diffoscope/comparators/utils/file.py

@@ -437,16 +437,18 @@ class File(object, metaclass=abc.ABCMeta):
                     )
             except subprocess.CalledProcessError as e:
                 difference = self.compare_bytes(other, source=source)
-                if e.output:
-                    output = re.sub(r'^', '    ', e.output, flags=re.MULTILINE)
-                else:
-                    output = '<none>'
                 cmd = ' '.join(e.cmd)
                 if difference is None:
                     return None
+                output = '<none>'
+                if e.output:
+                    output = '\n{}'.format(
+                        re.sub(r'^', '    ', e.output, flags=re.MULTILINE)
+                    )
                 difference.add_comment(
-                    "Command `%s` exited with %d. Output:\n%s"
-                    % (cmd, e.returncode, output)
+                    "Command `{}` exited with return code {}. Output: {}".format(
+                        cmd, e.returncode, output
+                    )
                 )
             except RequiredToolNotFound as e:
                 difference = self.compare_bytes(other, source=source)

diffoscope/feeders.py

@@ -92,8 +92,17 @@ def from_command(command):
             end_nl = feeder(out_file)
             returncode = command.returncode
         if returncode not in (0, -signal.SIGTERM):
+            # On error, default to displaying all lines of standard output.
+            output = command.stderr
+            if not output and command.stdout:
+                # ... but if we don't have, return the first line of the
+                # standard output.
+                output = '{}{}'.format(
+                    command.stdout[0].decode('utf-8', 'ignore').strip(),
+                    '\n[…]' if len(command.stdout) > 1 else '',
+                )
             raise subprocess.CalledProcessError(
-                returncode, command.cmdline(), output=command.stderr
+                returncode, command.cmdline(), output=output
             )
         return end_nl