Frequently asked questions

This document contains some common FAQ that may not have a place outside this documentation.

Janis Registry

  • Exception: Couldn’t find tool: ‘MyTool’

To ensure your tool is correctly found by the registry (JanisShed), you must ensure that:

  • The tool implements all abstract methods (can be initialised)
  • Available within two levels of the janis-pipelines.tool extension root. This means that it needs to be within one additional import from the base __init__.

For example:

|--     # imports my_directory
|-- my_directory/
|   |-- # imports tool from
|   |--

If you’re still having trouble, use janis spider --trace mytool to give you an indication of why your tool might be missing.

Command tools and the command line

  • Why is my input not being bound onto the command line?

    You need to provide a position or prefix to a :class:janis.ToolInput to be bound on the command line.

  • How do I prefix each individual element in an array for my command line?

    Set prefix_applies_to_all_elements=True on the :class:janis.ToolInput.

  • How do I make sure my file is in the execution directory? / How do I localise my file?

    Set localise_file=True on the :class:janis.ToolInput. Although the localise_file attribute is allowed for Array data types, the WDL translation will become disabled as this behaviour is not well defined.

  • How do I include environment variables within my execution environment?

    These are only available from within a CommandTool, and available by overriding the env_vars(self) method to return a dictionary of string to Union[str, Selector] key-value pairs.

  • Can a janis.ToolInput be marked as streamable?

    Currently there’s no support to mark ToolInput’s as streamable. Although the CWL specification. does support marking inputs as streamable, WDL does not and, there is no engine support.

  • How can I manipulate an input value?

    Janis only provides a limited way of manipulating input values, internally using a StringFormatter.


    • Initialisation
      • StringFormatter("Hello, {name}", name=InputSelector("username"))
    • Concatenation
      • "Hello, " + InputSelector("username")
      • InputSelector("greeting") + StringFormatter(", {name}", name=InputSelector("username"))
  • How do I specify an input multiple times on the command line?

    • Create a ToolInput for the input you want to create.
      • Omit any binding options, ie NO position and NO prefix.
    • Create a ToolArgument with the value `InputSelector(”nameofyourinput”) and the binding options:

    For example:

    class MyTool(CommandTool):
        def inputs(self):
            return [ToolInput("myInput", String)]
        def arguments(selfs):
            return [
                ToolArgument(InputSelector("myInput"), position=1),
                ToolArgument("transformed-" + InputSelector("myInput"), position=2, prefix="--name")
  • How do I ensure environment variables are set within my execution environment?

    You can include the following block within your CommandTool:

    # Within CommandTool
    def env_vars(self):
       return {
           "myvar1": InputSelector("myInput"),
           "myvar2": "constantvalue"
  • How do I make my generated filenames unique for a scatter or from different runs betweens tools?

    Long story short, you can’t.

    But there are a fun few reasons why that’s currently the case:

    • The filenames are generated at transpile time for a tool wrapper. This means that tasks that use this tool will get the same filename, including if your workflow scatters over this task.
    • WDL doesn’t really have a mechanism for achieving dynamic or generated components like this, and CWL was flaky at best.
    • Call caching in Cromwell relies on the command line being constructed, so the generated filenames currently break this, and a purely randomly generated filename would breat this further.
    • We have cascaded filename components on the roadmap, so your filename can be built from a collection of inputs (sort of possible anyway).

Running workflows

  • How do I detect a workflow’s status from the command line?

    You can request metadata for a workflow You can use the following bash command to detect the status.

    janis metadata <dir / wid> |grep status|tr -s ' '|cut -d ' ' -f 2

    Note that if Janis is killed suddenly, it might not have enough time to mark the workflow as failed or terminated. It’s worth checking the last_updated_time, if a workflow hasn’t been updated in a couple of hours, it’s likely Janis has been killed.

    The metadata will have the following keys:


    And the workflow status can have the following states:

  • How can I override a specific tools’ resources, like memory / ram, CPUs, or runtime (coming soon)?

    Long story short, lookup the override key with:

    janis inputs --resources <workflow>  

    It looks like yourworkflow_embeddedtool_runtime_memory: 8. More information: Configuring resources (CPU / Memory).


  • How do I override the container being used?

    You can override the container being used by specifying a container-override, there are two ways to do this:

    • CLI with the syntax: janis [translate|run] --container-override 'MyTool=myorganisation/mytool:0.10.0' mytool
    • API: include container override dictionary: mytool.translate("wdl", container_override={"mytool": "myorganisation/mytool:0.10.0"}), the dictionary has structure: {toolId: container}. The toolId can be an asterisk * to override all tools’ containers.