diff --git a/rust2rpm/__main__.py b/rust2rpm/__main__.py
index 0c29555..dc78828 100644
--- a/rust2rpm/__main__.py
+++ b/rust2rpm/__main__.py
@@ -7,6 +7,7 @@ import itertools
 import os
 import shlex
 import shutil
+import sys
 import tarfile
 import tempfile
 import time
@@ -185,6 +186,8 @@ def make_diff_metadata(crate, version, patch=False, store=False):
 def main():
     parser = argparse.ArgumentParser("rust2rpm",
                                      formatter_class=argparse.RawTextHelpFormatter)
+    parser.add_argument("--show-license-map", action="store_true",
+                        help="Print license mappings and exit")
     parser.add_argument("-", "--stdout", action="store_true",
                         help="Print spec and patches into stdout")
     parser.add_argument("-t", "--target", action="store",
@@ -196,10 +199,18 @@ def main():
                         help="Store crate in current directory")
     parser.add_argument("crate", help="crates.io name\n"
                                       "path/to/local.crate\n"
-                                      "path/to/project/")
+                                      "path/to/project/",
+                        nargs="?")
     parser.add_argument("version", nargs="?", help="crates.io version")
     args = parser.parse_args()
 
+    if args.show_license_map:
+        licensing.dump_sdpx_to_fedora_map(sys.stdout)
+        return
+
+    if args.crate is None:
+        parser.error('required crate/path argument missing')
+
     crate, diff, metadata = make_diff_metadata(args.crate, args.version,
                                                patch=args.patch,
                                                store=args.store_crate)
diff --git a/rust2rpm/licensing.py b/rust2rpm/licensing.py
index 1b9015e..964603e 100644
--- a/rust2rpm/licensing.py
+++ b/rust2rpm/licensing.py
@@ -21,6 +21,10 @@ def spdx_to_fedora_map():
                 for line in reader
                 if line['SPDX License Identifier']}
 
+def dump_sdpx_to_fedora_map(file):
+    for k,v in spdx_to_fedora_map().items():
+        print(f"{k} → {v}", file=file)
+
 def translate_license_fedora(license):
     comments = ''
     final = []