A new remote control command @env to change the default environment passed to newly created windows

Fixes #3529

docs/changelog.rst

@@ -11,6 +11,9 @@ To update |kitty|, :doc:`follow the instructions <binary>`.
   scrollback buffer onto the screen. Useful, for example, to restore
   the screen after showing completions below the shell prompt.
 
+- A new remote control command :ref:`at_env` to change the default
+  environment passed to newly created windows (:iss:`3529`)
+
 - Linux: Fix binary kitty builds not able to load fonts in WOFF2 format
   (:iss:`3506`)
 

kitty/rc/env.py

@@ -0,0 +1,49 @@
+#!/usr/bin/env python
+# vim:fileencoding=utf-8
+# License: GPLv3 Copyright: 2020, Kovid Goyal <kovid at kovidgoyal.net>
+
+from typing import Any, Optional
+
+from .base import (
+    ArgsType, Boss, PayloadGetType, PayloadType, RCOptions, RemoteCommand,
+    ResponseType, Window
+)
+
+
+class Env(RemoteCommand):
+
+    '''
+    env+: dictionary of environment variables to values. Empty values cause the variable to be deleted.
+    '''
+
+    short_desc = 'Change environment variables seen by future children'
+    desc = (
+        'Change the environment variables seen by processing in newly launched windows.'
+        ' Similar to the :opt:`env` option in kitty.conf, but affects running kitty instances.'
+        ' Empty values cause the environment variable to be removed.'
+    )
+    argspec = 'env_var1=val env_var2=val ...'
+
+    def message_to_kitty(self, global_opts: RCOptions, opts: Any, args: ArgsType) -> PayloadType:
+        if len(args) < 1:
+            self.fatal('Must specify at least one env var to set')
+        env = {}
+        for x in args:
+            key, val = x.split('=', 1)
+            env[key] = val
+        return {'env': env}
+
+    def response_from_kitty(self, boss: Boss, window: Optional[Window], payload_get: PayloadGetType) -> ResponseType:
+        from kitty.child import default_env, set_default_env
+        from kitty.utils import expandvars
+        new_env = payload_get('env') or {}
+        env = default_env().copy()
+        for k, v in new_env.items():
+            if v:
+                env[k] = expandvars(v, env)
+            else:
+                env.pop(k, None)
+        set_default_env(env)
+
+
+env = Env()