Read JSON file in Github Actions

Use the built-in fromJson(value) (see here: https://docs.github.com/en/actions/learn-github-actions/expressions#fromjson)

Reading a file depends on the shell you're using. Here's an example for sh:

name: Test linux job
on:
  push

jobs:
  testJob:
    name: Test
    runs-on: ubuntu-latest
    steps:
      - id: set_var
        run: |
          content=`cat ./path/to/package.json`
          # the following lines are only required for multi line json
          content="${content//'%'/'%25'}"
          content="${content//$'\n'/'%0A'}"
          content="${content//$'\r'/'%0D'}"
          # end of optional handling for multi line json
          echo "::set-output name=packageJson::$content"
      - run: |
          echo "${{fromJson(steps.set_var.outputs.packageJson).version}}"

Multi line JSON handling as per https://github.community/t5/GitHub-Actions/set-output-Truncates-Multiline-Strings/td-p/37870

GitHub issue about set-env / set-output multi line handling: https://github.com/actions/toolkit/issues/403

Below is a version of the example from Official GHA Docs that includes two changes:

1. Loads json from a file (./your.json)
2. Removes newline characters (Source)
3. Uses fromJson to parse the output and set a matrix variable.

name: build
on: push
jobs:
  job1:
    runs-on: ubuntu-latest
    outputs:
      matrix: ${{ steps.set-matrix.outputs.matrix }}
    steps:
    - id: set-matrix
      run: |
        JSON=$(cat ./your.json)
        echo "::set-output name=matrix::${JSON//'%'/'%25'}"

  job2:
    needs: job1
    runs-on: ubuntu-latest
    strategy:
      matrix: ${{fromJson(needs.job1.outputs.matrix)}}
    steps:
    - run: build

on: [push, pull_request] 
name: Build
jobs:
  build:
    name: Example
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v2
        with: 
          path: './'      
      - run: |
          echo "`jq '.base_config[0].value="Alpha-21"' config.json `" > config.json
          echo "`jq '.base_config[1].value="1.2.14"' config.json`" > config.json
          echo "`jq '.base_config[2].value="29/12/2020"' config.json `" > config.json
     
      - uses: EndBug/add-and-commit@v6
        with:
          message: 'Add the version and date'
          add: '*.json --force'
          cwd: './' 
          token: ${{ secrets.TOKEN }} 

Use a multi line environment variable:

- run: |
    echo 'PACKAGE_JSON<<EOF' >> $GITHUB_ENV
    cat ./package.json >> $GITHUB_ENV
    echo 'EOF' >> $GITHUB_ENV
- run: |
    echo '${{ fromJson(env.PACKAGE_JSON).version }}'

This avoids any need for escaping.

I once used this to get the value from the json data. Hope this helps

  - name: fetch the json value
    run: |
         githubjson=`cat $GITHUB_EVENT_PATH`
         echo $githubjson
         number=`echo $(jq -r '.number' <<< "$githubjson")`
         PRTitle=`echo $(jq -r '.pull_request.title' <<< "$githubjson")`
         PRUrl=`echo $(jq -r '.pull_request.html_url' <<< "$githubjson")`
         PRBody=`echo $(jq -r '.pull_request.body' <<< "$githubjson")`

With Powershell:

- name: Read json
  id: read-json
  shell: pwsh
  run: |
    $json = Get-Content yourfile.json | ConvertFrom-Json
    echo "::set-output name=prop::$(echo $json.prop)"

- run: echo ${{ steps.read-json.outputs.prop}}

Inspired by answer from @dastrobu which adds key/val to $GITHUB_ENV and using jq to transform/minify package.json to a single line:

- run: echo "PACKAGE_JSON=$(jq -c . < package.json)" >> $GITHUB_ENV
- run: echo '${{ fromJson(env.PACKAGE_JSON).version }}'