GitHub Actions step outputs

Guy Waldman

July 12, 2024 (2 months ago)

Time to read:

3 minutes

So you've created a GitHub Actions workflow and you want to reuse the output of one step in another step.

I present an easy way to do that.

Suppose we have the following workflow:

Note

This is a simplified example with better ways to do this, but it should hopefully convey the main idea.

1name: My workflow
2
3on:
4  pull_request:
5    branches: [dev, main, release-*]
6
7jobs:
8  test:
9    name: Run tests
10    steps:
11		  # ... (omitted)
12
13			- name: Run tests
14			  run: npm test
15				env:
16					CPU_CORES: 8 # TODO: Get this from the actual CI environment.
17
18			# ... (omitted)
19

OK, and now you want to handle that TODO, and populate CPU_CORES according to the actual system you're running the test on.
So you write a small script that does this in a cross-platform way (let's say only UNIX-like systems) and call it get-cpu-cores.sh.

1# ... (omitted)
2
3jobs:
4  test:
5    name: Run tests
6    steps:
7		  # ... (omitted)
8
9			- name: Get CPU cores
10				run: ./get-cpu-cores.sh
11
12			- name: Run tests
13			  run: npm test
14				env:
15					CPU_CORES: ??? # TODO: Get this from the actual CI environment.
16
17			# ... (omitted)
18

So now how do we get the output of the get-cpu-cores.sh script? If you want to run it and retain the output, you could do it in a number of ways, but GitHub Actions has a nice built-in way to pass along the outputs of a step:

1# ... (omitted)
2
3jobs:
4  test:
5    name: Run tests
6    steps:
7		  # ... (omitted)
8
9			- name: Get CPU cores
10				id: get-cpu-cores
11				run: ./get-cpu-cores.sh | { read cores; echo "cpu-cores=$cores" } >> $GITHUB_OUTPUT
12
13			- name: Run tests
14			  run: npm test
15				env:
16					CPU_CORES: ${{ steps.get-cpu-cores.outputs.cpu-cores }}
17
18			# ... (omitted)
19

As you can see, we're reading the output of the get-cpu-cores.sh script, and appending cpu-cores=<OUTPUT> (that's what the | { read cores; echo "cpu-cores=$cores" } part does) to the $GITHUB_OUTPUT variable.

You can think of this as an .env file which contains a key-value mapping of outputs from different stpes, and you can append to that. GitHub knows how to parse it and use it as an output for the step with the ID get-cpu-cores.

You can also get more sophisticated! This can now be used as other variables in GitHub Actions, for example only runing steps according to the output of the script:

1# ... (omitted)
2
3jobs:
4  test:
5    name: Run tests
6    steps:
7		  # ... (omitted)
8
9			- name: Get CPU cores
10				id: get-cpu-cores
11				run: ./get-cpu-cores.sh | { read cores; echo "cpu-cores=$cores" } >> $GITHUB_OUTPUT
12
13			- name: Run tests
14			  run: npm test
15				env:
16					CPU_CORES: ${{ steps.get-cpu-cores.outputs.cpu-cores }}
17
18			- name: Run heavy tests
19				run: npm run heavy-tests
20				if: steps.get-cpu-cores.outputs.cpu-cores > 8
21
22			# ... (omitted)
23

This is an example scenario where you run the CI on a matrix of different machines and you want to run the "heavy tests" only on more bulky ones.

GitHub Actions step outputs

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