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How npm handles the "scripts" field

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

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).

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:

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:

npm will default some script values based on package contents.

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.

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.

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.

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 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.

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

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"

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"
  }
}

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.

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.

Last modified June 02, 2015           Found a typo? Send a pull request!

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