module.run_command is intended to bypass the shell here, so can't do ">>"

lib/ansible/module_utils/basic.py

@@ -1089,6 +1089,12 @@ class AnsibleModule(object):
             self.fail_json(cmd=clean_args, rc=rc, stdout=out, stderr=err, msg=msg)
         return (rc, out, err)
 
+    def append_to_file(self, filename, str):
+        filename = os.path.expandvars(os.path.expanduser(filename))
+        fh = open(filename, 'a')
+        fh.write(str)
+        fh.close()
+
     def pretty_bytes(self,size):
         ranges = (
                 (1<<70L, 'ZB'),

lib/ansible/module_utils/known_hosts.py

@@ -91,8 +91,10 @@ def add_host_key(module, fqdn, key_type="rsa"):
     if not os.path.exists(os.path.expanduser("~/.ssh/")):
         module.fail_json(msg="%s does not exist" % os.path.expanduser("~/.ssh/"))
 
-    this_cmd = "%s -t %s %s >> ~/.ssh/known_hosts" % (keyscan_cmd, key_type, fqdn)
+    this_cmd = "%s -t %s %s" % (keyscan_cmd, key_type, fqdn)
+
     rc, out, err = module.run_command(this_cmd)
+    module.append_to_file("~/.ssh/known_hosts", out)
 
     return rc, out, err