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npm-scripts(1) -- How npm handles the "scripts" field

DESCRIPTION

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

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.
prepublish: Run BEFORE the package is published.
publish, postpublish: Run AFTER the package is published.
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 scrips can be run by doing npm run-script <stage> <pkg>.

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.

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" hash

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 npm-json(1) 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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