GitHub action - shell-with-post-hook. No NPM modules.

action.yml

name: 'Shell with Post Hook'
description: 'Run shell script with guaranteed post hook'
inputs:
  main:
    description: 'Main script to run (aborts job if exit code>10)'
    required: true
  post:
    description: 'Post script to run (if $MAIN_EXIT_CODE<11)'
    required: true
  shell:
    description: 'Shell'
    required: false
    default: 'bash'
outputs:
  main-exit:
    description: 'Exit code from main script'
runs:
  using: 'node20'
  main: 'index.js'
  post: 'index.js'

index.js

// index.js - ONE script for both main AND post
const { execSync } = require('child_process');

function shell(command) {
    try {
        execSync(command, { stdio: 'inherit' });
        return 0;
    } catch (error) {
        return error.status || 1;
    }
}

const bash = process.env.INPUT_SHELL || 'bash';
const actionId = process.env.GITHUB_ACTION || 'action';
console.log(`ActionId=${actionId}`);

const tempDir = process.env.GITHUB_TEMP || process.env.RUNNER_TEMP || '/tmp';
const scriptPrefix = `/${tempDir}/${actionId}-script-`;
const mainScriptFile = `${scriptPrefix}main.sh`;
const postScriptFile = `${scriptPrefix}post.sh`;

// Check which phase we're in: if there's a post file, run it.
const isPost = shell(`test -f ${postScriptFile}`) === 0;

if (isPost) {
    console.log(`Running post script... ${postScriptFile}`);
    shell(`${bash} ${postScriptFile}`);
    shell(`rm -f ${postScriptFile}`);
    console.log(`... post script run.`);
} else {
    console.log(`Running main script... ${mainScriptFile}`);
    shell(`cat > ${mainScriptFile} << 'EOF'
${process.env.INPUT_MAIN}
EOF`);
    const exitCode = shell(`${bash} ${mainScriptFile}`);
    shell(`rm -f ${mainScriptFile}`);
    console.log(`... main script run, writing post-hook script.`);

    shell(`cat > ${postScriptFile} << 'EOF'
export MAIN_EXIT_CODE=${exitCode}
${process.env.INPUT_POST}
EOF`);

    shell(`echo "main-exit=${exitCode}" >> $GITHUB_OUTPUT`);

    process.exit(exitCode >= 10 ? exitCode : 0);
}