Why is command output not passed to next command, github action? - github-actions

Question

I'm trying to set the output from a command to an other command in a github action in the following way:
- name: 'Checkout source code'
uses: actions/checkout#v2
with:
fetch-depth: 0
- name: 'Create delta packages for new, modified or deleted metadata'
run: |
mkdir changed-sources
echo "DIFF=$(git rev-parse --short origin/dev)" >> $GITHUB_ENV
echo "${{ env.DIFF }}"
sfdx sgd:source:delta --to "HEAD" --from ${{ env.DIFF }} --output changed-sources/ --generate-delta --source force-app/
But for some reason it's not working, I receive the following output:
steps:
mkdir changed-sources
echo "DIFF=$(git rev-parse --short origin/dev)" >> $GITHUB_ENV
echo ""
sfdx sgd:source:delta --to "HEAD" --from --output changed-sources/ --generate-delta --source force-app/
What I'm I missing, any help would be highly appreciated!

Answer 1

According to the official GitHub documentation:
If you generate a value in one step of a job, you can use the value in subsequent steps of the same job by assigning the value to an existing or new environment variable and then writing this to the GITHUB_ENV environment file.
In your example, you are using the env variable in the same step.
You should therefore access the env variable in another step instead of in the same one.
Example:
- name: 'Checkout source code'
uses: actions/checkout#v2
with:
fetch-depth: 0
- name: 'Create delta packages for new, modified or deleted metadata'
run: |
mkdir changed-sources
echo "DIFF=$(git rev-parse --short origin/dev)" >> $GITHUB_ENV
- name: NEW STEP
run: |
echo "${{ env.DIFF }}"
sfdx sgd:source:delta --to "HEAD" --from ${{ env.DIFF }} --output changed-sources/ --generate-delta --source force-app/