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# 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.
- Use a specific shell by providing its path/name and its string input switch,
e.g. `Deno.run({cmd: ["bash", "-c", '"ls -la"']});`
## 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)
```
|
