Add --user-config parser argument

locust/argument_parser.py

@@ -306,6 +306,13 @@ def setup_parser_arguments(parser):
         dest="list_commands",
         help="Show list of possible User classes and exit",
     )
+    parser.add_argument(
+        "--user-config",
+        type=str,
+        nargs="*",
+        help="User configuration as a JSON string or file. A list of arguments or an Array of JSON configuration may be provided",
+        env_var="LOCUST_USER_CONFIG",
+    )
 
     web_ui_group = parser.add_argument_group("Web UI options")
     web_ui_group.add_argument(

locust/main.py

@@ -3,6 +3,7 @@
 import errno
 import logging
 import os
+import json
 import signal
 import sys
 import time
@@ -358,6 +359,32 @@ def kill_workers(children):
         available_user_tasks=available_user_tasks,
     )
 
+    if options.user_config:
+        for json_user_config in options.user_config:
+            try:
+                if json_user_config.endswith(".json"):
+                    with open(json_user_config) as file:
+                        user_config = json.load(file)
+                else:
+                    user_config = json.loads(json_user_config)
+
+                if isinstance(user_config, list):
+                    for config in user_config:
+                        if "user_class_name" not in config:
+                            logger.error("The user config must specify a user_class_name")
+                            sys.exit(-1)
+
+                        environment.update_user_class(config)
+                else:
+                    if "user_class_name" not in user_config:
+                        logger.error("The user config must specify a user_class_name")
+                        sys.exit(-1)
+
+                    environment.update_user_class(user_config)
+            except Exception as e:
+                logger.error(f"The --user-config arugment must be in valid JSON string or file: {e}")
+                sys.exit(-1)
+
     if (
         shape_class
         and not shape_class.use_common_options