npm-scripts(7) -- How npm handles the "scripts" field
=====================================================

## DESCRIPTION

npm supports the "scripts" property of the package.json script, for the
following scripts:

* prepublish:
  Run BEFORE the package is published.  (Also run on local `npm
  install` without any arguments. See below.)
* prepare:
  Run both BEFORE the package is published, and on local `npm
  install` without any arguments. (See below.) This is run
  AFTER `prepublish`, but BEFORE `prepublishOnly`.
* prepublishOnly:
  Run BEFORE the package is published. (See below.)
* publish, postpublish:
  Run AFTER the package is published.
* preinstall:
  Run BEFORE the package is installed
* install, postinstall:
  Run AFTER the package is installed.
* preuninstall, uninstall:
  Run BEFORE the package is uninstalled.
* postuninstall:
  Run AFTER the package is uninstalled.
* preversion:
  Run BEFORE bumping the package version.
* version:
  Run AFTER bumping the package version, but BEFORE commit.
* postversion:
  Run AFTER bumping the package version, and AFTER commit.
* pretest, test, posttest:
  Run by the `npm test` command.
* prestop, stop, poststop:
  Run by the `npm stop` command.
* prestart, start, poststart:
  Run by the `npm start` command.
* prerestart, restart, postrestart:
  Run by the `npm restart` command. Note: `npm restart` will run the
  stop and start scripts if no `restart` script is provided.
* preshrinkwrap, shrinkwrap, postshrinkwrap:
  Run by the `npm shrinkwrap` command.

Additionally, arbitrary scripts can be executed by running `npm
run-script <stage>`. *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>`.

## PREPUBLISH AND PREPARE

### DEPRECATION NOTE

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).

**IMPORTANT:** As of `npm@5`, `prepublish` will _only_ be run for `npm
publish`.  This will make its behavior identical to `prepublishOnly`, so
`npm@6` or later may drop support for the use of `prepublishOnly`, and then
maybe we can all forget this embarrassing thing ever happened.

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.

## 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:

    { "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"

### 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:

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

and the server.js is this:

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

then the user could change the behavior by doing:

    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:

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

## EXAMPLES

For example, if your package.json contains this:

    { "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:

    { "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(5)` 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(1)
* package.json(5)
* npm-developers(7)
* npm-install(1)
