The shell or the command interpreter (we will hereafter refer to this as the shell) is the routine, which communicates with the user in a console based operating system like JOSH. The shell displays a prompt, gets user input, analyses it and performs the necessary action. Usually there are commands that the shell understands itself and provides the user with the necessary result - these commands are called internal commands. Sometimes the shell doesn't understand the command, and usually in this situation it tries to find a file with the command name and loads it for execution, if it is executable.
The shell is the interface between the operating system and the user and hence it is very important. Intuitiveness and usefulness of the shell determines if the OS is liked or hated by the user. The design of a shell takes into consideration many factors:
1. Should the shell be part of the kernel or should it be another application that the kernel loads automatically? - This is an important consideration as shell being part of the kernel will usually retain that space in memory throughout, resulting in reduced memory availability for user applications. The other option is better as the shell can be overwritten by the user application and re-loaded when the user application terminates. We will use the former approach as it is easy to code and we are aiming to provide only minimal functionality with the shell, which wouldn't take much space.
2. Length of the commands the shell can receive - We will set this limit as 255 characters. The shell will receive only 255 characters.
3. How many operands will a typical command have? - The operands can modify the command and sometimes will be parameters to the command. We will decide that the JOSH shell will accept a command with 4 operands and anything more will be discarded. The command and operands will be on a single string of characters and will be separated by spaces. The shell can identify leading/trailing/multiple-intermittent spaces and will remove them while deriving the command and the operands.
4. What internal commands will the shell support? - We will not define this at this moment. We will add internal commands as they are needed.
Having said so much let us delve into the design of the JOSH shell. Move on to the next chapter.