Docker containers were first developed on Linux for Linux. It is hence natural that the primary command-line tool used to work with Docker, also called a shell, is a Unix shell; remember, Linux derives from Unix. Most developers use the Bash shell. On some lightweight Linux distributions, such as Alpine, Bash is not installed and consequently one has to use the simpler Bourne shell, just called sh. Whenever we are working in a Linux environment, such as inside a container or on a Linux VM, we will use either /bin/bash or /bin/sh, depending on their availability.
Although Apple's macOS X is not a Linux OS, Linux and macOS X are both flavors of Unix and hence support the same set of tools. Among those tools are the shells. So, when working on macOS, you will probably be using the Bash shell.
In this book,...