diff --git a/package.json b/package.json
index 42dcf78..c332ed7 100644
--- a/package.json
+++ b/package.json
@@ -54,6 +54,7 @@
     "trailingComma": "es5"
   },
   "dependencies": {
+    "fast-deep-equal": "^2.0.1",
     "joycon": "^2.2.4",
     "md5-file": "^4.0.0",
     "mkdirp": "^0.5.1"
diff --git a/src/cli.js b/src/cli.js
index 155272b..03a80b0 100755
--- a/src/cli.js
+++ b/src/cli.js
@@ -9,11 +9,13 @@ function parseArgs() {
   return args.reduce(
     (acc, arg) => {
       if (arg === '--verbose') acc.verbose = true;
+      if (arg === '--force') acc.force = true;
       if (arg === '--use-yarn') acc.useYarn = true;
       return acc;
     },
     {
       verbose: false,
+      force: false,
       useYarn: false,
     }
   );
diff --git a/src/data.js b/src/data.js
index 0b4e420..3821c6a 100644
--- a/src/data.js
+++ b/src/data.js
@@ -29,6 +29,11 @@ exports.read = (fileRoot, ident) => {
 };
 
 exports.write = (fileRoot, ident, payload) => {
-  console.log('WRITE', { fileRoot, ident, payload });
-  throw new Error('Data write not implemented');
+  const filePath = path.join(fileRoot, CONFIG_FILENAME);
+  const existingCache = JSON.parse(fs.readFileSync(filePath));
+
+  // update the cache
+  existingCache[ident] = payload;
+
+  fs.writeFileSync(filePath, JSON.stringify(existingCache));
 };
diff --git a/src/index.js b/src/index.js
index 75a2bdf..019032a 100644
--- a/src/index.js
+++ b/src/index.js
@@ -1,6 +1,8 @@
+const path = require('path');
 const md5File = require('md5-file');
+const deepEqual = require('fast-deep-equal');
 const data = require('./data');
-// const utils = require('./utils');
+const utils = require('./utils');
 
 function getFileHash(file, verbose = false) {
   try {
@@ -12,9 +14,22 @@ function getFileHash(file, verbose = false) {
   }
 }
 
-module.exports = function mod(config, ident, opts = {}) {
-  const packageData = data.read(config.dataDir, ident);
-  const firstRun = Object.keys(packageData).length === 0;
+function execCommand(cmd) {
+  console.log('RUN:', cmd);
+}
+
+const defaultOptions = {
+  verbose: false,
+  force: false,
+};
+
+module.exports = function pkgcomp(config, ident, opts = defaultOptions) {
+  const packageHistory = data.read(config.dataDir, ident);
+  const firstRun = Object.keys(packageHistory).length === 0;
+  const doExec = !firstRun || opts.force;
+
+  // eslint-disable-next-line no-console
+  if (firstRun && !opts.force && opts.verbose) console.info('First run, command skipped');
 
   // create file hash object
   const hashes = config.checkFiles.reduce((acc, file) => {
@@ -22,9 +37,19 @@ module.exports = function mod(config, ident, opts = {}) {
     return acc;
   }, {});
 
-  console.log('First run?', firstRun);
+  // read dependencies from package.json
+  const pkg = utils.getFileContents(path.join(config.rootDir, 'package.json'), { format: 'json' });
 
-  data.write(config.dataDir, ident, {
+  const payload = {
     hashes,
-  });
+    dependencies: pkg.dependencies,
+    devDependencies: pkg.devDependencies,
+  };
+
+  // check if the command should be run
+  const runCmd = Boolean(opts.force || (doExec && !deepEqual(packageHistory, payload)));
+
+  if (runCmd) execCommand(config.cmd);
+
+  data.write(config.dataDir, ident, payload);
 };