shell: Add option to keep current environment

When doing interactive development via kas shell it is often desirable
to keep the user's customized configuration. The new --preserve-env
argument has been added to support this scenario via an opt in flag.

This flag is blocked  when not running from a TTY or via kas-container
and kas issues a warning to the user about potential unintended side
effects when invoked.

Signed-off-by: Ryan Fairfax <rfairfax@linux.microsoft.com>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

kas/plugins/shell.py

@@ -38,11 +38,12 @@
 """
 
 import logging
+import os
 import subprocess
 import sys
 from kas.context import create_global_context
 from kas.config import Config
-from kas.libcmds import Macro, Command
+from kas.libcmds import Macro, Command, SetupHome
 from kas.libkas import setup_parser_common_args
 
 __license__ = 'MIT'
@@ -70,6 +71,9 @@ class Shell:
         parser.add_argument('-c', '--command',
                             help='Run command',
                             default='')
+        parser.add_argument('-E', '--preserve-env',
+                            help='Keep current user enviornment block',
+                            action='store_true')
 
     def run(self, args):
         """
@@ -79,6 +83,25 @@ class Shell:
         ctx = create_global_context(args)
         ctx.config = Config(ctx, args.config)
 
+        if args.preserve_env:
+            # Warn if there's any settings that setup_home would apply
+            # but are now ignored
+            for var in SetupHome.ENV_VARS:
+                if var in os.environ:
+                    logging.warning('Environment variable "%s" ignored '
+                                    'because user environment is being used',
+                                    var)
+
+            if not os.isatty(sys.stdout.fileno()):
+                logging.error("Error: --preserve-env can only be "
+                              "run from a tty")
+                sys.exit(1)
+
+            ctx.environ = os.environ.copy()
+
+            logging.warning("Preserving the current environment block may "
+                            "have unintended side effects on the build.")
+
         if args.keep_config_unchanged:
             # Skip the tasks which would change the config of the build
             # environment