How-to Guide: Capturing stdout with CWL¶

This guide explains how to capture the standard output (stdout) of a CommandLineTool in CWL.

The example focuses on the stdout and outputs blocks to save the tool's output to a file and make it available for further use.

Objective¶

Capture the stdout of a command and make it accessible as a workflow output using:

stdout: message

outputs:

  info:

    type: string

    outputBinding:

      glob: message

      loadContents: true

      outputEval: $(self[0].contents)

Steps¶

Understand the stdout Block

The stdout field specifies the file where the standard output of the CommandLineTool should be redirected. In this example:

stdout: message

Understand the outputs Block

The outputs block defines how to extract information from the stdout file:

outputs:

  info:

    type: string

    outputBinding:

      glob: message

      loadContents: true

      outputEval: $(self[0].contents)

Review the CWL File

Here’s the complete example (capture-stdout.cwl):

cwlVersion: v1.2

$graph:

  - class: Workflow

    id: main

    inputs:

      tif:

        type: string

    outputs:

      info:

        outputSource: step-info/info

        type: string

    steps:

      step-info:

in:

          geotif: tif

        out:

          - info

        run: "#rio"

  - class: CommandLineTool

    id: rio

    requirements:

      NetworkAccess:

        networkAccess: true

      InlineJavascriptRequirement: {}

      DockerRequirement:

        dockerPull: ghcr.io/eoap/how-to/rio:1.0.0

    baseCommand: rio

    arguments:

      - info

      - $(inputs.geotif)

    inputs:

      geotif:

        type: string

    stdout: message

    outputs:

      info:

        type: string

        outputBinding:

          glob: message

          loadContents: true

          outputEval: $(self[0].contents)

It's graphical representation:

No description has been provided for this image

Provide an Input

For this example, use the input file (or command-line argument) to specify the GeoTIFF:

cwltool capture-stdout.cwl \
    --tif "https://sentinel-cogs.s3.us-west-2.amazonaws.com/sentinel-s2-l2a-cogs/10/T/FK/2021/7/S2B_10TFK_20210713_0_L2A/TCI.tif"

Execute the CWL Tool

Run the command in your terminal:

cwltool capture-stdout.cwl \
    --tif "https://sentinel-cogs.s3.us-west-2.amazonaws.com/sentinel-s2-l2a-cogs/10/T/FK/2021/7/S2B_10TFK_20210713_0_L2A/TCI.tif" > output.json

INFO /opt/hostedtoolcache/Python/3.13.3/x64/bin/cwltool 3.1.20250110105449

INFO Resolved '../cwl-workflows/capture-stdout.cwl' to 'file:///home/runner/work/how-to/how-to/cwl-workflows/capture-stdout.cwl'

INFO [workflow ] start

INFO [workflow ] starting step step-info

INFO [step step-info] start

INFO [job step-info] /tmp/0un8bj3n$ docker \

    run \

    -i \

    --mount=type=bind,source=/tmp/0un8bj3n,target=/xKzbcZ \

    --mount=type=bind,source=/tmp/3ddz4j98,target=/tmp \

    --workdir=/xKzbcZ \

    --read-only=true \

    --log-driver=none \

    --user=1001:118 \

    --rm \

    --cidfile=/tmp/690ht63y/20250620071648-784272.cid \

    --env=TMPDIR=/tmp \

    --env=HOME=/xKzbcZ \

    ghcr.io/eoap/how-to/rio:1.0.0 \

    rio \

    info \

    https://sentinel-cogs.s3.us-west-2.amazonaws.com/sentinel-s2-l2a-cogs/10/T/FK/2021/7/S2B_10TFK_20210713_0_L2A/TCI.tif > /tmp/0un8bj3n/message

INFO [job step-info] Max memory used: 39MiB

INFO [job step-info] completed success

INFO [step step-info] completed success

INFO [workflow ] completed success

INFO Final process status is success

Expected Output¶

The rio info command processes the GeoTIFF file, and its output is written to the message file. The workflow then captures this file’s contents as the info output.

Sample output:

  "info": "{\"blockxsize\": 1024, \"blockysize\": 1024, \"bounds\": [600000.0, 4390200.0, 709800.0, 4500000.0], \"colorinterp\": [\"red\", \"green\", \"blue\"], \"compress\": \"deflate\", \"count\": 3, \"crs\": \"EPSG:32610\", \"descriptions\": [null, null, null], \"driver\": \"GTiff\", \"dtype\": \"uint8\", \"height\": 10980, \"indexes\": [1, 2, 3], \"interleave\": \"pixel\", \"lnglat\": [-121.18160686746373, 40.1419732942866], \"mask_flags\": [[\"nodata\"], [\"nodata\"], [\"nodata\"]], \"nodata\": 0.0, \"res\": [10.0, 10.0], \"shape\": [10980, 10980], \"tiled\": true, \"transform\": [10.0, 0.0, 600000.0, 0.0, -10.0, 4500000.0, 0.0, 0.0, 1.0], \"units\": [null, null, null], \"width\": 10980}\n"

Key Takeaways¶

stdout Field: Redirects the standard output to a specified file.
outputs Block:
- Uses glob to locate the file.
- Loads the file's contents into memory with loadContents: true.
- Extracts and evaluates the contents with outputEval.

By focusing on the stdout and outputs blocks, this guide helps you capture and use command outputs in CWL workflows.