What is a Viash Component?

We have provided a simple Hello World component as an introduction to Viash. Follow the steps below to learn how to run a component and get a result back from it.


To follow along with this tutorial, you need to have this software installed on your machine:

Running the component

You can run a simple ‘Hello World’ component by running the following command:

viash run $URL
Hello world!

Every component accepts –help as an option, which outputs a description of the component and a list of accepted options. Run the command below to see the help for the ‘Hello World’ component:

viash run $URL -- --help
hello_world <not versioned>
A very simple 'Hello world' component.

        type: string, multiple values allowed

        type: string
        default: Hello world!

As you can see, the ‘Hello World’ component accepts several string arguments and a --greeter option. Run the command below and replace NAME with your name to see what happens:

viash run $URL -- NAME. --greeter="Hello there,"
Hello there, NAME.

How does the hello world component work?

When you call ‘viash run’, Viash parses the config.vsh.yaml file, which is a meta description of the component written in the yaml serialization language:

  name: hello_world
  description: A very simple 'Hello world' component.
  - type: string
    name: input
    multiple: true
    multiple_sep: " "
  - type: string
    name: --greeter
    default: "Hello world!"
  - type: bash_script
    path: script.sh
  - type: bash_script
    path: test.sh
  - type: native
  - type: docker
    image: bash:4.0
  - type: docker
    id: alpine
    image: alpine
      - type: apk
        packages: [ bash ]

This config file describes the behavior of a script and the platform it runs on. Every config file consists of two main sections: functionality and platforms.


The functionality attribute describes the core functionality of the component, such as its inputs, outputs, arguments, and extra resources. The ‘Hello World’ component accepts two arguments:

  • A string named input that accepts multiple space-separated arguments.
  • A second string named –greeter which defaults to “Hello world!”.

These arguments are passed on to the resources. In this case, there’s a single reference to a file named script.sh. This file is the ‘brain’ of the component, it’s small Bash script which prints out two environment values: par_input and par_greeter:

par_input="I am debug!"
par_greeter="Hello world!"

echo $par_greeter $par_input

Any variables defined in the config file will be automatically generated between the ## VIASH START and ## VIASH END lines. You can add pre-defined values here for debugging purposes by adding the variables and adding the par_ prefix, their values will automatically be replaced at runtime with parameter values from the CLI.

Finally, there’s a tests section to put your test scripts. It’s a good practice to write tests and run these every time you update your component and/or the resources. The component creating tutorials on the left include an example.


The platforms attribute specifies the requirements to execute the component on one or more platforms:

  - type: native
  - type: docker
    image: bash:4.0
  - type: docker
    id: alpine
    image: alpine
      - type: apk
        packages: [ bash ]

The list of currently supported platforms are Native, Docker, and Nextflow. If no platforms are specified, a native platform with no system requirements is assumed.

You can specify what platform a component should run on by passing the -p or --platform option. For example, try running the following command:

viash run -p native $URL -- NAME. --greeter="Hello there,"
Hello there, NAME.

The results should be exactly the same as Viash automatically picks the first platform when you don’t pass the platform option, in this case that’s native.

Exporting a component as an executable

Components can be exported to executables, making it easy to share scripted functionality without the need to have Viash installed on the target system.
Run the following command to make Viash parse the config file and export the result to an executable called hello_world in a (new) folder named my_hello_world:

viash build $URL -o my_hello_world

You can now run the following command to run the generated executable:

my_hello_world/hello_world NAME. --greeter="Hello there,"
Hello there, NAME.

What’s next?

Now that you understand the basics of how Viash works, take a look at how to write your own component: