# Creating a subprocess

## Concepts

- Deno is capable of spawning a subprocess via
  [Deno.run](https://doc.deno.land/builtin/stable#Deno.run)
- `--allow-run` permission is required to spawn a subprocess
- Spawned subprocesses do not run in a security sandbox
- Communicate with the subprocess via the
  [stdin](https://doc.deno.land/builtin/stable#Deno.stdin),
  [stdout](https://doc.deno.land/builtin/stable#Deno.stdout) and
  [stderr](https://doc.deno.land/builtin/stable#Deno.stderr) streams

## Simple example

This example is the equivalent of running `'echo hello'` from the command line.

```ts
/**
 * subprocess_simple.ts
 */

// create subprocess
const p = Deno.run({
  cmd: ["echo", "hello"],
});

// await its completion
await p.status();
```

Run it:

```shell
$ deno run --allow-run ./subprocess_simple.ts
hello
```

## Security

The `--allow-run` permission is required for creation of a subprocess. Be aware
that subprocesses are not run in a Deno sandbox and therefore have the same
permissions as if you were to run the command from the command line yourself.

## Communicating with subprocesses

By default when you use `Deno.run()` the subprocess inherits `stdin`, `stdout`
and `stderr` of the parent process. If you want to communicate with started
subprocess you can use `"piped"` option.

```ts
/**
 * subprocess.ts
 */
const fileNames = Deno.args;

const p = Deno.run({
  cmd: [
    "deno",
    "run",
    "--allow-read",
    "https://deno.land/std@$STD_VERSION/examples/cat.ts",
    ...fileNames,
  ],
  stdout: "piped",
  stderr: "piped",
});

const { code } = await p.status();

if (code === 0) {
  const rawOutput = await p.output();
  await Deno.stdout.write(rawOutput);
} else {
  const rawError = await p.stderrOutput();
  const errorString = new TextDecoder().decode(rawError);
  console.log(errorString);
}

Deno.exit(code);
```

When you run it:

```shell
$ deno run --allow-run ./subprocess.ts <somefile>
[file content]

$ deno run --allow-run ./subprocess.ts non_existent_file.md

Uncaught NotFound: No such file or directory (os error 2)
    at DenoError (deno/js/errors.ts:22:5)
    at maybeError (deno/js/errors.ts:41:12)
    at handleAsyncMsgFromRust (deno/js/dispatch.ts:27:17)
```