Index: src/__phutil_library_map__.php
===================================================================
--- src/__phutil_library_map__.php
+++ src/__phutil_library_map__.php
@@ -767,6 +767,7 @@
     'LiskIsolationTestDAOException' => 'infrastructure/storage/lisk/__tests__/LiskIsolationTestDAOException.php',
     'LiskMigrationIterator' => 'infrastructure/storage/lisk/LiskMigrationIterator.php',
     'LiskRawMigrationIterator' => 'infrastructure/storage/lisk/LiskRawMigrationIterator.php',
+    'LocalCommandBuildStepImplementation' => 'applications/harbormaster/step/LocalCommandBuildStepImplementation.php',
     'ManiphestActionMenuEventListener' => 'applications/maniphest/event/ManiphestActionMenuEventListener.php',
     'ManiphestBatchEditController' => 'applications/maniphest/controller/ManiphestBatchEditController.php',
     'ManiphestCapabilityBulkEdit' => 'applications/maniphest/capability/ManiphestCapabilityBulkEdit.php',
@@ -2199,6 +2200,7 @@
     'SleepBuildStepImplementation' => 'applications/harbormaster/step/SleepBuildStepImplementation.php',
     'SlowvoteEmbedView' => 'applications/slowvote/view/SlowvoteEmbedView.php',
     'SlowvoteRemarkupRule' => 'applications/slowvote/remarkup/SlowvoteRemarkupRule.php',
+    'VariableBuildStepImplementation' => 'applications/harbormaster/step/VariableBuildStepImplementation.php',
   ),
   'function' =>
   array(
@@ -3016,6 +3018,7 @@
     'LiskIsolationTestDAOException' => 'Exception',
     'LiskMigrationIterator' => 'PhutilBufferedIterator',
     'LiskRawMigrationIterator' => 'PhutilBufferedIterator',
+    'LocalCommandBuildStepImplementation' => 'VariableBuildStepImplementation',
     'ManiphestActionMenuEventListener' => 'PhabricatorEventListener',
     'ManiphestBatchEditController' => 'ManiphestController',
     'ManiphestCapabilityBulkEdit' => 'PhabricatorPolicyCapability',
@@ -4662,5 +4665,6 @@
     'SleepBuildStepImplementation' => 'BuildStepImplementation',
     'SlowvoteEmbedView' => 'AphrontView',
     'SlowvoteRemarkupRule' => 'PhabricatorRemarkupRuleObject',
+    'VariableBuildStepImplementation' => 'BuildStepImplementation',
   ),
 ));
Index: src/applications/harbormaster/controller/HarbormasterStepEditController.php
===================================================================
--- src/applications/harbormaster/controller/HarbormasterStepEditController.php
+++ src/applications/harbormaster/controller/HarbormasterStepEditController.php
@@ -63,6 +63,11 @@
     $form = id(new AphrontFormView())
       ->setUser($viewer);
 
+    $instructions = $implementation->getSettingRemarkupInstructions();
+    if ($instructions !== null) {
+      $form->appendRemarkupInstructions($instructions);
+    }
+
     // We need to render out all of the fields for the settings that
     // the implementation has.
     foreach ($implementation->getSettingDefinitions() as $name => $opt) {
Index: src/applications/harbormaster/step/BuildStepImplementation.php
===================================================================
--- src/applications/harbormaster/step/BuildStepImplementation.php
+++ src/applications/harbormaster/step/BuildStepImplementation.php
@@ -85,4 +85,11 @@
   public function getSettingDefinitions() {
     return array();
   }
+
+  /**
+   * Return relevant setting instructions as Remarkup.
+   */
+  public function getSettingRemarkupInstructions() {
+    return null;
+  }
 }
Index: src/applications/harbormaster/step/LocalCommandBuildStepImplementation.php
===================================================================
--- /dev/null
+++ src/applications/harbormaster/step/LocalCommandBuildStepImplementation.php
@@ -0,0 +1,53 @@
+<?php
+
+final class LocalCommandBuildStepImplementation
+  extends VariableBuildStepImplementation {
+
+  public function getName() {
+    return pht('Run Local Command');
+  }
+
+  public function getGenericDescription() {
+    return pht('Run a command on the local web server.');
+  }
+
+  public function getDescription() {
+    $settings = $this->getSettings();
+
+    return pht('Run \'%s\' on the local web server.', $settings['command']);
+  }
+
+  public function execute(HarbormasterBuild $build) {
+    $settings = $this->getSettings();
+
+    list($err, $stdout, $stderr) = exec_manual(
+      '%C',
+      $this->mergeVariables($build, $settings['command']));
+
+    if ($err) {
+      // TODO: Create a log entry with the command output.
+      $build->setBuildStatus(HarbormasterBuild::STATUS_FAILED);
+    }
+  }
+
+  public function validateSettings() {
+    $settings = $this->getSettings();
+
+    if ($settings['command'] === null) {
+      return false;
+    }
+    if (!is_string($settings['command'])) {
+      return false;
+    }
+    return true;
+  }
+
+  public function getSettingDefinitions() {
+    return array(
+      'command' => array(
+        'name' => 'Command',
+        'description' => 'The command to execute on the local web server.',
+        'type' => BuildStepImplementation::SETTING_TYPE_STRING));
+  }
+
+}
Index: src/applications/harbormaster/step/VariableBuildStepImplementation.php
===================================================================
--- /dev/null
+++ src/applications/harbormaster/step/VariableBuildStepImplementation.php
@@ -0,0 +1,64 @@
+<?php
+
+abstract class VariableBuildStepImplementation extends BuildStepImplementation {
+
+  public function retrieveVariablesFromBuild(HarbormasterBuild $build) {
+    $results = array(
+      'revision' => null,
+      'commit' => null,
+      'repository' => null,
+      'vcs' => null,
+      'uri' => null);
+
+    $buildable = $build->getBuildable();
+    $object = $buildable->getBuildableObject();
+
+    $repo = null;
+    if ($object instanceof DifferentialRevision) {
+      $results['revision'] = $object->getID();
+      $repo = $object->getRepository();
+    } else if ($object instanceof PhabricatorRepositoryCommit) {
+      $results['commit'] = $object->getCommitIdentifier();
+      $repo = $object->getRepository();
+    }
+
+    $results['repository'] = $repo->getCallsign();
+    $results['vcs'] = $repo->getVersionControlSystem();
+    $results['uri'] = $repo->getPublicRemoteURI();
+
+    return $results;
+  }
+
+  public function mergeVariables(HarbormasterBuild $build, $string) {
+    $variables = $this->retrieveVariablesFromBuild($build);
+    foreach ($variables as $name => $value) {
+      if ($value === null) {
+        $value = '';
+      }
+      $string = str_replace('#{'.$name.'}', $value, $string);
+    }
+    return $string;
+  }
+
+  public function getAvailableVariables() {
+    return array(
+      'revision' => pht('The differential revision ID, if applicable.'),
+      'commit' => pht('The commit identifier, if applicable.'),
+      'repository' => pht('The callsign of the repository in Phabricator.'),
+      'vcs' => pht('The version control system, either "svn", "hg" or "git".'),
+      'uri' => pht('The URI to clone or checkout the repository from.'));
+  }
+
+  public function getSettingRemarkupInstructions() {
+    $text = '';
+    $text .= pht('The following variables are available: ')."\n";
+    $text .= "\n";
+    foreach ($this->getAvailableVariables() as $name => $desc) {
+      $text .= '  - `'.$name.'`: '.$desc."\n";
+    }
+    $text .= "\n";
+    $text .= "Use `#{name}` to merge a variable into a setting.";
+    return $text;
+  }
+
+}