Build Systems are output-only, meaning it is not possible to execute some code that will request user input using the build system. The code will just block waiting for input forever. To work around this, many people use a REPL plugin.

JSON keys:

• shell_cmd specifies the exact command to run in the shell, and has support for variable placeholders (like ${file} that refers to the currently open file).
• result_file_regex and result_line_regex are used to parse the output from a failed build and show the errors in such a way that it is possible to navigate the editor to where an error occurred.
• selector is a scope selector that defines what syntax the build is relevant for. There's no point running a Python interpreter on PHP code, for example.
• variants can be used to allow a different command to be run, for example to just check the syntax of the file rather than execute it. The variants can also specify or override the same JSON keys, and they will apply only to that variant.
• syntax is used to set the syntax definition file which will be applied to the output panel, thus giving it syntax highlighting.

Package Control is the Sublime Text package manager. It includes a list of over 2,500 packages available for install, and users can add any GitHub or BitBucket repository themselves. Once installed, packages are kept up-to-date automatically.

The example code creates the Installed Packages folder for you (if necessary), and then downloads the Package Control.sublime-package into it. The download will be done over HTTP instead of HTTPS due to Python standard library limitations, however the file will be validated using SHA-256.