diff --git a/src/main-process/main.js b/src/main-process/main.js
index dca2bc2e7..28871b661 100644
--- a/src/main-process/main.js
+++ b/src/main-process/main.js
@@ -1,115 +1,36 @@
-global.shellStartTime = Date.now()
+const startTime = Date.now()
 
-process.on('uncaughtException', function (error = {}) {
-  if (error.message != null) {
-    console.log(error.message)
-  }
-
-  if (error.stack != null) {
-    console.log(error.stack)
-  }
-})
-
-const {app} = require('electron')
-const fs = require('fs-plus')
+const electron = require('electron')
+const fs = require('fs')
 const path = require('path')
-const temp = require('temp')
-const parseCommandLine = require('./parse-command-line')
-const startCrashReporter = require('../crash-reporter-start')
-const previousConsoleLog = console.log
-console.log = require('nslog')
+const yargs = require('yargs')
 
-function start () {
-  const args = parseCommandLine(process.argv.slice(1))
-  setupAtomHome(args)
-  setupCompileCache()
+const args =
+  yargs(process.argv)
+    .alias('d', 'dev')
+    .alias('t', 'test')
+    .argv
 
-  if (handleStartupEventWithSquirrel()) {
-    return
-  } else if (args.test && args.mainProcess) {
-    app.setPath('userData', temp.mkdirSync('atom-user-data-dir-for-main-process-tests'))
-    console.log = previousConsoleLog
-    app.on('ready', function () {
-      const testRunner = require(path.join(args.resourcePath, 'spec/main-process/mocha-test-runner'))
-      testRunner(args.pathsToOpen)
-    })
-    return
-  }
+let resourcePath
 
-  // NB: This prevents Win10 from showing dupe items in the taskbar
-  app.setAppUserModelId('com.squirrel.atom.atom')
+if (args.resourcePath) {
+  resourcePath = args.resourcePath
+} else {
+  const stableResourcePath = path.dirname(path.dirname(__dirname))
+  const defaultRepositoryPath = path.join(electron.app.getPath('home'), 'github', 'atom')
 
-  function addPathToOpen (event, pathToOpen) {
-    event.preventDefault()
-    args.pathsToOpen.push(pathToOpen)
-  }
-
-  function addUrlToOpen (event, urlToOpen) {
-    event.preventDefault()
-    args.urlsToOpen.push(urlToOpen)
-  }
-
-  app.on('open-file', addPathToOpen)
-  app.on('open-url', addUrlToOpen)
-  app.on('will-finish-launching', startCrashReporter)
-
-  if (args.userDataDir != null) {
-    app.setPath('userData', args.userDataDir)
-  } else if (args.test) {
-    app.setPath('userData', temp.mkdirSync('atom-test-data'))
-  }
-
-  app.on('ready', function () {
-    app.removeListener('open-file', addPathToOpen)
-    app.removeListener('open-url', addUrlToOpen)
-    const AtomApplication = require(path.join(args.resourcePath, 'src', 'main-process', 'atom-application'))
-    AtomApplication.open(args)
-  })
-}
-
-function handleStartupEventWithSquirrel () {
-  if (process.platform !== 'win32') {
-    return false
-  }
-
-  const SquirrelUpdate = require('./squirrel-update')
-  const squirrelCommand = process.argv[1]
-  return SquirrelUpdate.handleStartupEvent(app, squirrelCommand)
-}
-
-function setupAtomHome ({setPortable}) {
-  if (process.env.ATOM_HOME) {
-    return
-  }
-
-  let atomHome = path.join(app.getPath('home'), '.atom')
-  const AtomPortable = require('./atom-portable')
-
-  if (setPortable && !AtomPortable.isPortableInstall(process.platform, process.env.ATOM_HOME, atomHome)) {
-    try {
-      AtomPortable.setPortable(atomHome)
-    } catch (error) {
-      console.log(`Failed copying portable directory '${atomHome}' to '${AtomPortable.getPortableAtomHomePath()}'`)
-      console.log(`${error.message} ${error.stack}`)
+  if (args.dev || args.test) {
+    if (process.env.ATOM_DEV_RESOURCE_PATH) {
+      resourcePath = process.env.ATOM_DEV_RESOURCE_PATH
+    } else if (fs.statSyncNoException(defaultRepositoryPath)) {
+      resourcePath = defaultRepositoryPath
+    } else {
+      resourcePath = stableResourcePath
     }
+  } else {
+    resourcePath = stableResourcePath
   }
-
-  if (AtomPortable.isPortableInstall(process.platform, process.env.ATOM_HOME, atomHome)) {
-    atomHome = AtomPortable.getPortableAtomHomePath()
-  }
-
-  try {
-    atomHome = fs.realpathSync(atomHome)
-  } catch (e) {
-    // Don't throw an error if atomHome doesn't exist.
-  }
-
-  process.env.ATOM_HOME = atomHome
 }
 
-function setupCompileCache () {
-  const CompileCache = require('../compile-cache')
-  CompileCache.setAtomHomeDirectory(process.env.ATOM_HOME)
-}
-
-start()
+const start = require(path.join(resourcePath, 'src', 'main-process', 'start'))
+start(resourcePath, startTime)
diff --git a/src/main-process/start.js b/src/main-process/start.js
new file mode 100644
index 000000000..125f3ed54
--- /dev/null
+++ b/src/main-process/start.js
@@ -0,0 +1,115 @@
+const {app} = require('electron')
+const fs = require('fs-plus')
+const nslog = require('nslog')
+const path = require('path')
+const temp = require('temp')
+const parseCommandLine = require('./parse-command-line')
+const startCrashReporter = require('../crash-reporter-start')
+
+module.exports = function start (resourcePath, startTime) {
+  global.shellStartTime = startTime
+
+  process.on('uncaughtException', function (error = {}) {
+    if (error.message != null) {
+      console.log(error.message)
+    }
+
+    if (error.stack != null) {
+      console.log(error.stack)
+    }
+  })
+
+  const previousConsoleLog = console.log
+  console.log = nslog
+
+  const args = parseCommandLine(process.argv.slice(1))
+  setupAtomHome(args)
+  setupCompileCache()
+
+  if (handleStartupEventWithSquirrel()) {
+    return
+  } else if (args.test && args.mainProcess) {
+    app.setPath('userData', temp.mkdirSync('atom-user-data-dir-for-main-process-tests'))
+    console.log = previousConsoleLog
+    app.on('ready', function () {
+      const testRunner = require(path.join(args.resourcePath, 'spec/main-process/mocha-test-runner'))
+      testRunner(args.pathsToOpen)
+    })
+    return
+  }
+
+  // NB: This prevents Win10 from showing dupe items in the taskbar
+  app.setAppUserModelId('com.squirrel.atom.atom')
+
+  function addPathToOpen (event, pathToOpen) {
+    event.preventDefault()
+    args.pathsToOpen.push(pathToOpen)
+  }
+
+  function addUrlToOpen (event, urlToOpen) {
+    event.preventDefault()
+    args.urlsToOpen.push(urlToOpen)
+  }
+
+  app.on('open-file', addPathToOpen)
+  app.on('open-url', addUrlToOpen)
+  app.on('will-finish-launching', startCrashReporter)
+
+  if (args.userDataDir != null) {
+    app.setPath('userData', args.userDataDir)
+  } else if (args.test) {
+    app.setPath('userData', temp.mkdirSync('atom-test-data'))
+  }
+
+  app.on('ready', function () {
+    app.removeListener('open-file', addPathToOpen)
+    app.removeListener('open-url', addUrlToOpen)
+    const AtomApplication = require(path.join(args.resourcePath, 'src', 'main-process', 'atom-application'))
+    AtomApplication.open(args)
+  })
+}
+
+function handleStartupEventWithSquirrel () {
+  if (process.platform !== 'win32') {
+    return false
+  }
+
+  const SquirrelUpdate = require('./squirrel-update')
+  const squirrelCommand = process.argv[1]
+  return SquirrelUpdate.handleStartupEvent(app, squirrelCommand)
+}
+
+function setupAtomHome ({setPortable}) {
+  if (process.env.ATOM_HOME) {
+    return
+  }
+
+  let atomHome = path.join(app.getPath('home'), '.atom')
+  const AtomPortable = require('./atom-portable')
+
+  if (setPortable && !AtomPortable.isPortableInstall(process.platform, process.env.ATOM_HOME, atomHome)) {
+    try {
+      AtomPortable.setPortable(atomHome)
+    } catch (error) {
+      console.log(`Failed copying portable directory '${atomHome}' to '${AtomPortable.getPortableAtomHomePath()}'`)
+      console.log(`${error.message} ${error.stack}`)
+    }
+  }
+
+  if (AtomPortable.isPortableInstall(process.platform, process.env.ATOM_HOME, atomHome)) {
+    atomHome = AtomPortable.getPortableAtomHomePath()
+  }
+
+  try {
+    atomHome = fs.realpathSync(atomHome)
+  } catch (e) {
+    // Don't throw an error if atomHome doesn't exist.
+  }
+
+  process.env.ATOM_HOME = atomHome
+}
+
+function setupCompileCache () {
+  const CompileCache = require('../compile-cache')
+  CompileCache.setAtomHomeDirectory(process.env.ATOM_HOME)
+}