cond_exec.nf

#!/usr/bin/env nextflow

params.samplel = false

process test_sample {

        when:
        params.sample

        script:
        """
        echo "sample"
        """
}

process test_sample_list {

        when:
        params.samplel

        script:
        """
        echo "sample list"
        """
}

//Other example:
// There is the option of running the script with a single sample passed
// in command line or passing a file with sample ids
params.sample = false
params.samplel = false

process test_sample {

        when:
        params.sample

        script:
        """
        echo "sample"
        """
}

if (params.samplel) {
   Channel.fromPath(params.samplel)
        .splitText()
        .set { sample_list }

        process test_sample_list {
                input:
                val s from sample_list
                when:
                params.samplel

                script:
                """
                echo "sample list: $s"
                """
        }
}