core: add --url_file flag for scripting purposes

tensorboard/plugins/core/core_plugin.py

@@ -447,6 +447,25 @@ def define_flags(self, parser):
 means keep all samples of that type. For instance "scalars=500,images=0"
 keeps 500 scalars and all images. Most users should not need to set this
 flag.\
+''')
+
+    parser.add_argument(
+        '--url_file',
+        metavar='FILE',
+        type=str,
+        help='''\
+An optional file to which to write a URL from which TensorBoard should
+be reachable. The URL will include scheme, hostname, and port, and
+possibly path if `--path_prefix` is set. Useful when setting `--port=0`,
+which automatically selects an unused port. Primarily intended for use
+by scripts; for human consumers, this same value is already printed to
+stderr.
+
+If provided, this should point to an existing file (which will be
+truncated) or a nonexistent file within a directory that exists.
+
+If TensorBoard fails to start (e.g., because the specified port is
+already in use), this file will not be written.
 ''')
 
   def fix_flags(self, flags):

tensorboard/program.py

@@ -207,9 +207,13 @@ def main(self, ignored_argv=('',)):
       return 0
     try:
       server = self._make_server()
+      url = server.get_url()
       sys.stderr.write('TensorBoard %s at %s (Press CTRL+C to quit)\n' %
-                       (version.VERSION, server.get_url()))
+                       (version.VERSION, url))
       sys.stderr.flush()
+      if self.flags.url_file is not None:
+        with open(self.flags.url_file, "w") as outfile:
+          outfile.write("%s\n" % server.get_url())
       server.serve_forever()
       return 0
     except TensorBoardServerException as e: