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Pro SSH Workspace
Allows you to connect directly to your external server via SSH. Modifying files directly on your server using a cloud based editor allows you to have the portability of the a cloud based workspace with the control of your own server (including complete DNS control).
Pro Can be hosted on own server
Since Cloud9 is an open source project with source code available on GitHub, it can be run as a self-hosted solution on own hardware and behind a firewall as long as it's being used on non-commercial projects.
Pro Terminal with root access
Cloud9 gives full terminal access to home directory. In their hosted Linux Ubuntu environment it has sudo powers. No UNIX commands have been blocked - npm, ifconfig, chmod, chown, tar, etc work. All commands can be accessed and any package can be installed. However, certain advanced features are inaccessible (one cannot run Docker within the terminal).
And if the terminal is used when using Remote SSH feature it connects directly to the server and runs the commands on that server.
Pro Provides with own runtime environment
Cloud9 can connect to a dedicated VM to provide a powerful Ubuntu runtime environment in the cloud using Docker. Apps can be either run from the run panel where a selection of runners is provided or from a terminal.
Pro Git & Mercurial support
Git and hg commands can be run in the command-line, the same way as in a local terminal. There are also built-in add-on services for GitHub, BitBucket and GitLab.
Pro Capable editor
Cloud9 uses their own editor called ACE. Besides the basics, it covers most important advanced code editor features such as code folding, converting cases, auto-completion, code analysis and refactoring, regex search and offers easy access to relevant documentation.
It also gives access to the CLI, has support for Vim and Emacs keybindings, includes multiple cursors and zen coding mode that removes all distractions and allows focusing on code.
Pro Enables real-time online collaboration
An important feature of Cloud9 is the real-time collaboration ability. It allows pairing programs or perform code reviews really easily as well as simultaneously text chat.
Pro Runs any language
The runner has built-in functionality fo Apache, Node, Python, Ruby, Ruby on Rails, Go, CoffeeScript, Julia, Mocha and Shell script, but any other language can be used by creating a runner for it.
Breakpoints allow specifying a stopping points in the execution of the application. When these breakpoints are hit, the application will stop executing and give the ability to examine data such as local variables, run commands and control the execution flow of the application.
Pro Support for most databases
In addition to launching a server to run code, Cloud9 will also host a database to develop against. Support for MySQL, PostgreSQL, MongoDB and SQLite.
Pro Provides with a simple way to deploy apps
Pro Ability to clone multiple repos in one project
Cloud9 provides one free private workspace. However, I can host multiple projects there by cloning as many repositories into the root project directory, thanks to the full access terminal.
Pro Package manager
Similar to package managers for the desktop, Cloud9 also includes their own package manager, c9pm, which allows adding new software from a list of available utilities.
Apt-get can be used in the project's workspace terminal to install/update/upgrade software. Composer, Bower or any other utilities of choice can be installed to manage dependencies and packages.
Pro Offline editing
By installing and running a client application that syncs the local file system and cloud storage Cloud9 can be run locally. Great alternative for situations when the Internet connection is unreliable.
Pro Great documentation
Pro Browser testing support
Cloud9 integrates with Sauce Labs a browser testing suite that allows previewing the app in any desktop or mobile browser.
Pro Bitbucket integration
You can launch into C9 directly from the Bitbucket UI via a plugin available on Bitbucket. If you set up your Git repo for a project in Bitbucket and open it in C9 you can push to Git via the CLI from the start.
Pro Built-in image editor
Includes a very basic image preview and editing tool.
For more than one private workspace, the starting price is $19/month.
Con Lacks a built-in Java builder and runner
Con Non free/libre
Con Cannot run "chroot"
This breaks many developer tools like Docker and Bazel. This is the result of the fact that the hosted environment runs within Docker which limits access to this capability in order to prevent breaking out of the container.
Con You need a credit card to sign up
You need a credit card to sign up for Cloud9, which is problematic for users in countries where credit cards are not the norm.
They won't bill you (for the free account), but still require it. You can buy a VISA gift card and sign up with that.