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Chef DK is a software development kit providing a bundle of tools for developing and testing Chef cookbooks.
Prior to the arrival of Chef DK, setting up a toolkit for chef was a difficult process. A developer might need as many as a dozen separate tools from the chef ecosystem; each of these tools has its own requirements for the Ruby interpreter version and Ruby gems. Keeping a mutually compatible set of tools installed was very difficult. Chef DK provides an internally bundled Ruby installation, with all needed Ruby gems already installed and known to be mutually compatible. When running a tool, you use the chef exec command to run the tool in the provided environment.
There are many ways to layout cookbooks, and many ways to choose which components of Chef to use. Newcomers are often confused about what approach to take. Chef DK provides some guidance in these areas.