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# execa [](https://travis-ci.org/sindresorhus/execa) [](https://ci.appveyor.com/project/sindresorhus/execa/branch/master) [](https://coveralls.io/github/sindresorhus/execa?branch=master)
> A better [`child_process`](https://nodejs.org/api/child_process.html)
## Why
- Promise interface.
- [Strips EOF](https://github.com/sindresorhus/strip-eof) from the output so you don't have to `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. 10 MB instead of 200 KB.
- [Executes locally installed binaries by name.](#preferlocal)
- [Cleans up spawned processes when the parent process dies.](#cleanup)
## Install
```
$ npm install --save execa
```
## Usage
```js
const execa = require('execa');
execa('echo', ['unicorns']).then(result => {
console.log(result.stdout);
//=> 'unicorns'
});
// pipe the child process stdout to the current stdout
execa('echo', ['unicorns']).stdout.pipe(process.stdout);
execa.shell('echo unicorns').then(result => {
console.log(result.stdout);
//=> 'unicorns'
});
// example of catching an error
execa.shell('exit 3').catch(error => {
console.log(error);
/*
{
message: 'Command failed: /bin/sh -c exit 3'
killed: false,
code: 3,
signal: null,
cmd: '/bin/sh -c exit 3',
stdout: '',
stderr: '',
timedOut: false
}
*/
});
```
## API
### execa(file, [arguments], [options])
Execute a file.
Think of this as a mix of `child_process.execFile` and `child_process.spawn`.
Returns a [`child_process` instance](https://nodejs.org/api/child_process.html#child_process_class_childprocess), which is enhanced to also be a `Promise` for a result `Object` with `stdout` and `stderr` properties.
### execa.stdout(file, [arguments], [options])
Same as `execa()`, but returns only `stdout`.
### execa.stderr(file, [arguments], [options])
Same as `execa()`, but returns only `stderr`.
### execa.shell(command, [options])
Execute a command through the system shell. Prefer `execa()` whenever possible, as it's both faster and safer.
Returns a [`child_process` instance](https://nodejs.org/api/child_process.html#child_process_class_childprocess).
The `child_process` instance is enhanced to also be promise for a result object with `stdout` and `stderr` properties.
### execa.sync(file, [arguments], [options])
Execute a file synchronously.
Returns the same result object as [`child_process.spawnSync`](https://nodejs.org/api/child_process.html#child_process_child_process_spawnsync_command_args_options).
This method throws an `Error` if the command fails.
### execa.shellSync(file, [options])
Execute a command synchronously through the system shell.
Returns the same result object as [`child_process.spawnSync`](https://nodejs.org/api/child_process.html#child_process_child_process_spawnsync_command_args_options).
### options
Type: `Object`
#### cwd
Type: `string`<br>
Default: `process.cwd()`
Current working directory of the child process.
#### env
Type: `Object`<br>
Default: `process.env`
Environment key-value pairs. Extends automatically from `process.env`. Set `extendEnv` to `false` if you don't want this.
#### extendEnv
Type: `boolean`<br>
Default: `true`
Set to `false` if you don't want to extend the environment variables when providing the `env` property.
#### argv0
Type: `string`
Explicitly set the value of `argv[0]` sent to the child process. This will be set to `command` or `file` if not specified.
#### stdio
Type: `Array` `string`<br>
Default: `pipe`
Child's [stdio](https://nodejs.org/api/child_process.html#child_process_options_stdio) configuration.
#### 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`<br>
Default: `false`
If `true`, runs `command` 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.
#### stripEof
Type: `boolean`<br>
Default: `true`
[Strip EOF](https://github.com/sindresorhus/strip-eof) (last newline) from the output.
#### preferLocal
Type: `boolean`<br>
Default: `true`
Prefer locally installed binaries when looking for a binary to execute.<br>
If you `$ npm install foo`, you can then `execa('foo')`.
#### localDir
Type: `string`<br>
Default: `process.cwd()`
Preferred path to find locally installed binaries in (use with `preferLocal`).
#### input
Type: `string` `Buffer` `stream.Readable`
Write some input to the `stdin` of your binary.<br>
Streams are not allowed when using the synchronous methods.
#### reject
Type: `boolean`<br>
Default: `true`
Setting this to `false` resolves the promise with the error instead of rejecting it.
#### cleanup
Type: `boolean`<br>
Default: `true`
Keep track of the spawned process and `kill` it when the parent process exits.
#### encoding
Type: `string`<br>
Default: `utf8`
Specify the character encoding used to decode the `stdout` and `stderr` output.
#### timeout
Type: `number`<br>
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`<br>
Default: `10000000` (10MB)
Largest amount of data in bytes allowed on `stdout` or `stderr`.
#### killSignal
Type: `string` `number`<br>
Default: `SIGTERM`
Signal value to be used when the spawned process will be killed.
#### stdin
Type: `string` `number` `Stream` `undefined` `null`<br>
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` `null`<br>
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` `null`<br>
Default: `pipe`
Same options as [`stdio`](https://nodejs.org/dist/latest-v6.x/docs/api/child_process.html#child_process_options_stdio).
## Tips
### 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 getStream = require('get-stream');
const stream = execa('echo', ['foo']).stdout;
stream.pipe(process.stdout);
getStream(stream).then(value => {
console.log('child output:', value);
});
```
## License
MIT © [Sindre Sorhus](https://sindresorhus.com)