The basic tool to the linux kernel for executing commands. I have used this tool so much that I have created a space for holding all of my notes and templates related to the shell covering both bash and zsh for now. Its possible in the future that I will use other shells, but zsh has fit the bill for now. I have tried tcsh and fish and they were great, but I did cross some incompatibilies with community scripts and decided it was not really worth my time to try something for now.