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> Process execution for humans
---
Netlify is looking for a senior/expert Node.js backend engineer to join a fully remote, international, diverse (44% women and non-binary) and friendly team. This is for the team where one of the co-maintainers [@ehmicky](https://github.com/ehmicky) is working. The job description is [here](https://boards.greenhouse.io/netlify/jobs/4832483002).
---
## Why
This package improves [`child_process`](https://nodejs.org/api/child_process.html) methods with:
- Promise interface.
- [Strips the final newline](#stripfinalnewline) from the output so you don't have to do `stdout.trim()`.
- Supports [shebang](https://en.wikipedia.org/wiki/Shebang_(Unix)) binaries cross-platform.
- [Improved Windows support.](https://github.com/IndigoUnited/node-cross-spawn#why)
- Higher max buffer. 100 MB instead of 200 KB.
- [Executes locally installed binaries by name.](#preferlocal)
- [Cleans up spawned processes when the parent process dies.](#cleanup)
- [Get interleaved output](#all) from `stdout` and `stderr` similar to what is printed on the terminal. [*(Async only)*](#execasyncfile-arguments-options)
- [Can specify file and arguments as a single string without a shell](#execacommandcommand-options)
- More descriptive errors.
## Install
```
$ npm install execa
```
## Usage
```js
const execa = require('execa');
(async () => {
const {stdout} = await execa('echo', ['unicorns']);
console.log(stdout);
//=> 'unicorns'
})();
```
### Pipe the child process stdout to the parent
```js
const execa = require('execa');
execa('echo', ['unicorns']).stdout.pipe(process.stdout);
```
### Handling Errors
```js
const execa = require('execa');
(async () => {
// Catching an error
try {
await execa('unknown', ['command']);
} catch (error) {
console.log(error);
/*
{
message: 'Command failed with ENOENT: unknown command spawn unknown ENOENT',
errno: -2,
code: 'ENOENT',
syscall: 'spawn unknown',
path: 'unknown',
spawnargs: ['command'],
originalMessage: 'spawn unknown ENOENT',
shortMessage: 'Command failed with ENOENT: unknown command spawn unknown ENOENT',
command: 'unknown command',
stdout: '',
stderr: '',
all: '',
failed: true,
timedOut: false,
isCanceled: false,
killed: false
}
*/
}
})();
```
### Cancelling a spawned process
```js
const execa = require('execa');
(async () => {
const subprocess = execa('node');
setTimeout(() => {
subprocess.cancel();
}, 1000);
try {
await subprocess;
} catch (error) {
console.log(subprocess.killed); // true
console.log(error.isCanceled); // true
}
})()
```
### Catching an error with the sync method
```js
try {
execa.sync('unknown', ['command']);
} catch (error) {
console.log(error);
/*
{
message: 'Command failed with ENOENT: unknown command spawnSync unknown ENOENT',
errno: -2,
code: 'ENOENT',
syscall: 'spawnSync unknown',
path: 'unknown',
spawnargs: ['command'],
originalMessage: 'spawnSync unknown ENOENT',
shortMessage: 'Command failed with ENOENT: unknown command spawnSync unknown ENOENT',
command: 'unknown command',
stdout: '',
stderr: '',
all: '',
failed: true,
timedOut: false,
isCanceled: false,
killed: false
}
*/
}
```
### Kill a process
Using SIGTERM, and after 2 seconds, kill it with SIGKILL.
```js
const subprocess = execa('node');
setTimeout(() => {
subprocess.kill('SIGTERM', {
forceKillAfterTimeout: 2000
});
}, 1000);
```
## API
### execa(file, arguments, options?)
Execute a file. Think of this as a mix of [`child_process.execFile()`](https://nodejs.org/api/child_process.html#child_process_child_process_execfile_file_args_options_callback) and [`child_process.spawn()`](https://nodejs.org/api/child_process.html#child_process_child_process_spawn_command_args_options).
No escaping/quoting is needed.
Unless the [`shell`](#shell) option is used, no shell interpreter (Bash, `cmd.exe`, etc.) is used, so shell features such as variables substitution (`echo $PATH`) are not allowed.
Returns a [`child_process` instance](https://nodejs.org/api/child_process.html#child_process_class_childprocess) which:
- is also a `Promise` resolving or rejecting with a [`childProcessResult`](#childProcessResult).
- exposes the following additional methods and properties.
#### kill(signal?, options?)
Same as the original [`child_process#kill()`](https://nodejs.org/api/child_process.html#child_process_subprocess_kill_signal) except: if `signal` is `SIGTERM` (the default value) and the child process is not terminated after 5 seconds, force it by sending `SIGKILL`.
##### options.forceKillAfterTimeout
Type: `number | false`\
Default: `5000`
Milliseconds to wait for the child process to terminate before sending `SIGKILL`.
Can be disabled with `false`.
#### cancel()
Similar to [`childProcess.kill()`](https://nodejs.org/api/child_process.html#child_process_subprocess_kill_signal). This is preferred when cancelling the child process execution as the error is more descriptive and [`childProcessResult.isCanceled`](#iscanceled) is set to `true`.
#### all
Type: `ReadableStream | undefined`
Stream combining/interleaving [`stdout`](https://nodejs.org/api/child_process.html#child_process_subprocess_stdout) and [`stderr`](https://nodejs.org/api/child_process.html#child_process_subprocess_stderr).
This is `undefined` if either:
- the [`all` option](#all-2) is `false` (the default value)
- both [`stdout`](#stdout-1) and [`stderr`](#stderr-1) options are set to [`'inherit'`, `'ipc'`, `Stream` or `integer`](https://nodejs.org/dist/latest-v6.x/docs/api/child_process.html#child_process_options_stdio)
### execa.sync(file, arguments?, options?)
Execute a file synchronously.
Returns or throws a [`childProcessResult`](#childProcessResult).
### execa.command(command, options?)
Same as [`execa()`](#execafile-arguments-options) except both file and arguments are specified in a single `command` string. For example, `execa('echo', ['unicorns'])` is the same as `execa.command('echo unicorns')`.
If the file or an argument contains spaces, they must be escaped with backslashes. This matters especially if `command` is not a constant but a variable, for example with `__dirname` or `process.cwd()`. Except for spaces, no escaping/quoting is needed.
The [`shell` option](#shell) must be used if the `command` uses shell-specific features, as opposed to being a simple `file` followed by its `arguments`.
### execa.commandSync(command, options?)
Same as [`execa.command()`](#execacommand-command-options) but synchronous.
Returns or throws a [`childProcessResult`](#childProcessResult).
### execa.node(scriptPath, arguments?, options?)
Execute a Node.js script as a child process.
Same as `execa('node', [scriptPath, ...arguments], options)` except (like [`child_process#fork()`](https://nodejs.org/api/child_process.html#child_process_child_process_fork_modulepath_args_options)):
- the current Node version and options are used. This can be overridden using the [`nodePath`](#nodepath-for-node-only) and [`nodeOptions`](#nodeoptions-for-node-only) options.
- the [`shell`](#shell) option cannot be used
- an extra channel [`ipc`](https://nodejs.org/api/child_process.html#child_process_options_stdio) is passed to [`stdio`](#stdio)
### childProcessResult
Type: `object`
Result of a child process execution. On success this is a plain object. On failure this is also an `Error` instance.
The child process [fails](#failed) when:
- its [exit code](#exitcode) is not `0`
- it was [killed](#killed) with a [signal](#signal)
- [timing out](#timedout)
- [being canceled](#iscanceled)
- there's not enough memory or there are already too many child processes
#### command
Type: `string`
The file and arguments that were run.
#### exitCode
Type: `number`
The numeric exit code of the process that was run.
#### stdout
Type: `string | Buffer`
The output of the process on stdout.
#### stderr
Type: `string | Buffer`
The output of the process on stderr.
#### all
Type: `string | Buffer | undefined`
The output of the process with `stdout` and `stderr` interleaved.
This is `undefined` if either:
- the [`all` option](#all-2) is `false` (the default value)
- `execa.sync()` was used
#### failed
Type: `boolean`
Whether the process failed to run.
#### timedOut
Type: `boolean`
Whether the process timed out.
#### isCanceled
Type: `boolean`
Whether the process was canceled.
#### killed
Type: `boolean`
Whether the process was killed.
#### signal
Type: `string | undefined`
The name of the signal that was used to terminate the process. For example, `SIGFPE`.
If a signal terminated the process, this property is defined and included in the error message. Otherwise it is `undefined`.
#### signalDescription
Type: `string | undefined`
A human-friendly description of the signal that was used to terminate the process. For example, `Floating point arithmetic error`.
If a signal terminated the process, this property is defined and included in the error message. Otherwise it is `undefined`. It is also `undefined` when the signal is very uncommon which should seldomly happen.
#### message
Type: `string`
Error message when the child process failed to run. In addition to the [underlying error message](#originalMessage), it also contains some information related to why the child process errored.
The child process [stderr](#stderr) then [stdout](#stdout) are appended to the end, separated with newlines and not interleaved.
#### shortMessage
Type: `string`
This is the same as the [`message` property](#message) except it does not include the child process stdout/stderr.
#### originalMessage
Type: `string | undefined`
Original error message. This is the same as the `message` property except it includes neither the child process stdout/stderr nor some additional information added by Execa.
This is `undefined` unless the child process exited due to an `error` event or a timeout.
### options
Type: `object`
#### cleanup
Type: `boolean`\
Default: `true`
Kill the spawned process when the parent process exits unless either:
- the spawned process is [`detached`](https://nodejs.org/api/child_process.html#child_process_options_detached)
- the parent process is terminated abruptly, for example, with `SIGKILL` as opposed to `SIGTERM` or a normal exit
#### preferLocal
Type: `boolean`\
Default: `false`
Prefer locally installed binaries when looking for a binary to execute.\
If you `$ npm install foo`, you can then `execa('foo')`.
#### localDir
Type: `string`\
Default: `process.cwd()`
Preferred path to find locally installed binaries in (use with `preferLocal`).
#### execPath
Type: `string`\
Default: `process.execPath` (Current Node.js executable)
Path to the Node.js executable to use in child processes.
This can be either an absolute path or a path relative to the [`cwd` option](#cwd).
Requires [`preferLocal`](#preferlocal) to be `true`.
For example, this can be used together with [`get-node`](https://github.com/ehmicky/get-node) to run a specific Node.js version in a child process.
#### buffer
Type: `boolean`\
Default: `true`
Buffer the output from the spawned process. When set to `false`, you must read the output of [`stdout`](#stdout-1) and [`stderr`](#stderr-1) (or [`all`](#all) if the [`all`](#all-2) option is `true`). Otherwise the returned promise will not be resolved/rejected.
If the spawned process fails, [`error.stdout`](#stdout), [`error.stderr`](#stderr), and [`error.all`](#all) will contain the buffered data.
#### input
Type: `string | Buffer | stream.Readable`
Write some input to the `stdin` of your binary.\
Streams are not allowed when using the synchronous methods.
#### stdin
Type: `string | number | Stream | undefined`\
Default: `pipe`
Same options as [`stdio`](https://nodejs.org/dist/latest-v6.x/docs/api/child_process.html#child_process_options_stdio).
#### stdout
Type: `string | number | Stream | undefined`\
Default: `pipe`
Same options as [`stdio`](https://nodejs.org/dist/latest-v6.x/docs/api/child_process.html#child_process_options_stdio).
#### stderr
Type: `string | number | Stream | undefined`\
Default: `pipe`
Same options as [`stdio`](https://nodejs.org/dist/latest-v6.x/docs/api/child_process.html#child_process_options_stdio).
#### all
Type: `boolean`\
Default: `false`
Add an `.all` property on the [promise](#all) and the [resolved value](#all-1). The property contains the output of the process with `stdout` and `stderr` interleaved.
#### reject
Type: `boolean`\
Default: `true`
Setting this to `false` resolves the promise with the error instead of rejecting it.
#### stripFinalNewline
Type: `boolean`\
Default: `true`
Strip the final [newline character](https://en.wikipedia.org/wiki/Newline) from the output.
#### extendEnv
Type: `boolean`\
Default: `true`
Set to `false` if you don't want to extend the environment variables when providing the `env` property.
---
Execa also accepts the below options which are the same as the options for [`child_process#spawn()`](https://nodejs.org/api/child_process.html#child_process_child_process_spawn_command_args_options)/[`child_process#exec()`](https://nodejs.org/api/child_process.html#child_process_child_process_exec_command_options_callback)
#### cwd
Type: `string`\
Default: `process.cwd()`
Current working directory of the child process.
#### env
Type: `object`\
Default: `process.env`
Environment key-value pairs. Extends automatically from `process.env`. Set [`extendEnv`](#extendenv) to `false` if you don't want this.
#### argv0
Type: `string`
Explicitly set the value of `argv[0]` sent to the child process. This will be set to `file` if not specified.
#### stdio
Type: `string | string[]`\
Default: `pipe`
Child's [stdio](https://nodejs.org/api/child_process.html#child_process_options_stdio) configuration.
#### serialization
Type: `string`\
Default: `'json'`
Specify the kind of serialization used for sending messages between processes when using the [`stdio: 'ipc'`](#stdio) option or [`execa.node()`](#execanodescriptpath-arguments-options):
- `json`: Uses `JSON.stringify()` and `JSON.parse()`.
- `advanced`: Uses [`v8.serialize()`](https://nodejs.org/api/v8.html#v8_v8_serialize_value)
Requires Node.js `13.2.0` or later.
[More info.](https://nodejs.org/api/child_process.html#child_process_advanced_serialization)
#### detached
Type: `boolean`
Prepare child to run independently of its parent process. Specific behavior [depends on the platform](https://nodejs.org/api/child_process.html#child_process_options_detached).
#### uid
Type: `number`
Sets the user identity of the process.
#### gid
Type: `number`
Sets the group identity of the process.
#### shell
Type: `boolean | string`\
Default: `false`
If `true`, runs `file` inside of a shell. Uses `/bin/sh` on UNIX and `cmd.exe` on Windows. A different shell can be specified as a string. The shell should understand the `-c` switch on UNIX or `/d /s /c` on Windows.
We recommend against using this option since it is:
- not cross-platform, encouraging shell-specific syntax.
- slower, because of the additional shell interpretation.
- unsafe, potentially allowing command injection.
#### encoding
Type: `string | null`\
Default: `utf8`
Specify the character encoding used to decode the `stdout` and `stderr` output. If set to `null`, then `stdout` and `stderr` will be a `Buffer` instead of a string.
#### timeout
Type: `number`\
Default: `0`
If timeout is greater than `0`, the parent will send the signal identified by the `killSignal` property (the default is `SIGTERM`) if the child runs longer than timeout milliseconds.
#### maxBuffer
Type: `number`\
Default: `100_000_000` (100 MB)
Largest amount of data in bytes allowed on `stdout` or `stderr`.
#### killSignal
Type: `string | number`\
Default: `SIGTERM`
Signal value to be used when the spawned process will be killed.
#### windowsVerbatimArguments
Type: `boolean`\
Default: `false`
If `true`, no quoting or escaping of arguments is done on Windows. Ignored on other platforms. This is set to `true` automatically when the `shell` option is `true`.
#### windowsHide
Type: `boolean`\
Default: `true`
On Windows, do not create a new console window. Please note this also prevents `CTRL-C` [from working](https://github.com/nodejs/node/issues/29837) on Windows.
#### nodePath *(For `.node()` only)*
Type: `string`\
Default: [`process.execPath`](https://nodejs.org/api/process.html#process_process_execpath)
Node.js executable used to create the child process.
#### nodeOptions *(For `.node()` only)*
Type: `string[]`\
Default: [`process.execArgv`](https://nodejs.org/api/process.html#process_process_execargv)
List of [CLI options](https://nodejs.org/api/cli.html#cli_options) passed to the Node.js executable.
## Tips
### Retry on error
Gracefully handle failures by using automatic retries and exponential backoff with the [`p-retry`](https://github.com/sindresorhus/p-retry) package:
```js
const pRetry = require('p-retry');
const run = async () => {
const results = await execa('curl', ['-sSL', 'https://sindresorhus.com/unicorn']);
return results;
};
(async () => {
console.log(await pRetry(run, {retries: 5}));
})();
```
### Save and pipe output from a child process
Let's say you want to show the output of a child process in real-time while also saving it to a variable.
```js
const execa = require('execa');
const subprocess = execa('echo', ['foo']);
subprocess.stdout.pipe(process.stdout);
(async () => {
const {stdout} = await subprocess;
console.log('child output:', stdout);
})();
```
### Redirect output to a file
```js
const execa = require('execa');
const subprocess = execa('echo', ['foo'])
subprocess.stdout.pipe(fs.createWriteStream('stdout.txt'))
```
### Redirect input from a file
```js
const execa = require('execa');
const subprocess = execa('cat')
fs.createReadStream('stdin.txt').pipe(subprocess.stdin)
```
### Execute the current package's binary
```js
const {getBinPathSync} = require('get-bin-path');
const binPath = getBinPathSync();
const subprocess = execa(binPath);
```
`execa` can be combined with [`get-bin-path`](https://github.com/ehmicky/get-bin-path) to test the current package's binary. As opposed to hard-coding the path to the binary, this validates that the `package.json` `bin` field is correctly set up.
## Related
- [gulp-execa](https://github.com/ehmicky/gulp-execa) - Gulp plugin for `execa`
- [nvexeca](https://github.com/ehmicky/nvexeca) - Run `execa` using any Node.js version
- [sudo-prompt](https://github.com/jorangreef/sudo-prompt) - Run commands with elevated privileges.
## Maintainers
- [Sindre Sorhus](https://github.com/sindresorhus)
- [@ehmicky](https://github.com/ehmicky)
---
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