What is PowerShell?

While we assume in this book that you have a basic understanding of PowerShell command-line use and scripting, before we go too much into DSC, it is helpful to describe what PowerShell is compared to PowerShell DSC.

First released in 2006, PowerShell is a scripting language and command-line shell built on the .NET framework. PowerShell provides complete access to COM, WMI, and .NET and also provides a large set of commands called cmdlets to perform administrative tasks on both local and remote systems.

PowerShell can execute PowerShell scripts, PowerShell cmdlets, and standalone executable programs or other language files. PowerShell also provides a hosting API that allows programs to run PowerShell natively inside their program, enabling scripting or automation scenarios. Being both a shell language and a scripting language allows it to be quick as well as terse on the command line, along with being verbose and consistent in scripts.

Over the years, PowerShell has become the de facto way to administer and automate Windows OS and software. As computing environments grow larger and engineering teams become smaller, it is paramount in automating processes and procedures that used to be done by hand. PowerShell provides a consistent command-line language to automate the administration of a large number of scenarios, which are growing every day and were previously not available on Windows. Because of PowerShell's hosting API, applications such as Microsoft Exchange have enabled a command line, first, GUI—second, mode of development, which enables quick deployment and management using automated tools.

PowerShell not only enables the automation at the single system level, but it also scales out to the multi-node environment. Being able to automate your system from the command line or script is fine, but if you have to manually run that script on every system in your environment by hand, then we still have a bottleneck on efficiency. Using an industry standard protocol, PowerShell provides PowerShell remoting as a way of running commands or scripts on any number of remote hosts in parallel. Thousands of computers can be managed at the same time, consistently, in a repeatable and automated manner.

Consistent repeatable automation is important, but PowerShell is also extensible, which is not only essential, but it also leads us into DSC. PowerShell is a both a typed and dynamic scripting language, which means that it supports both static typed objects (System.IO.FileInfo) and objects with methods and properties that are defined at runtime (PSCustomObject and Add-Member). This enables PowerShell to be extended to suit the needs of the user. You do this every day to an extent, by creating functions and scripts to wrap common operations into reusable components or modules. Taking this a step further, PowerShell can be extended to support specific scenarios that were not envisioned when the product was made. DSC is such an extension, as it builds on the existing PowerShell language and infrastructure to enable new uses of the program.