Blocks and Directives

Overview

• Tutorial: 10 min

  Objectives:

  1. Learn the difference between blocks and directives in NextFlow.

In Nextflow, blocks and directives serve different purposes in defining and controlling workflow execution.

What are Blocks?

Blocks define logical sections of a Nextflow script, such as process, workflow, and channel. They contain execution logic and define how tasks are structured.

Common Blocks:

• process: Defines a computational task.

• workflow: Manages the execution flow of processes.

• channel: Defines input/output data channels.

Example of Blocks:

process sayHello {
    script:
    """
    echo "Hello, Nextflow!" > output.txt
    """
}

workflow {
    sayHello()
}

What are Directives?

Directives are configuration instructions inside a process block. They control execution behavior, such as input/output handling and resource allocation.

Common Directives:

• input: Defines input parameters/files.

• output: Specifies expected output files or channels.

• script: Specifies the execution script.

• publishDir: Defines where output files are stored.

• cpus: Specifies the number of CPU cores.

• memory: Allocates the required memory.

Example of Directives:

process sayHello {
    output:
        path 'output.txt'  // Defines output file

    script:
    """
    echo "Hello, Nextflow!" > output.txt
    """
}

Key Differences

Blocks vs. Directives in Nextflow

Feature	Blocks	Directives
Purpose	Define script sections.	Configure process execution.
Scope	Used at the top level (e.g., process, workflow).	Used inside process blocks.
Function	Controls execution flow.	Controls execution settings.
Example	process, workflow, channel.	input, output, script, cpus, memory.