nipype
diff --git a/‎pydra/engine/task.py
Lines changed: 8 additions & 2 deletions b/‎pydra/engine/task.py
Lines changed: 8 additions & 2 deletions
diff --git a/‎pydra/engine/tests/test_shelltask.py
Lines changed: 34 additions & 0 deletions b/‎pydra/engine/tests/test_shelltask.py
Lines changed: 34 additions & 0 deletions
@@ -458,7 +458,11 @@ def _command_pos_args(self, field):
                 cmd_add.append(argstr)
         else:
             sep = field.metadata.get("sep", " ")
-            if argstr.endswith("...") and isinstance(value, list):
+            if (
+                argstr.endswith("...")
+                and isinstance(value, ty.Iterable)
+                and not isinstance(value, (str, bytes))
+            ):
                 argstr = argstr.replace("...", "")
                 # if argstr has a more complex form, with "{input_field}"
                 if "{" in argstr and "}" in argstr:
@@ -474,7 +478,9 @@ def _command_pos_args(self, field):
             else:
                 # in case there are ... when input is not a list
                 argstr = argstr.replace("...", "")
-                if isinstance(value, list):
+                if isinstance(value, ty.Iterable) and not isinstance(
+                        value, (str, bytes)
+                ):
                     cmd_el_str = sep.join([str(val) for val in value])
                     value = cmd_el_str
                 # if argstr has a more complex form, with "{input_field}"
 
@@ -1674,6 +1674,40 @@ def template_function(inputs):
     assert shelly.output_dir == res.output.file_copy.parent
 
 
+def test_shell_cmd_inputspec_with_iterable():
+    """Test formatting of argstr with different iterable types."""
+
+    input_spec = SpecInfo(
+        name="Input",
+        fields=[
+            (
+                "iterable_1",
+                ty.Iterable[int],
+                {
+                    "help_string": "iterable input 1",
+                    "argstr": "--in1",
+                },
+            ),
+            (
+                "iterable_2",
+                ty.Iterable[str],
+                {
+                    "help_string": "iterable input 2",
+                    "argstr": "--in2...",
+                },
+            ),
+        ],
+        bases=(ShellSpec,),
+    )
+
+    task = ShellCommandTask(name="test", input_spec=input_spec, executable="test")
+
+    for iterable_type in (list, tuple, set):
+        task.inputs.iterable_1 = iterable_type(range(3))
+        task.inputs.iterable_2 = iterable_type(["bar", "foo"])
+        assert task.cmdline == "test --in1 0 1 2 --in2 bar --in2 foo"
+
+
 @pytest.mark.parametrize("results_function", [result_no_submitter, result_submitter])
 def test_shell_cmd_inputspec_copyfile_1(plugin, results_function, tmpdir):
     """shelltask changes a file in place,