Kept existing options and re-wrote the test case.

shell.js

@@ -1102,8 +1102,8 @@ function _exec(command, options, callback) {
   }
 
   // Callback is defined with options.
-  if (typeof callback === 'function') {
-    options = { async: true };
+  if (typeof options === 'object' && typeof callback === 'function') {
+      options.async = true;
   }
 
   options = extend({

test/exec.js

@@ -91,16 +91,17 @@ shell.exec('node -e \"console.log(5678);\"', function(code, output) {
     assert.equal(code, 0);
     assert.ok(output === '5566\n' || output === '5566\nundefined\n');  // 'undefined' for v0.4
 
-    shell.exit(123);  
+    //
+    // callback as 3rd argument (slient:true)
+    //
+    shell.exec('node -e \"console.log(5678);\"', {silent:true}, function(code, output) {
+      assert.equal(code, 0);
+      assert.ok(output === '5678\n' || output === '5678\nundefined\n');  // 'undefined' for v0.4
 
-  });
+      shell.exit(123);
+
+    });
 
-  //
-  // callback as 3rd argument (slient:true)
-  //
-  shell.exec('node -e \"console.log(5678);\"', {silent:true}, function(code, output) {
-    assert.equal(code, 0);
-    assert.ok(output === '5678\n' || output === '5678\nundefined\n');  // 'undefined' for v0.4
   });
 
 });