[Core V3] add instance listing cli flag (#1079)

* [Core] add instance listing cli flag

* Alphabetically sort instance names

redbot/__main__.py

@@ -6,7 +6,8 @@ import sys
 import discord
 from redbot.core.bot import Red, ExitCodes
 from redbot.core.cog_manager import CogManagerUI
-from redbot.core.data_manager import load_basic_configuration
+from redbot.core.data_manager import load_basic_configuration, config_file
+from redbot.core.json_io import JsonIO
 from redbot.core.global_checks import init_global_checks
 from redbot.core.events import init_events
 from redbot.core.sentry_setup import init_sentry_logging
@@ -81,8 +82,27 @@ async def _get_prefix_and_token(red, indict):
     indict['enable_sentry'] = await red.db.enable_sentry()
 
 
+def list_instances():
+    if not config_file.exists():
+        print("No instances have been configured! Configure one "
+              "using `redbot-setup` before trying to run the bot!")
+        sys.exit(1)
+    else:
+        data = JsonIO(config_file)._load_json()
+        text = "Configured Instances:\n\n"
+        for instance_name in sorted(data.keys()):
+            text += "{}\n".format(instance_name)
+        print(text)
+        sys.exit(0)
+
+
 def main():
     cli_flags = parse_cli_flags(sys.argv[1:])
+    if cli_flags.list_instances:
+        list_instances()
+    elif not cli_flags.instance_name:
+        print("Error: No instance name was provided!")
+        sys.exit(1)
     load_basic_configuration(cli_flags.instance_name)
     log, sentry_log = init_loggers(cli_flags)
     description = "Red - Version {}".format(__version__)