app.js

var fs = require('fs');
fs.writeFile('env.temp', `DATA_DIR=${process.env.DATA_DIR}`, function (err) {
  if (err) {
    return console.log(err);
  }

  console.log('The file was saved!');
});

server.js

var pm2 = require('pm2');
process.env.DATA_DIR = '/var/lib/data';

pm2.connect(function (err) {
  if (err) {
    console.error(err);
    process.exit(2);
  }

  pm2.start({
    name: 'app',
    script: 'app.js',         // Script to be run
    exec_mode: 'fork',        // Allows your app to be clustered
    instances: 1,                // Optional: Scales your app by 4
    max_memory_restart: '100M'   // Optional: Restarts your app if it reaches 100Mo
  }, function (err, apps) {
    pm2.disconnect();   // Disconnects from PM2
    if (err) throw err
  });
});