How can I access GitHub Action environment variables within a Bash script run by the Action?

You can use env at any level also in jobs and steps.

I wrote a test action and a test script to validate it:

The action file:

name: Env tests
on: push

env:
  FOO_ROOT: bar on root

jobs:
  test:
    runs-on: ubuntu-latest
    env:
      FOO_JOB: bar on job
    steps:
      - uses: actions/checkout@v1
      - name: Test envs
        run: ./env-test.sh
        env:
          FOO_STEP: bar on step

The script file:

#!/usr/bin/env bash

echo "FOO_ROOT: $FOO_ROOT"
echo "FOO_JOB: $FOO_JOB"
echo "FOO_STEP: $FOO_STEP"
echo " "
printenv

The results:

FOO_ROOT: bar on root
FOO_JOB: bar on job
FOO_STEP: bar on step

LEIN_HOME=/usr/local/lib/lein
M2_HOME=/usr/share/apache-maven-3.6.3
...

Check my results and, in fact, I don't know why it didn't work on your side because it must work.

If some one else is searching for a solution you have to explicitly pass the environment variables to the bash script. Otherwise they are not available:

name: x-pull-request
on: pull_request
env:
  FOO: bar
jobs:
  test:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v1
      - name: does a thing
        run: ./scripts/do-a-thing.sh $FOO

and in the script you have to map the parameter to a variable.

X=$1
echo "X: $X" # X: default

I know this is old, but I came across this while writing my own GitHub Actions code and figured I'd point out what I believe is the actual issue you're running into.

The problem is you're using the wrong syntax for default variable values.

Incorrect:

myVal=${ENV_VAL:default}

Correct:

# Notice the addition of the hyphen
myVal=${ENV_VAL:-default}

Thus, your script should use:

X=${FOO:-default}
echo "X: $X" # X: default

More details can be found in the Parameter Expansion subsection of the docs.

If you've to execute your script with sudo privileges then make sure to use -E flag so that sudo passes all envs to the script.

name: x-pull-request
on: pull_request
env:
  FOO: bar
jobs:
  test:
    runs-on: ubuntu-latest
    env:
      SOME_ENV: some_val
    steps:
      - uses: actions/checkout@v1
      - name: does a thing
        run: sudo -E ./scripts/do-a-thing.sh