ShellJob: Raise when < or > are specified in arguments

README.md

@@ -153,6 +153,9 @@ print(results['stdout'].get_content())
 ```
 which prints `string a`.
 
+N.B.: one might be tempted to simply define the `arguments` as `['<', '{input}']`, but this won't work as the `<` symbol will be quoted and will be read as a literal command line argument, not as the redirection symbol.
+This is why passing the `<` in the `arguments` input will result in a validation error.
+
 ### Defining output files
 When the shell command is executed, AiiDA captures by default the content written to the stdout and stderr file descriptors.
 The content is wrapped in a `SinglefileData` node and attached to the `ShellJob` with the `stdout` and `stderr` link labels, respectively.

src/aiida_shell/calculations/shell.py

@@ -30,7 +30,9 @@ def define(cls, spec: CalcJobProcessSpec):  # type: ignore[override]
         super().define(spec)
         spec.input_namespace('nodes', valid_type=Data, required=False, validator=cls.validate_nodes)
         spec.input('filenames', valid_type=Dict, required=False, serializer=to_aiida_type)
-        spec.input('arguments', valid_type=List, required=False, serializer=to_aiida_type)
+        spec.input(
+            'arguments', valid_type=List, required=False, serializer=to_aiida_type, validator=cls.validate_arguments
+        )
         spec.input('outputs', valid_type=List, required=False, serializer=to_aiida_type, validator=cls.validate_outputs)
         spec.input(
             'metadata.options.filename_stdin',
@@ -87,6 +89,24 @@ def validate_nodes(cls, value: t.Mapping[str, Data], _) -> str | None:
             except Exception as exception:  # pylint: disable=broad-except
                 return f'Casting `value` to `str` for `{key}` in `nodes` excepted: {exception}'
 
+    @classmethod
+    def validate_arguments(cls, value: List, _) -> str | None:
+        """Validate the ``arguments`` input."""
+        if not value:
+            return None
+
+        elements = value.get_list()
+
+        if any(not isinstance(element, str) for element in elements):
+            return 'all elements of the `arguments` input should be strings'
+
+        if '<' in elements:
+            var = 'metadata.options.filename_stdin'
+            return f'`<` cannot be specified in the `arguments`; to redirect a file to stdin, use the `{var}` input.'
+
+        if '>' in elements:
+            return 'the symbol `>` cannot be specified in the `arguments`; stdout is automatically redirected.'
+
     @classmethod
     def validate_outputs(cls, value: List, _) -> str | None:
         """Validate the ``outputs`` input."""

tests/calculations/test_shell.py

@@ -207,6 +207,20 @@ def value_raises(self):
         generate_calc_job('core.shell', {'code': generate_code(), 'nodes': nodes})
 
 
+@pytest.mark.parametrize(
+    'arguments, message', (
+        (['string', 1], r'.*all elements of the `arguments` input should be strings'),
+        (['string', {input}], r'.*all elements of the `arguments` input should be strings'),
+        (['<', '{filename}'], r'`<` cannot be specified in the `arguments`.*'),
+        (['{filename}', '>'], r'the symbol `>` cannot be specified in the `arguments`.*'),
+    )
+)
+def test_validate_arguments(generate_calc_job, generate_code, arguments, message):
+    """Test the validator for the ``arguments`` argument."""
+    with pytest.raises(ValueError, match=message):
+        generate_calc_job('core.shell', {'code': generate_code(), 'arguments': arguments})
+
+
 def test_build_process_label(generate_calc_job, generate_code):
     """Test the :meth:`~aiida_shell.calculations.shell_job.ShellJob.build_process_label` method."""
     computer = 'localhost'