Output Specification

As it was mentioned in Shell Command Tasks, the user can customize the input and output for the ShellCommandTask. In this section, the output specification will be covered.

Instead of using field with output_file_template in the customized input_spec to specify an output field, a customized output_spec can be used, e.g.:

output_spec = SpecInfo(
                    "output_file_template": "{inp1}",
                    "help_string": "output file",
                    "requires": ["inp1", "inp2"]


Similarly as for input_spec, in order to create an output specification, a new SpecInfo object has to be created. The field name specifies the type of the spec and it should be always “Output” for the output specification. The field bases specifies the “base specification” you want to use (can think about it as a parent class) and it will usually contains ShellOutSpec only, unless you want to build on top of your other specification (this will not be cover in this section). The part that should be always customised is the fields part. Each element of the fields is a separate output field that is added to the specification. In this example, a three-elements tuple - with name, type and dictionary with additional information - is used. See Input Specification for other recognized syntax for specification’s fields and possible types.


The metadata dictionary for output_spec can include:

help_string (str, mandatory):

A short description of the input field. The same as in input_spec.

mandatory (bool, default: False):

If True the output file has to exist, otherwise an error will be raised.

output_file_template (str):

If provided the output file name (or list of file names) is created using the template. The template can use other fields, e.g. {file1}. The same as in input_spec.

output_field_name (str, used together with output_file_template)

If provided the field is added to the output spec with changed name. The same as in input_spec.

keep_extension (bool, default: True):

A flag that specifies if the file extension should be removed from the field value. The same as in input_spec.

requires (list):

List of field names that are required to create a specific output. The fields do not have to be a part of the output_file_template and if any field from the list is not provided in the input, a NOTHING is returned for the specific output. This has a different meaning than the requires form the input_spec.

callable (function):

If provided the output file name (or list of file names) is created using the function. The function can take field (the specific output field will be passed to the function), output_dir (task output_dir will be used), stdout, stderr (stdout and stderr of the task will be sent) inputs (entire inputs will be passed) or any input field name (a specific input field will be sent).