Plugin Development

About 418 wordsAbout 1 min

In Cloud Native Build, a plugin is essentially a Docker image. Below is how to develop an image plugin from scratch using Bash.

This example plugin prints hello world. Basic Docker knowledge is assumed.

Designing the Plugin

Parameter Design

Design the plugin parameters:

text: The text content to be printed to the console

These parameters are passed to the container as environment variables, converted to uppercase with a PLUGIN_ prefix.

Supported Parameter Types

Parameter types support string, number, boolean, one-dimensional array, and plain object.

Configuration example:

.cnb.yml
main:
  push:
    - stages:
        - name: hello world
          image: cnbcool/hello-world
          settings:
            text: hello world
            boolean: true
            number: 123
            array: [hello, world]
            map:
              key: value

These parameters are converted to the following environment variables inside the container:

PLUGIN_TEXT='hello world'
PLUGIN_BOOLEAN='true'
PLUGIN_NUMBER='123'
PLUGIN_ARRAY='hello,world'
PLUGIN_MAP='{"key":"value"}'

Arrays are comma-separated when passed to the container
Plain objects are converted to JSON strings

If the parameter structure is overly complex, consider storing them in a file and loading at runtime, simplifying the design, or splitting into multiple plugins.

Writing the Script

Write a Bash script to print the parameters:

#!/bin/sh
echo "$PLUGIN_TEXT"

Building the Plugin Image

The plugin is packaged as a Docker image for distribution. Create a Dockerfile to package the script and set it as the Entrypoint.

FROM alpine

ADD entrypoint.sh /bin/
RUN chmod +x /bin/entrypoint.sh

ENTRYPOINT /bin/entrypoint.sh

Build the image:

docker build -t cnbcool/hello-world .

Testing the Plugin

Test the plugin locally using docker run, passing parameters via environment variables:

docker run --rm \
  -e PLUGIN_TEXT="hello world" \
  cnbcool/hello-world

Testing Filesystem Access

The plugin can read the build process workspace directory. By default, the build directory is mapped to a plugin directory and set as the workspace:

docker run --rm \
  -e PLUGIN_TEXT="hello world" \
  -v $(pwd):$(pwd) \
  -w $(pwd) \
  cnbcool/hello-world

docker push cnbcool/hello-world