using killPort

scripts/clean.js

 run();
 
 function run() {
-  exec.execSync('lsof -t -i tcp:8081 | xargs kill || true');
+  exec.killPort(8081);
   exec.execSync(`watchman watch-del-all || true`);
   exec.execSync(`adb reverse tcp:8081 tcp:8081 || true`);
   exec.execSync(`rm -rf lib/ios/DerivedData/`);

scripts/start.js

 run();
 
 function run() {
-  exec.kill('8081');
+  exec.killPort(8081);
   exec.execSync(`watchman watch-del-all || true`);
   exec.execSync(`adb reverse tcp:8081 tcp:8081 || true`);
   exec.execSync(`node ./node_modules/react-native/local-cli/cli.js start --root=./playground`);