---
section: using-npm
title: scripts
description: How npm handles the "scripts" field
---

# scripts(7)

## How npm handles the "scripts" field

### Description

The `"scripts"` property of of your `package.json` file supports a number of built-in scripts and their preset life cycle events as well as arbitrary scripts. These all can be executed by running `npm run-script <stage>` or `npm run <stage>` for short. *Pre* and *post* commands with matching names will be run for those as well (e.g. `premyscript`, `myscript`, `postmyscript`). Scripts from dependencies can be run with `npm explore <pkg> -- npm run <stage>`.

### Pre & Post Scripts

To create "pre" or "post" scripts for any scripts defined in the `"scripts"` section of the `package.json`, simply create another script *with a matching name* and add "pre" or "post" to the beginning of them.

```json
{
  "scripts": {
    "precompress": "{{ executes BEFORE the `compress` script }}",
    "compress": "{{ run command to compress files }}",
    "postcompress": "{{ executes AFTER `compress` script }}"
  }
}
```

### Life Cycle Scripts

There are some special life cycle scripts that happen only in certain situations. These scripts happen in addtion to the "pre" and "post" script.
* `prepare`, `prepublish`, `prepublishOnly`, `prepack`, `postpack`

**prepare** (since `npm@4.0.0`)
* Runs BEFORE the package is packed
* Runs BEFORE the package is published
* Runs on local `npm install` without any arguments
* Run AFTER `prepublish`, but BEFORE `prepublishOnly`
* NOTE: If a package being installed through git contains a `prepare` script, its `dependencies` and `devDependencies` will be installed, and the prepare script will be run, before the package is packaged and installed.

**prepublish** (DEPRECATED)
* Same as `prepare`

**prepublishOnly**
* Runs BEFORE the package is prepared and packed, ONLY on `npm publish`.

**prepack**
* Runs BEFORE a tarball is packed (on "`npm pack`", "`npm publish`", and when installing a git dependencies).
* NOTE: "`npm run pack`" is NOT the same as "`npm pack`". "`npm run pack`" is an arbitrary user defined script name, where as, "`npm pack`" is a CLI defined command.

**postpack**
* Runs AFTER the tarball has been generated and moved to its final destination.

#### Prepare and Prepublish

**Deprecation Note: prepublish**

Since `npm@1.1.71`, the npm CLI has run the `prepublish` script for both `npm publish` and `npm install`, because it's a convenient way to prepare a package for use (some common use cases are described in the section below).  It has also turned out to be, in practice, [very confusing](https://github.com/npm/npm/issues/10074).  As of `npm@4.0.0`, a new event has been introduced, `prepare`, that preserves this existing behavior. A _new_ event, `prepublishOnly` has been added as a transitional strategy to allow users to avoid the confusing behavior of existing npm versions and only run on `npm publish` (for instance, running the tests one last time to ensure they're in good shape).

See <https://github.com/npm/npm/issues/10074> for a much lengthier justification, with further reading, for this change.

**Use Cases**

If you need to perform operations on your package before it is used, in a way that is not dependent on the operating system or architecture of the target system, use a `prepublish` script. This includes tasks such as:

* Compiling CoffeeScript source code into JavaScript.
* Creating minified versions of JavaScript source code.
* Fetching remote resources that your package will use.

The advantage of doing these things at `prepublish` time is that they can be done once, in a single place, thus reducing complexity and variability. Additionally, this means that:

* You can depend on `coffee-script` as a `devDependency`, and thus
  your users don't need to have it installed.
* You don't need to include minifiers in your package, reducing
  the size for your users.
* You don't need to rely on your users having `curl` or `wget` or
  other system tools on the target machines.

### Life Cycle Operation Order

#### [`npm publish`](/cli-commands/npm-publish)

* `prepublishOnly`
* `prepare`
* `prepublish`
* `publish`
* `postpublish`

#### [`npm pack`](/cli-commands/npm-pack)

* `prepack`
* `postpack`

#### [`npm install`](/cli-commands/npm-install)

* `preinstall`
* `install`
* `postinstall`

Also triggers

* `prepublish` (when on local)
* `prepare` (when on local)

#### [`npm start`](/cli-commands/npm-start)

`npm run start` has an `npm start` shorthand.

* `prestart`
* `start`
* `poststart`

### Default Values
npm will default some script values based on package contents.

* `"start": "node server.js"`:

  If there is a `server.js` file in the root of your package, then npm
  will default the `start` command to `node server.js`.

* `"install": "node-gyp rebuild"`:

  If there is a `binding.gyp` file in the root of your package and you
  haven't defined your own `install` or `preinstall` scripts, npm will
  default the `install` command to compile using node-gyp.

### User

If npm was invoked with root privileges, then it will change the uid
to the user account or uid specified by the `user` config, which
defaults to `nobody`.  Set the `unsafe-perm` flag to run scripts with
root privileges.

### Environment

Package scripts run in an environment where many pieces of information
are made available regarding the setup of npm and the current state of
the process.


#### path

If you depend on modules that define executable scripts, like test
suites, then those executables will be added to the `PATH` for
executing the scripts.  So, if your package.json has this:

```json
{ 
  "name" : "foo", 
  "dependencies" : { 
    "bar" : "0.1.x" 
  }, 
  "scripts": { 
    "start" : "bar ./test" 
  } 
}
```

then you could run `npm start` to execute the `bar` script, which is
exported into the `node_modules/.bin` directory on `npm install`.

#### package.json vars

The package.json fields are tacked onto the `npm_package_` prefix. So,
for instance, if you had `{"name":"foo", "version":"1.2.5"}` in your
package.json file, then your package scripts would have the
`npm_package_name` environment variable set to "foo", and the
`npm_package_version` set to "1.2.5".  You can access these variables 
in your code with `process.env.npm_package_name` and 
`process.env.npm_package_version`, and so on for other fields.

#### configuration

Configuration parameters are put in the environment with the
`npm_config_` prefix. For instance, you can view the effective `root`
config by checking the `npm_config_root` environment variable.

#### Special: package.json "config" object

The package.json "config" keys are overwritten in the environment if
there is a config param of `<name>[@<version>]:<key>`.  For example,
if the package.json has this:

```json
{ 
  "name" : "foo", 
  "config" : { 
    "port" : "8080" 
  }, 
  "scripts" : { 
    "start" : "node server.js" 
  } 
}
```

and the server.js is this:

```javascript
http.createServer(...).listen(process.env.npm_package_config_port)
```

then the user could change the behavior by doing:

```bash
  npm config set foo:port 80
  ```

#### current lifecycle event

Lastly, the `npm_lifecycle_event` environment variable is set to
whichever stage of the cycle is being executed. So, you could have a
single script used for different parts of the process which switches
based on what's currently happening.

Objects are flattened following this format, so if you had
`{"scripts":{"install":"foo.js"}}` in your package.json, then you'd
see this in the script:

```bash
process.env.npm_package_scripts_install === "foo.js"
```

### Examples

For example, if your package.json contains this:

```json
{ 
  "scripts" : { 
    "install" : "scripts/install.js", 
    "postinstall" : "scripts/install.js", 
    "uninstall" : "scripts/uninstall.js"
  }
}
```

then `scripts/install.js` will be called for the install
and post-install stages of the lifecycle, and `scripts/uninstall.js`
will be called when the package is uninstalled.  Since
`scripts/install.js` is running for two different phases, it would
be wise in this case to look at the `npm_lifecycle_event` environment
variable.

If you want to run a make command, you can do so.  This works just
fine:

```json
{ 
  "scripts" : { 
    "preinstall" : "./configure", 
    "install" : "make && make install", 
    "test" : "make test"
  }
}
```

### Exiting

Scripts are run by passing the line as a script argument to `sh`.

If the script exits with a code other than 0, then this will abort the
process.

Note that these script files don't have to be nodejs or even
javascript programs. They just have to be some kind of executable
file.

### Hook Scripts

If you want to run a specific script at a specific lifecycle event for
ALL packages, then you can use a hook script.

Place an executable file at `node_modules/.hooks/{eventname}`, and
it'll get run for all packages when they are going through that point
in the package lifecycle for any packages installed in that root.

Hook scripts are run exactly the same way as package.json scripts.
That is, they are in a separate child process, with the env described
above.

### Best Practices

* Don't exit with a non-zero error code unless you *really* mean it.
  Except for uninstall scripts, this will cause the npm action to
  fail, and potentially be rolled back.  If the failure is minor or
  only will prevent some optional features, then it's better to just
  print a warning and exit successfully.
* Try not to use scripts to do what npm can do for you.  Read through
  [`package.json`](/configuring-npm/package-json) to see all the things that you can specify and enable
  by simply describing your package appropriately.  In general, this
  will lead to a more robust and consistent state.
* Inspect the env to determine where to put things.  For instance, if
  the `npm_config_binroot` environment variable is set to `/home/user/bin`, then
  don't try to install executables into `/usr/local/bin`.  The user
  probably set it up that way for a reason.
* Don't prefix your script commands with "sudo".  If root permissions
  are required for some reason, then it'll fail with that error, and
  the user will sudo the npm command in question.
* Don't use `install`. Use a `.gyp` file for compilation, and `prepublish`
  for anything else. You should almost never have to explicitly set a
  preinstall or install script. If you are doing this, please consider if
  there is another option. The only valid use of `install` or `preinstall`
  scripts is for compilation which must be done on the target architecture.

### See Also

* [npm run-script](/cli-commands/npm-run-script)
* [package.json](/configuring-npm/package-json)
* [npm developers](/using-npm/developers)
* [npm install](/cli-commands/npm-install)
