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Don't mark the Build as failed if a given step would fail (ignore the error of the Step)

To do this you have to switch to bitrise.yml mode (open the Workflow Editor on bitrise.io -> left side: click on bitrise.yml to switch to the interactive bitrise.yml editor).

All you have to do is: add an is_skippable: true flag to the Step.

An example script step which, although fails, it won't "break" the build:

- script:
    is_skippable: true
    inputs:
    - content: |-
        #!/bin/bash
        echo "Failing Step."
        exit 1
        # exit 1 would mark this step as Failed, but it won't break the Build
        #  because of the is_skippable: true flag / property

You can find more examples on GitHub.