Dump only salient differences from env_dump.py.

build/env_dump.py

Index: build/env_dump.py
diff --git a/build/env_dump.py b/build/env_dump.py
index d6a18c83224cf1d142c1b50b3135b6c056e9fdc4..1534b377bd63c80b9763d29c167a6162bafd0403 100755
--- a/build/env_dump.py
+++ b/build/env_dump.py
@@ -20,7 +20,7 @@ def main():
                     help='File to dump the environment as JSON into.')
   parser.add_option(
       '-d', '--dump-mode', action='store_true',
-      help='Dump the environment to the file and exit immediately.')
+      help='Dump the environment to sys.stdout and exit immediately.')

[iannucci, 2013/09/18 00:55:37]

just 'stdout' in help string :)

 
   options, args = parser.parse_args()
   if not options.output_json:
@@ -29,8 +29,7 @@ def main():
   if options.dump_mode:

[iannucci, 2013/09/18 00:55:37]

we should make it an error to pass options.output_json here

     if args:
       parser.error("Cannot specify args with --dump-mode")
-    with open(options.output_json, 'w') as f:
-      json.dump(dict(os.environ), f)
+    json.dump(dict(os.environ), sys.stdout)
   else:
     envsetup_cmd = ' '.join(map(pipes.quote, args))
     full_cmd = [
@@ -38,9 +37,19 @@ def main():
         '. %s; %s -d -f %s' % (envsetup_cmd, os.path.abspath(__file__),

[iannucci, 2013/09/18 00:55:37]

I don't think we need to pass -f any more, since it's ignored.

                                options.output_json)
     ]
-    ret = subprocess.call(full_cmd)
-    if ret:
-      sys.exit('Error running %s and dumping env' % envsetup_cmd)
+    try:
+      output = subprocess.check_output(full_cmd)
+    except Exception as e:
+      sys.exit('Error running % and dumping environment.' % envsetup_cmd)

[iannucci, 2013/09/18 00:55:37]

missing %s

+
+    env_diff = {}
+    new_env = json.loads(output)
+    for k, val in new_env.items():
+      if k == '_' or (k in os.environ and os.environ[k] == val):
+        continue
+      env_diff[k] = val
+    with open(options.output_json, 'w') as f:
+      json.dump(env_diff, f)
 
 
 if __name__ == '__main__':