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Pow User's Manual

Pow is a zero-configuration Rack server for Mac OS X. It makes developing Rails and Rack applications as frictionless as possible. You can install it in ten seconds and have your first app up and running in under a minute. No mucking around with /etc/hosts, no compiling Apache modules, no editing configuration files or installing preference panes. And running multiple apps with multiple versions of Ruby is trivial.

How does it work? A few simple conventions eliminate the need for tedious configuration. Pow runs as your user on an unprivileged port, and includes both an HTTP and a DNS server. The installation process sets up a firewall rule to forward incoming requests on port 80 to Pow. It also sets up a system hook so that all DNS queries for a special top-level domain (.test) resolve to your local machine.

To serve a Rack app, just symlink it into your ~/.pow directory. Let's say you're working on an app that lives in ~/Projects/myapp. You'd like to access it at http://myapp.test/. Setting it up is as easy as:

$ cd ~/.pow
$ ln -s ~/Projects/myapp

That's it! The name of the symlink (myapp) determines the hostname you use (myapp.test) to access the application it points to (~/Projects/myapp).

1 Installation

Pow requires Mac OS X version 10.9 or newer. To install or upgrade Pow, just open a terminal and run this command:

$ curl | sh

You can review the install script yourself before running it, if you'd like. Always a good idea.

The installer unpacks the latest Pow version into ~/Library/Application Support/Pow/Versions and points the ~/Library/Application Support/Pow/Current symlink there. It also installs launchd scripts for your user (the Pow server itself) and for the system (to set up the ipfw rule), if necessary. Then it boots the server.

Note: The firewall rule installed by Pow redirects all incoming traffic on port 80 to port 20559, where Pow runs. This means if you have another web server running on port 80, like the Apache that comes with Mac OS X, it will be inaccessible without either disabling the firewall rule or updating that server's configuration to listen on another port.

1.1 Installing From Source

To install Pow from source, you'll need Node 0.10.0 or higher and npm:

$ git clone
$ cd pow
$ npm --global install
$ npm --global run-script pow restart

For detailed instructions on installing Pow from source, including instructions on how to install Node and npm, see the Installation wiki page.

1.2 Uninstalling Pow

If you decide Pow's not for you, uninstallation is just as easy:

$ curl | sh

(Review the uninstall script.)

2 Managing Applications

Pow deals primarily with Rack applications. For the purposes of this document, a Rack application is a directory with a rackup file (and optionally a public subdirectory containing static assets). For more information on rackup files, see the Rack::Builder documentation.

Pow automatically spawns a worker process for an application the first time it's accessed, and will keep up to two workers running for each application. Workers are automatically terminated after 15 minutes of inactivity.

2.1 Using Virtual Hosts and the .test Domain

A virtual host specifies a mapping between a hostname and an application. To install a virtual host, symlink a Rack application into your ~/.pow directory. The name of the symlink tells Pow which hostname you want to use to access the application. For example, a symlink named myapp will be accessible at http://myapp.test/.

Note: The Pow installer creates ~/.pow as a convenient symlink to ~/Library/Application Support/Pow/Hosts, the actual location from which virtual host symlinks are read.

2.1.1 Subdomains

Once a virtual host is installed, it's also automatically accessible from all subdomains of the named host. For example, the myapp virtual host described above could also be accessed at http://www.myapp.test/ and http://assets.www.myapp.test/. You can override this behavior to, say, point www.myapp.test to a different application — just create another virtual host symlink named www.myapp for the application you want.

2.1.2 Multiple Virtual Hosts

You might want to serve the same application from multiple hostnames. In Pow, an application may have more than one virtual host. Multiple symlinks that point to the same application will share the same worker processes.

2.1.3 The Default Virtual Host

If you attempt to access a domain that Pow doesn't understand, like http://localhost/, you'll see a page letting you know that Pow is installed and working correctly, with instructions on how to set up an application.

You can override this behavior to serve all requests for unhandled domains with a particular Rack application. Create a symlink in ~/.pow named default and point it to the application of your choice.

2.1.4 Port Proxying

Pow's port proxying feature lets you route all web traffic on a particular hostname to another port or IP address. To use it, just create a file in ~/.pow (instead of a symlink) with the destination port number as its contents.

For example, to forward all traffic for http://proxiedapp.test/ to port 8080 on your localhost:

$ echo 8080 > ~/.pow/proxiedapp

To forward traffic for http://proxiedapp.test/ to an IP address other than localhost (such as a virtual machine), create a file with the destination protocol, IP, and port:

$ echo > ~/.pow/proxiedapp

You can also use port proxying to access web apps written for other runtimes such as Python or Node.js. Remember that services behind the proxy won't automatically be started or stopped like Rack apps.

2.1.5 Accessing Virtual Hosts from Other Computers

Sometimes you need to access your Pow virtual hosts from another computer on your local network — for example, when testing your application on a mobile device or from a Windows or Linux VM.

The .test domain will only work on your development computer. However, you can use the special domain to remotely access your Pow virtual hosts.

Construct your domain by appending your application's name to your LAN IP address followed by For example, if your development computer's LAN IP address is, you can visit myapp.test from another computer on your local network using the URL

2.2 Customizing Environment Variables

Pow lets you customize the environment in which worker processes run. Before an application boots, Pow attempts to execute two scripts — first .powrc, then .powenv — in the application's root. Any environment variables exported from these scripts are passed along to Rack.

For example, if you wanted to adjust the Ruby load path for a particular application, you could modify RUBYLIB in .powrc:

export RUBYLIB="app:lib:$RUBYLIB"

2.2.1 Choosing the Right Environment Script

Pow supports two separate environment scripts with the intention that one may be checked into your source control repository, leaving the other free for any local overrides. If this sounds like something you need, you'll want to keep .powrc under version control, since it's loaded first.

2.3 Working With Different Versions of Ruby

Pow invokes each application's Ruby processes in an isolated environment. This design makes it possible to use different Ruby runtimes on a per-application basis.

There are three ways to specify which version of Ruby to use for a particular application.

2.3.1 Specifying Ruby Versions with rbenv

You can use rbenv to specify per-application Ruby versions for use with Pow.

The rbenv local command lets you set a per-project Ruby version by saving an .rbenv-version file in the current directory. For example, to configure a particular application to run with Ruby 1.9.3-p194, change to the application's directory and run:

$ rbenv local 1.9.3-p194

Assuming you have set up rbenv in your login environment, there is no additional configuration necessary to use it with Pow.

For more information, see the rbenv documentation.

2.3.2 Specifying Ruby Versions with RVM

RVM is another option for specifying per-application Ruby versions for use with Pow.

You can create a project .rvmrc file to specify an application's Ruby version. For example, to configure your application to run with Ruby 1.8.7, add the following to .rvmrc in the application's root directory:

rvm 1.8.7

Because RVM works by injecting itself into your shell, you must first load it in each application's .powrc or .powenv file using the following code:

if [ -f "$rvm_path/scripts/rvm" ] && [ -f ".rvmrc" ]; then
  source "$rvm_path/scripts/rvm"
  source ".rvmrc"

For more information, see the RVM web site.

2.3.3 Specifying Ruby Versions by Setting $PATH

You can adjust the PATH environment variable in .powrc or .powenv to select Ruby versions on a per-application basis. For example, if you have Ruby installed into /opt/ruby-1.8.7, you can add the following to .powenv:

export PATH="/opt/ruby-1.8.7/bin:$PATH"

When Pow loads your application, it will find and use the first ruby binary in your PATH — in this case /opt/ruby-1.8.7/bin/ruby.

2.4 Serving Static Files

Pow automatically serves static files in the public directory of your application. It's possible to serve a completely static site without a file as long as it has a public directory. If you have a static site and want to keep your files in the root of your project (i.e. not in a public directory), you can do the following:

$ cd ~/.pow
$ mkdir your-app-domain
$ cd !$
$ ln -s ~/Projects/your-app public

2.5 Restarting Applications

You can tell Pow to restart an application the next time it's accessed. Simply save a file named restart.txt in the tmp directory of your application (you'll need to create the directory first if it doesn't exist). The easiest way to do this is with the touch command:

$ touch tmp/restart.txt

Restarting an application will also reload any environment scripts (.powrc, .powenv, or .rvmrc) before booting the app, so don't forget to touch restart.txt if you make changes to these scripts.

It's also fine to kill worker processes manually — they'll restart the next time you access the virtual host. A handy way to do this is with OS X's Activity Monitor. Select "All Processes, Hierarchically" from the dropdown at the top of the Activity Monitor window. Then find the pow process, expand the disclosure triangle, find the Ruby worker process you want to kill, and choose "Quit Process." (You can click "Inspect" on a worker process and choose "Open Files and Ports" to determine which application the process is serving.)

2.5.1 Restarting Before Every Request

It can be useful during development to reload your application with each request, and frameworks like Rails will handle such reloading for you. For pure Rack apps, or when using frameworks like Sinatra that don't manage code reloading, Pow can help.

If the tmp/always_restart.txt file is present in your application's root, Pow will automatically reload the application before each request.

Note: tmp/always_restart.txt will only reload the application, not its environment scripts. To reload .powrc, .powenv, or .rvmrc, you must touch tmp/restart.txt first.

2.6 Viewing Log Files

Pow stores log files in the ~/Library/Logs/Pow directory so they can be viewed easily with OS X's Console application. Each incoming request URL is logged, along with its hostname and HTTP method, in the access.log file. The stdout and stderr streams for each worker process are captured and logged to the apps directory in a file matching the name of the application.

Note: Rails logger output does not appear in Pow's logs. You'll want to tail -f log/development.log to see those entries.

3 Configuring Pow

Pow is designed so that most people will never need to configure it. Sometimes, though, you can't avoid having to adjust a setting or two.

When Pow boots, it executes the .powconfig script in your home directory if it's present. You can use this script to export environment variables that will override Pow's default settings.

For example, this ~/.powconfig file tells Pow to kill idle applications after 5 minutes (300 seconds) and spawn 3 workers per app:

export POW_TIMEOUT=300
export POW_WORKERS=3

See the Configuration class documentation for a full list of settings that you can change.

Note: After modifying a setting in ~/.powconfig, you'll need to restart Pow for the change to take effect. See the Restarting Pow section below.

3.1 Configuring Top-Level Domains

The POW_DOMAINS environment variable specifies a comma-separated list of top-level domains for which Pow will serve DNS queries and HTTP requests. The default value for this list are the two test,dev domains, meaning Pow will configure your system to resolve *.test and *.dev to and serve apps in ~/.pow under the .test and .dev domains.

The .test domain is preferred since Google owns the .dev TLD and has recently enabled HSTS, forcing all requests to use HTTPS. Pow supports .dev by default for backward compatibility only.

You can add additional domains to POW_DOMAINS:

export POW_DOMAINS=test,local

If you want Pow to serve apps under additional top-level domains, but not serve DNS queries for those domains, use the POW_EXT_DOMAINS variable. Entries in POW_EXT_DOMAINS will not be configured with the system resolver, so you must make sure they point to your computer by other means.

Note: If you change the value of POW_DOMAINS, you must reinstall Pow using curl | sh. This is because the relevant files in /etc/resolver/ are created at install time.

WARNING: Adding top-level domains like ".com" to POW_DOMAINS can be hazardous to your health! In the (likely) event that at some point you lock yourself out of these domains, you will be unable to reach important remote addresses like (where you can find the source code) and S3 (where Pow's installation and uninstallation scripts are hosted). Do not panic! Delete the files Pow has created in /etc/resolver/ and DNS activity will return to normal. (You can safely use POW_EXT_DOMAINS for these domains instead.)

3.2 Reading the Current Configuration

If you are writing software that interfaces with Pow, you can inspect the running server's status and configuration via HTTP. To access this information, open a connection to localhost and issue a GET request with the header Host: pow. The available endpoints are:

Example of requesting an endpoint with curl:

$ curl -H host:pow localhost/status.json

Alternatively, if you know the path to the Pow binary, you can generate an eval-safe version of the local configuration by invoking Pow with the --print-config option (useful for shell scripts):

$ eval $(~/Library/Application\ Support/Pow/Current/bin/pow \

3.3 Restarting Pow

Pow runs as a Mac OS X Launch Agent. If the Pow server process terminates, the OS will restart it automatically.

You may need to manually restart Pow if you make configuration changes to ~/.powconfig or modify your login environment. To tell Pow to quit and restart, touch the global restart.txt file:

$ touch ~/.pow/restart.txt

Alternatively, you can use the Activity Monitor application. Find the pow process in the process listing, select it, and click the Quit Process button.

4 Third-Party Tools

5 Contributing

Pow is written in Node.js with CoffeeScript. You can read the annotated source code to learn about how it works internally. Please report bugs on the GitHub issue tracker.

If you're interested in contributing to Pow, first start by installing Pow from source.

Make your changes and use cake to run the test suite:

$ cake test

Then submit a pull request on GitHub. Your patch is more likely to be merged if it's well-documented and well-tested. Read through the closed issues to get a feel for what's already been proposed and what a good patch looks like.

6 Version History

7 License

(The MIT License)

Copyright © 2017 Sam Stephenson, Basecamp

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.