit
+  local EXPECTED_HELP_MESSAGE=$(cat << EOM
+It seems you already have a "pre-commit" hook.
+To enable the git-duet hook, please append:
+
+  exec git duet-pre-commit "\$@"
+
+to your $PWD/.git/hooks/pre-commit file.
+EOM
+)
+  run git duet-install-hook -q pre-commit
+  assert_output "$EXPECTED_HELP_MESSAGE"
+  assert_failure
+}
+
 @test "does not overwrite existing prepare-commit-msg hook file with other contents" {
   echo "Some content" > .git/hooks/prepare-commit-msg
+  local EXPECTED_HELP_MESSAGE=$(cat << EOM
+It seems you already have a "prepare-commit-msg" hook.
+To enable the git-duet hook, please append:
+
+  exec git duet-prepare-commit-msg "\$@"
+
+to your $PWD/.git/hooks/prepare-commit-msg file.
+EOM
+)
   run git duet-install-hook -q prepare-commit-msg
+  assert_output "$EXPECTED_HELP_MESSAGE"
   assert_failure
 }
 
 @test "does not overwrite existing post-commit hook file with other contents" {
   echo "Some content" > .git/hooks/post-commit
+  local EXPECTED_HELP_MESSAGE=$(cat << EOM
+It seems you already have a "post-commit" hook.
+To enable the git-duet hook, please append:
+
+  exec git duet-post-commit "\$@"
+
+to your $PWD/.git/hooks/post-commit file.
+EOM
+)
   run git duet-install-hook -q post-commit
+  assert_output "$EXPECTED_HELP_MESSAGE"
   assert_failure
 }