Run Claude Code from a script

The steps

One step at a time. When a step’s check matches, tick it and the next one opens.

1. 01

   Step 1 of 4: Ask one question

   In the project folder, pass a question after `claude -p`.

   TerminalCopy

   Run: $ claude -p "Which command runs this project's tests?"

   – What happens: Claude reads the project, prints its answer and exits.

   Check it worked

   The answer names the test command (npm test in the practice project). echo $? (PowerShell: $LASTEXITCODE) prints 0.

   It worked — next step Not yet
2. 02

   Step 2 of 4: Pipe another command’s output in

   Send a command’s output to Claude on stdin and ask about it.

   claude -p reads stdin, so any command’s output can go to Claude. Here it is a quote; in your own project, pipe a log or git diff.

   TerminalCopy

   Run: $ node src/cli.js design:10 dev:24 --discount 10 | claude -p "Check the maths in this quote. Reply OK or name the wrong line."

   Check it worked

   Claude replies OK. The sample quote is correct.

   It worked — next step Not yet Back
3. 03

   Step 3 of 4: Pre-approve one tool and get JSON back

   List the tools Claude may use without asking, and ask for JSON output.

   With -p, nobody is there to answer a permission prompt, so anything that would prompt is refused. --allowedTools approves exactly what the task needs.

   TerminalCopy

   Run: $ claude -p "Run the tests and say how many passed" --allowedTools "Bash(npm test *)" --output-format json

   – What happens: One JSON object: the answer in result, plus session_id and an estimated cost in total_cost_usd.

   Check it worked

   The JSON’s result field says how many tests passed.

   It worked — next step Not yet Back
4. 04

   Step 4 of 4: Let a script read the answer

   Run the kit script, which asks for JSON in a fixed shape.

   The kit’s script pipes the quote to claude -p with --json-schema, then reads ok and problems from the structured_output field.

   scripts/check-quote.mjs · JavaScriptCopy

   /**
    * Day 22: run Claude Code from a script.
    *
    *   node scripts/check-quote.mjs
    *
    * Prints the sample quote with src/cli.js, pipes it to `claude -p` and asks
    * Claude to check the arithmetic. `--output-format json` with `--json-schema`
    * makes Claude Code return the answer in the `structured_output` field, so
    * this script reads two fields instead of parsing prose.
    *
    * Exit codes: 0 the quote checks out, 1 Claude found a problem, 2 Claude
    * could not be run or gave no answer in the expected shape.
    *
    * Flags and fields: https://code.claude.com/docs/en/headless (checked
    * 9 October 2026).
    *
    * On Windows, Node starts `claude.exe` from the native installer. An npm
    * install provides `claude.cmd`, which Node can't start without a shell.
    */
   import { spawnSync } from 'node:child_process';
   import { fileURLToPath } from 'node:url';
   
   const ROOT = fileURLToPath(new URL('..', import.meta.url));
   const SAMPLE = ['src/cli.js', 'design:10', 'dev:24', '--discount', '10'];
   
   const SCHEMA = JSON.stringify({
     type: 'object',
     properties: {
       ok: { type: 'boolean' },
       problems: { type: 'array', items: { type: 'string' } },
     },
     required: ['ok', 'problems'],
   });
   
   const PROMPT = [
     'The text on stdin is a price quote.',
     'Each line is hours times an hourly rate.',
     'The discount is a percentage of the subtotal.',
     'VAT is a percentage of the amount after the discount.',
     'Check every line and every total.',
     'Set ok to true only if all of them are right.',
     'List each wrong line in problems, with the value it should have.',
   ].join(' ');
   
   function fail(message) {
     console.error(message);
     process.exit(2);
   }
   
   const quote = spawnSync(process.execPath, SAMPLE, { cwd: ROOT, encoding: 'utf8' });
   if (quote.status !== 0) fail(`Could not print the sample quote:\n${quote.stderr}`);
   
   const run = spawnSync('claude', ['-p', PROMPT, '--output-format', 'json', '--json-schema', SCHEMA], {
     cwd: ROOT,
     input: quote.stdout,
     encoding: 'utf8',
     maxBuffer: 10 * 1024 * 1024,
   });
   
   if (run.error) fail(`Could not start claude (${run.error.code}). Check that \`claude --version\` works in this terminal.`);
   if (run.status !== 0) fail(`claude -p exited with code ${run.status}:\n${(run.stdout + run.stderr).trim()}`);
   
   let answer;
   try {
     answer = JSON.parse(run.stdout).structured_output;
   } catch {
     fail(`claude -p did not print JSON:\n${run.stdout.trim()}`);
   }
   if (!answer || typeof answer.ok !== 'boolean' || !Array.isArray(answer.problems)) {
     fail(`No answer in the expected shape:\n${run.stdout.trim()}`);
   }
   
   if (answer.ok) {
     console.log('Quote checked: no problems found.');
   } else {
     console.log('Quote checked: Claude found problems.');
     for (const problem of answer.problems) console.log(`- ${problem}`);
     process.exitCode = 1;
   }
   

   TerminalCopy

   Run: $ node scripts/check-quote.mjs

   Output: Quote checked: no problems found.

   Check it worked

   The script prints Quote checked: no problems found. and exits with 0.

   If it didn’t work

   The script says it could not start claude

   Check that claude --version works in the same terminal. On Windows, Node can start the native installer’s claude.exe but not npm’s claude.cmd: use the Day 1 installer.

   The result says you are not logged in

   /login doesn’t work with -p. Start claude once, run /login, then try again.

   It worked — finish Not yet Back