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Run a subprocess with custom env and cwd

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A subprocess normally inherits the environment variables and working directory of its parent. The spawn options can override both, which is useful for passing configuration to a child or running a tool inside a specific directory. The child here is deno eval, so the example is portable.

Deno.spawn takes the command, its arguments, and options. The env option adds variables on top of the inherited environment, and the child's stdout stream offers the same convenience methods as a fetch Response.
const withEnv = Deno.spawn(Deno.execPath(), [
  "eval",
  "console.log(Deno.env.get('GREETING'))",
], {
  env: { GREETING: "hello from parent" },
  stdout: "piped",
});
console.log((await withEnv.stdout.text()).trim()); // hello from parent
await withEnv.status;
With clearEnv set to true the child does not inherit anything, it only gets the variables listed in env. Even PATH is gone, so this is best combined with an absolute path to the executable. Note that the operating system may still inject a few variables of its own.
const cleared = Deno.spawn(Deno.execPath(), [
  "eval",
  "console.log('has PATH:', Deno.env.has('PATH'))",
], {
  env: { GREETING: "hello again" },
  clearEnv: true,
  stdout: "piped",
});
console.log((await cleared.stdout.text()).trim()); // has PATH: false
await cleared.status;
The cwd option sets the working directory the child starts in. The child reports its own Deno.cwd() to prove it. Deno.spawnAndWait runs the command to completion and collects everything in one call.
const withCwd = await Deno.spawnAndWait(Deno.execPath(), [
  "eval",
  "console.log(Deno.cwd())",
], {
  cwd: "/",
  stdout: "piped",
});
console.log(new TextDecoder().decode(withCwd.stdout).trim()); // /
The parent's own working directory is untouched by the child's cwd.
console.log(Deno.cwd() === "/"); // false
The same options exist on the underlying Deno.Command API, which is the right tool when you need to configure a command before spawning it.

Run this example locally using the Deno CLI:

deno run --allow-run https://docs.deno.com/examples/scripts/subprocess_env_cwd.ts

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