Creating Files at Runtime


Teaching: 10 min
Exercises: 0 min
  • How do I create required input files from input parameters at runtime?

  • How do I invoke a script rather than just a simple command line?

  • How do I make inputs available to my script?

  • Learn how to create files on the fly during runtime.

  • Learn how to use expressions in bash scripts.

Sometimes you need to create a file on the fly from input parameters, such as tools which expect to read their input configuration from a file rather than the command line parameters, or need a small wrapper shell script.

To generate such files we can use the InitialWorkDirRequirement.


class: CommandLineTool
cwlVersion: v1.0
baseCommand: ["sh", ""]

      - entryname:
        entry: |-
          PREFIX='Message is:'
          MSG="\${PREFIX} $(inputs.message)"
          echo \${MSG}

  message: string
    type: stdout
stdout: output.txt

Any expressions like $(inputs.message) are expanded by the CWL engine before creating the file; here inserting the value at the input message.

Tip: The CWL expressions are independent of any shell variables used later during command line tool invocation. That means that any genuine need for the character $ should be escaped with \, for instance \${PREFIX} above is expanded to ${PREFIX} in the generated file to be evaluated by the shell script instead of the CWL engine.

To test the above CWL tool use this job to provide the input value message:


message: Hello world!

Now invoke cwl-runner with the tool wrapper and the input object on the command line:

$ cwl-runner createfile.cwl echo-job.yml
[job createfile.cwl] /private/tmp/docker_tmphrqxxcdl$ sh \ > /private/tmp/docker_tmphrqxxcdl/output.txt
Could not collect memory usage, job ended before monitoring began.
[job createfile.cwl] completed success
    "example_out": {
        "location": "file:///home/example/output.txt",
        "basename": "output.txt",
        "class": "File",
        "checksum": "sha1$9045abe4bd04dd8ccfe50c6ff61820b784b64aa7",
        "size": 25,
        "path": "/home/example/output.txt"
Final process status is success
$ cat output.txt
Message is: Hello world!

Key Points

  • Use InitialWorkDirRequirement to specify input files that need to be created during tool runtime.