Design And Implement A Shell Supporting Command Execution

Design and implement a shell supporting command execution, redirection, and piping

You will be writing a program that presents its user with a shell in which they can type commands and see their results. This shell should support changing directories, running commands, redirecting input and/or output, and piping output from one command to the input of another command.

You are to take the provided Command class (which can read commands from the keyboard) and use it to repeatedly read in and process commands until the user types in “exit” as a command. With the exception of the “cd” and “exit” commands, each command should create a new process and execute the command in the new process. The “exit” command should cause your shell program to end, printing a message indicating such. The “cd” command should use the chdir() system call to change directories in the current shell. Other commands should be executed using an execvp() call.

A command with an input redirect should use the contents of the specified input file as if it is standard input. For example, wc -l < data.txt will count the number of lines in data.txt. A command with an output redirect should use the contents of the specified output file as if it is standard output, overwriting the file if it exists. For example, wc -l < data.txt > output.txt.

A command specified to run in the background (by adding an &) should immediately allow another command to be entered at a new prompt, and when the background process completes, a summary including its name and PID should be printed. Commands without & should wait for completion before prompting again. A pipe (<|>) should redirect output from one command into the input of another, e.g., cat somefile.txt | sort.

Upon exit, your shell should print a message indicating that it has completed.

Sample Paper For Above instruction

The implementation of a custom shell in Unix-like operating systems is a fundamental exercise in understanding process control, inter-process communication, and system call utilization. In this context, the shell must support core functionalities such as command execution, directory management, input/output redirection, background processing, and piping, reflecting a simplified yet robust command interpreter.

The primary requirement is to develop a command-reading loop that utilizes a Command class capable of parsing user input into executable commands, arguments, and redirection specifications. The shell repeatedly prompts the user for input until the "exit" command is issued. At each prompt, input is analyzed, and the appropriate action is taken based on the command and its components.

For the "cd" command, the shell invokes the chdir() system call to change the current working directory of the process. This allows subsequent commands to be executed relative to the new directory. The "exit" command causes the shell to terminate gracefully, providing user feedback that the session has ended.

All other commands are forked into new processes where the child executes the command via execvp(). Before executing, the command undergoes redirection handling: input streams are redirected from files if specified (using the "" operator). For input redirection, the file specified replaces standard input; similarly, for output redirection, the file replaces standard output, ensuring data flow control.

Background processes are managed by detecting an "&" suffix in the command line. Such processes are forked, and the parent does not wait for their completion, instead logging their PID and command name once they terminate. Synchronous commands that run in the foreground cause the shell to wait using waitpid() until the process completes before prompting the user again.

Piping is managed by creating a pipe (via pipe()), and redirecting the output of the first command to the pipe's write end and the input of the second command to the pipe's read end. This setup allows for chained commands where data flows seamlessly from one process to the next, mimicking Unix shell behavior.

The shell's robustness includes error handling for system calls, validation of command input, and ensuring process resources are managed correctly to prevent zombie processes. Upon termination, the shell outputs a message confirming its shutdown, preserving a user-friendly interface that mirrors conventional command-line environments.

This project embodies key system programming concepts, providing a practical platform for understanding process management, file descriptor manipulation, and command parsing, essential for systems programming and operating systems coursework.

References

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