THE CODE MUST BE IN C ONLY. Every shell needs to support a number of built-in commands, which are functions in the shell itself, not external programs. Shells directly make system calls to execute built-in commands, instead of forking a child process to handle them.
The requirement states that the shell code must be written in the programming language C and should support a set of built-in commands. These built-in commands are functions that are part of the shell itself and do not rely on external programs. Instead of creating a new process for executing these commands, the shell directly makes system calls to handle them.
By implementing built-in commands within the shell, we can achieve more efficient execution since we do not need to create a new process for each command. This eliminates the overhead of process creation, leading to faster execution times.
To meet these requirements, we can design the shell program to have a set of functions that correspond to each built-in command. These functions can be defined within the shell source code and called directly when the user enters one of the built-in commands.
For instance, if we want to implement a built-in command like “cd” (change directory), we can define a function called “builtin_cd” within our shell code. This function would contain the logic to handle changing the current working directory.
When the user enters the “cd” command in the shell, the shell program would first check if this is a built-in command. If it is, it would call the corresponding function “builtin_cd” to execute the command.
To make system calls within the shell code, we can utilize the system call interface provided by the operating system. In the case of Unix-like systems, the “syscall” function can be used to make system calls.
For example, if we want to implement a built-in command like “pwd” (print working directory), we can define a function called “builtin_pwd”. Inside this function, we can use the system call interface to retrieve the current working directory and then print it to the console.
Once we have implemented all the necessary built-in commands as functions within the shell code, we can integrate them into the shell’s command execution flow. When the user enters a command in the shell, the program would check if it is a built-in command by comparing it with the list of supported commands. If it is a built-in command, the corresponding function would be called to handle the command. If it is not a built-in command, the shell would fork a child process and execute the external program corresponding to the command.
In conclusion, the requirement to implement built-in commands in a C-based shell program involves defining functions within the shell code to handle these commands and making system calls directly instead of creating new processes. This approach allows for more efficient execution of the built-in commands and faster response times.