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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.)
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.
preupdate: Run BEFORE the package is updated with the update command.
update, postupdate: Run AFTER the package is updated with the update command.
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.

Additionally, arbitrary scripts can be executed by running npm run-script <pkg> <stage>. Pre and post commands with matching names will be run for those as well (e.g. premyscript, myscript, postmyscript).

NOTE: INSTALL SCRIPTS ARE AN ANTIPATTERN

tl;dr 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. In early versions of node, this was often done using the node-waf scripts, or a standalone Makefile, and early versions of npm required that it be explicitly set in package.json. This was not portable, and harder to do properly.

In the current version of node, the standard way to do this is using a .gyp file. If you have a file with a .gyp extension in the root of your package, then npm will run the appropriate node-gyp commands automatically at install time. This is the only officially supported method for compiling binary addons, and does not require that you add anything to your package.json file.

If you have to do other things before your package is used, in a way that is not dependent on the operating system or architecture of the target system, then use a prepublish script instead. This includes tasks such as:

Compile CoffeeScript source code into JavaScript.
Create minified versions of JavaScript source code.
Fetching remote resources that your package will use.

The advantage of doing these things at prepublish time instead of preinstall or install time is that they can be done once, in a single place, and thus greatly reduce 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 the 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.
"preinstall": "node-waf clean || true; node-waf configure build":

If there is a wscript file in the root of your package, npm will default the preinstall command to compile using node-waf.

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 the scripts/install.js will be called for the install, post-install, stages of the lifecycle, and the scripts/uninstall.js would be called when the package is uninstalled. Since scripts/install.js is running for three 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 environ 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.

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