1---
2section: using-npm
3title: scripts
4description: How npm handles the "scripts" field
5---
6
7# scripts(7)
8
9## How npm handles the "scripts" field
10
11### Description
12
13The `"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>`.
14
15### Pre & Post Scripts
16
17To 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.
18
19```json
20{
21  "scripts": {
22    "precompress": "{{ executes BEFORE the `compress` script }}",
23    "compress": "{{ run command to compress files }}",
24    "postcompress": "{{ executes AFTER `compress` script }}"
25  }
26}
27```
28
29### Life Cycle Scripts
30
31There are some special life cycle scripts that happen only in certain situations. These scripts happen in addtion to the "pre" and "post" script.
32* `prepare`, `prepublish`, `prepublishOnly`, `prepack`, `postpack`
33
34**prepare** (since `npm@4.0.0`)
35* Runs BEFORE the package is packed
36* Runs BEFORE the package is published
37* Runs on local `npm install` without any arguments
38* Run AFTER `prepublish`, but BEFORE `prepublishOnly`
39* 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.
40
41**prepublish** (DEPRECATED)
42* Same as `prepare`
43
44**prepublishOnly**
45* Runs BEFORE the package is prepared and packed, ONLY on `npm publish`.
46
47**prepack**
48* Runs BEFORE a tarball is packed (on "`npm pack`", "`npm publish`", and when installing a git dependencies).
49* 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.
50
51**postpack**
52* Runs AFTER the tarball has been generated and moved to its final destination.
53
54#### Prepare and Prepublish
55
56**Deprecation Note: prepublish**
57
58Since `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).
59
60See <https://github.com/npm/npm/issues/10074> for a much lengthier justification, with further reading, for this change.
61
62**Use Cases**
63
64If 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:
65
66* Compiling CoffeeScript source code into JavaScript.
67* Creating minified versions of JavaScript source code.
68* Fetching remote resources that your package will use.
69
70The 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:
71
72* You can depend on `coffee-script` as a `devDependency`, and thus
73  your users don't need to have it installed.
74* You don't need to include minifiers in your package, reducing
75  the size for your users.
76* You don't need to rely on your users having `curl` or `wget` or
77  other system tools on the target machines.
78
79### Life Cycle Operation Order
80
81#### [`npm publish`](/cli-commands/npm-publish)
82
83* `prepublishOnly`
84* `prepare`
85* `prepublish`
86* `publish`
87* `postpublish`
88
89#### [`npm pack`](/cli-commands/npm-pack)
90
91* `prepack`
92* `postpack`
93
94#### [`npm install`](/cli-commands/npm-install)
95
96* `preinstall`
97* `install`
98* `postinstall`
99
100Also triggers
101
102* `prepublish` (when on local)
103* `prepare` (when on local)
104
105#### [`npm start`](/cli-commands/npm-start)
106
107`npm run start` has an `npm start` shorthand.
108
109* `prestart`
110* `start`
111* `poststart`
112
113### Default Values
114npm will default some script values based on package contents.
115
116* `"start": "node server.js"`:
117
118  If there is a `server.js` file in the root of your package, then npm
119  will default the `start` command to `node server.js`.
120
121* `"install": "node-gyp rebuild"`:
122
123  If there is a `binding.gyp` file in the root of your package and you
124  haven't defined your own `install` or `preinstall` scripts, npm will
125  default the `install` command to compile using node-gyp.
126
127### User
128
129If npm was invoked with root privileges, then it will change the uid
130to the user account or uid specified by the `user` config, which
131defaults to `nobody`.  Set the `unsafe-perm` flag to run scripts with
132root privileges.
133
134### Environment
135
136Package scripts run in an environment where many pieces of information
137are made available regarding the setup of npm and the current state of
138the process.
139
140
141#### path
142
143If you depend on modules that define executable scripts, like test
144suites, then those executables will be added to the `PATH` for
145executing the scripts.  So, if your package.json has this:
146
147```json
148{
149  "name" : "foo",
150  "dependencies" : {
151    "bar" : "0.1.x"
152  },
153  "scripts": {
154    "start" : "bar ./test"
155  }
156}
157```
158
159then you could run `npm start` to execute the `bar` script, which is
160exported into the `node_modules/.bin` directory on `npm install`.
161
162#### package.json vars
163
164The package.json fields are tacked onto the `npm_package_` prefix. So,
165for instance, if you had `{"name":"foo", "version":"1.2.5"}` in your
166package.json file, then your package scripts would have the
167`npm_package_name` environment variable set to "foo", and the
168`npm_package_version` set to "1.2.5".  You can access these variables
169in your code with `process.env.npm_package_name` and
170`process.env.npm_package_version`, and so on for other fields.
171
172#### configuration
173
174Configuration parameters are put in the environment with the
175`npm_config_` prefix. For instance, you can view the effective `root`
176config by checking the `npm_config_root` environment variable.
177
178#### Special: package.json "config" object
179
180The package.json "config" keys are overwritten in the environment if
181there is a config param of `<name>[@<version>]:<key>`.  For example,
182if the package.json has this:
183
184```json
185{
186  "name" : "foo",
187  "config" : {
188    "port" : "8080"
189  },
190  "scripts" : {
191    "start" : "node server.js"
192  }
193}
194```
195
196and the server.js is this:
197
198```javascript
199http.createServer(...).listen(process.env.npm_package_config_port)
200```
201
202then the user could change the behavior by doing:
203
204```bash
205  npm config set foo:port 80
206  ```
207
208#### current lifecycle event
209
210Lastly, the `npm_lifecycle_event` environment variable is set to
211whichever stage of the cycle is being executed. So, you could have a
212single script used for different parts of the process which switches
213based on what's currently happening.
214
215Objects are flattened following this format, so if you had
216`{"scripts":{"install":"foo.js"}}` in your package.json, then you'd
217see this in the script:
218
219```bash
220process.env.npm_package_scripts_install === "foo.js"
221```
222
223### Examples
224
225For example, if your package.json contains this:
226
227```json
228{
229  "scripts" : {
230    "install" : "scripts/install.js",
231    "postinstall" : "scripts/install.js",
232    "uninstall" : "scripts/uninstall.js"
233  }
234}
235```
236
237then `scripts/install.js` will be called for the install
238and post-install stages of the lifecycle, and `scripts/uninstall.js`
239will be called when the package is uninstalled.  Since
240`scripts/install.js` is running for two different phases, it would
241be wise in this case to look at the `npm_lifecycle_event` environment
242variable.
243
244If you want to run a make command, you can do so.  This works just
245fine:
246
247```json
248{
249  "scripts" : {
250    "preinstall" : "./configure",
251    "install" : "make && make install",
252    "test" : "make test"
253  }
254}
255```
256
257### Exiting
258
259Scripts are run by passing the line as a script argument to `sh`.
260
261If the script exits with a code other than 0, then this will abort the
262process.
263
264Note that these script files don't have to be nodejs or even
265javascript programs. They just have to be some kind of executable
266file.
267
268### Hook Scripts
269
270If you want to run a specific script at a specific lifecycle event for
271ALL packages, then you can use a hook script.
272
273Place an executable file at `node_modules/.hooks/{eventname}`, and
274it'll get run for all packages when they are going through that point
275in the package lifecycle for any packages installed in that root.
276
277Hook scripts are run exactly the same way as package.json scripts.
278That is, they are in a separate child process, with the env described
279above.
280
281### Best Practices
282
283* Don't exit with a non-zero error code unless you *really* mean it.
284  Except for uninstall scripts, this will cause the npm action to
285  fail, and potentially be rolled back.  If the failure is minor or
286  only will prevent some optional features, then it's better to just
287  print a warning and exit successfully.
288* Try not to use scripts to do what npm can do for you.  Read through
289  [`package.json`](/configuring-npm/package-json) to see all the things that you can specify and enable
290  by simply describing your package appropriately.  In general, this
291  will lead to a more robust and consistent state.
292* Inspect the env to determine where to put things.  For instance, if
293  the `npm_config_binroot` environment variable is set to `/home/user/bin`, then
294  don't try to install executables into `/usr/local/bin`.  The user
295  probably set it up that way for a reason.
296* Don't prefix your script commands with "sudo".  If root permissions
297  are required for some reason, then it'll fail with that error, and
298  the user will sudo the npm command in question.
299* Don't use `install`. Use a `.gyp` file for compilation, and `prepublish`
300  for anything else. You should almost never have to explicitly set a
301  preinstall or install script. If you are doing this, please consider if
302  there is another option. The only valid use of `install` or `preinstall`
303  scripts is for compilation which must be done on the target architecture.
304
305### See Also
306
307* [npm run-script](/cli-commands/npm-run-script)
308* [package.json](/configuring-npm/package-json)
309* [npm developers](/using-npm/developers)
310* [npm install](/cli-commands/npm-install)
311