Difference between PUT and OUTPUT steps in Concourse - continuous-integration

Question

Could someone tell me the difference between the PUT step and the OUTPUT step in Concourse? For example, in the following type of YAML files why do we need a put step after a get? Can't we use output instead of put? If not what are the purposes of each two?
jobs:
- name: PR-Test
plan:
- get: some-git-pull-request
trigger: true
- put: some-git-pull-request
params:
context: tests
path: some-git-pull-request
status: pending
....
<- some more code to build ->
....

Answer 1

The purpose of a PUT step is to push to the given resource while an OUTPUT is the result of TASK step.
A task can configure outputs to produce artifacts that can then be propagated to either a put step or to another task step in the same plan.
This means that you send the resource that you are specifying on the GET step to the task as an input, to perform wherever build or scripts executions and the output of that task
is a modified resource that you can later pass to your put step or to another TASK if you don't want to use PUT.
It would also depend on the nature of the defined resource in your pipeline. I'm assuming that you have a git type resource like this:
resources:
- name: some-git-pull-request
type: git
source:
branch: ((credentials.git.branch))
uri: ((credentials.git.uri))
username: ((credentials.git.username))
password: ((credentials.git.pass))
If this is true, the GET step will pull that repo so you can use it as an input for your tasks and if you use PUT against that same resource as you are describing in your sample code, that will push changes to your repo.
Really it depends on the workflow that you want to write but to give an idea it would look something like this:
jobs:
- name: PR-Test
plan:
- get: some-git-pull-request
trigger: true
- task: test-code
config:
platform: linux
image_resource:
type: docker-image
source:
repository: yourRepo/yourImage
tag: latest
inputs:
- name: some-git-pull-request
run:
path: bash
args:
- -exc
- |
cd theNameOfYourRepo
npm install -g mocha
npm test
outputs:
- name: some-git-pull-request-output
Then you can use it on either a PUT
- put: myCloud
params:
manifest: some-git-pull-request-output/manifest.yml
path: some-git-pull-request-output
or a another task whitin the same plan
- task: build-code
config:
platform: linux
image_resource:
type: docker-image
source:
repository: yourRepo/yourImage
tag: latest
inputs:
- name: some-git-pull-request-output
run:
path: bash
args:
- -exc
- |
cd some-git-pull-request-output/
npm install
gulp build
outputs:
- name: your-code-build-output
Hope it helps!