# Incremental migration from cross-spawn

> Swap cross-spawn for bellpull/cross-spawn in one import, then move call sites to run() one at a time, and know what changes at each step.

Source: https://bellpull.interlace.tools/docs/recipes/incremental-migration

## 1. Change the import

```diff
- import spawn from 'cross-spawn';
+ import spawn from 'bellpull/cross-spawn';
```

`bellpull/cross-spawn` passes cross-spawn 7.0.6's own suite, 68 of 68, and `require()` works
too. The dependency tree goes from four packages to none. `npx burgee migrate --dry-run` lists
the imports it would rewrite.

## 2. Move a call site to run()

The drop-in returns a `ChildProcess`, and the caller collects output and waits for `close`.
`run()` does that and returns the result:

```js title="migrated.mjs"
import spawn from 'bellpull/cross-spawn';
import { run } from 'bellpull';

import { install, runtimeWith } from './tools.mjs';

install();
const runtime = runtimeWith('bin');

// Before: a ChildProcess, and the bookkeeping around it.
const child = spawn('greet', ['1'], { env: runtime.env });
let out = '';
child.stdout.on('data', (chunk) => (out += chunk));
const code = await new Promise((resolve) => child.on('close', resolve));
console.log('spawn:', code, out.trim());

// After: one value.
const result = await run('greet', ['1'], { runtime });
console.log('run:  ', result.code, result.stdout.trim(), result.ok);
```

```text title="node migrated.mjs"
spawn: 1 hello from bin
run:   1 hello from bin false
```

What changes at a call site that moves:

- **A deadline appears.** `run()` kills a child after 30 s unless you pass `timeout`; a
  long-running child — a dev server, a watcher — needs `timeout: 0`.
- **A missing command rejects** with `NotFoundError`, where `spawn` emitted an `error` event.
- **Output is collected.** For a child whose output should go straight to the terminal, pass
  `stdio: 'inherit'`; the result then has empty `stdout` and `stderr`.
- **The environment is explicit.** The runtime's `env` and `cwd` are the child's, unless the
  call passes its own.

## From which

```diff
- import which from 'which';
+ import which from 'bellpull/node-which';
```

graded 5 of 5 by node-which's suite. Moving on to `bellpull/which` changes the answer's shape
(`{ path, from, ext }` instead of a string) and stops searching empty and relative `PATH`
entries — see [Finding the executable](/docs/guides/resolution).