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Process Promise

The $ returns a ProcessPromise instance. When resolved, it becomes a ProcessOutput.

js
const p = $`command` // ProcessPromise
const o = await p    // ProcessOutput

By default, $ spawns a new process immediately, but you can delay the start to trigger in manually.

ts
const p = $({halt: true})`command`
const o = await p.run()

stage

Shows the current process stage: initial | halted | running | fulfilled | rejected

ts
const p = $`echo foo`
p.stage // 'running'
await p
p.stage // 'fulfilled'

stdin

Returns a writable stream of the stdin process. Accessing this getter will trigger execution of a subprocess with stdio('pipe').

Do not forget to end the stream.

js
const p = $`while read; do echo $REPLY; done`
p.stdin.write('Hello, World!\n')
p.stdin.end()

By default, each process is created with stdin in inherit mode.

stdout/stderr

Returns a readable streams of stdout/stderr process.

js
const p = $`npm init`
for await (const chunk of p.stdout) {
  echo(chunk)
}

exitCode

Returns a promise which resolves to the exit code of the process.

js
if (await $`[[ -d path ]]`.exitCode == 0) {
...
}

json(), text(), lines(), buffer(), blob()

Output formatters collection.

js
const p = $`echo 'foo\nbar'`

await p.text()        // foo\n\bar\n
await p.text('hex')   //  666f6f0a0861720a
await p.buffer()      //  Buffer.from('foo\n\bar\n')
await p.lines()       // ['foo', 'bar']
await $`echo '{"foo": "bar"}'`.json() // {foo: 'bar'}

[Symbol.asyncIterator]

Returns an async iterator of the stdout process.

js
const p = $`echo "Line1\nLine2\nLine3"`
for await (const line of p) {
  console.log()
}

pipe()

Redirects the output of the process.

js
await $`echo "Hello, stdout!"`
  .pipe(fs.createWriteStream('/tmp/output.txt'))

await $`cat /tmp/output.txt`

pipe() accepts any kind Writable, ProcessPromise or a file path. You can pass a string to pipe() to implicitly create a receiving file. The previous example is equivalent to:

js
await $`echo "Hello, stdout!"`
  .pipe('/tmp/output.txt')

Chained streams becomes thenables, so you can await them:

js
const p = $`echo "hello"`
  .pipe(getUpperCaseTransform())
  .pipe(fs.createWriteStream(tempfile()))  // <- stream
const o = await p

And the ProcessPromise itself is compatible with the standard Stream.pipe API:

js
const { stdout } = await fs
  .createReadStream(await fs.writeFile(file, 'test'))
  .pipe(getUpperCaseTransform())
  .pipe($`cat`)

Pipes can be used to show a real-time output of the process:

js
await $`echo 1; sleep 1; echo 2; sleep 1; echo 3;`
  .pipe(process.stdout)

And the time machine is in stock! You can pipe the process at any phase: on start, in the middle, or even after the end. All chunks will be buffered and processed in the right order.

js
const result = $`echo 1; sleep 1; echo 2; sleep 1; echo 3`
const piped1 = result.pipe`cat`
let piped2

setTimeout(() => { piped2 = result.pipe`cat` }, 1500)
  
(await piped1).toString()  // '1\n2\n3\n'
(await piped2).toString()  // '1\n2\n3\n'

The pipe() method can combine $ processes. Same as | in bash:

js
const greeting = await $`printf "hello"`
  .pipe($`awk '{printf $1", world!"}'`)
  .pipe($`tr '[a-z]' '[A-Z]'`)

echo(greeting)

Use combinations of pipe() and nothrow():

js
await $`find ./examples -type f -print0`
  .pipe($`xargs -0 grep ${'missing' + 'part'}`.nothrow())
  .pipe($`wc -l`)

And literals! Pipe does support them too:

js
await $`printf "hello"`
  .pipe`awk '{printf $1", world!"}'`
  .pipe`tr '[a-z]' '[A-Z]'`

By default, pipe() API operates with stdout stream, but you can specify stderr as well:

js
const p = $`echo foo >&2; echo bar`
const o1 = (await p.pipe.stderr`cat`).toString()  // 'foo\n'
const o2 = (await p.pipe.stdout`cat`).toString()  // 'bar\n'

Btw, the signal, if specified, will be transmitted through pipeline.

js
const ac = new AbortController()
const { signal } = ac
const p = $({ signal, nothrow: true })`echo test`.pipe`sleep 999`
setTimeout(() => ac.abort(), 50)

try {
  await p
} catch ({ message }) {
  message // The operation was aborted
}

In short, combine anything you want:

js
const getUpperCaseTransform = () => new Transform({
  transform(chunk, encoding, callback) {
    callback(null, String(chunk).toUpperCase())
  },
})

// $ > stream (promisified) > $
const o1 = await $`echo "hello"`
  .pipe(getUpperCaseTransform())
  .pipe($`cat`)

o1.stdout //  'HELLO\n'

// stream > $
const file = tempfile()
await fs.writeFile(file, 'test')
const o2 = await fs
  .createReadStream(file)
  .pipe(getUpperCaseTransform())
  .pipe($`cat`)

o2.stdout //  'TEST'

kill()

Kills the process and all children.

By default, signal SIGTERM is sent. You can specify a signal via an argument.

js
const p = $`sleep 999`
setTimeout(() => p.kill('SIGINT'), 100)
await p

abort()

Terminates the process via an AbortController signal.

js
const ac = new AbortController()
const {signal} = ac
const p = $({signal})`sleep 999`

setTimeout(() => ac.abort('reason'), 100)
await p

If ac or signal is not provided, it will be autocreated and could be used to control external processes.

js
const p = $`sleep 999`
const {signal} = p

const res = fetch('https://example.com', {signal})
p.abort('reason')

stdio()

Specifies a standard input-output for the process.

js
const h$ = $({halt: true})
const p1 = h$`read`.stdio('inherit', 'pipe', null).run()
const p2 = h$`read`.stdio('pipe').run() // sets ['pipe', 'pipe', 'pipe']

Keep in mind, stdio should be set before the process is started, so the preset syntax might be preferable:

js
await $({stdio: ['pipe', 'pipe', 'pipe']})`read`

nothrow()

Changes behavior of $ to not throw an exception on non-zero exit codes.

js
await $`grep something from-file`.nothrow()

// Inside a pipe():

await $`find ./examples -type f -print0`
  .pipe($`xargs -0 grep something`.nothrow())
  .pipe($`wc -l`)

If only the exitCode is needed, you can use exitCode directly:

js
if (await $`[[ -d path ]]`.exitCode == 0) {
...
}

// Equivalent of:

if ((await $`[[ -d path ]]`.nothrow()).exitCode == 0) {
...
}

quiet()

Changes behavior of $ to enable suppress mode.

js
// Command output will not be displayed.
await $`grep something from-file`.quiet()

$.quiet = true
await $`echo foo`.quiet(false) // Disable for the specific command

verbose()

Enables verbose output. Pass false to disable.

js
await $`grep something from-file`.verbose()

$.verbose = true
await $`echo foo`.verbose(false) // Turn off verbose mode once

timeout()

Kills the process after a specified timeout.

js
await $`sleep 999`.timeout('5s')

// Or with a specific signal.
await $`sleep 999`.timeout('5s', 'SIGKILL')

Disclaimer: This is not an officially supported Google product.