Write Your Own Shell

Authors

• Alina Hlazkova (346316979)
• Sergey Nazaryev (342537685)

Usage

make shell
./shell

Introduction

The object of this assignment is to gain experience with some advanced programming techniques such as process creation, termination, and overriding an existing process (fork, wait, exec, and additional system calls). To do this, you will be writing your own command shell — much like bash.

What is Shell?

A shell program is an application that allows interacting with the computer. In a shell, the user can run programs and also redirect the input and output. Shell additionally offers features such as line editing, history, file completion, etc.

"A shell is an interface that allows you to interact with the kernel of an operating system."

Shell Workflow

A shell parses commands entered by the user and executes them. The workflow of the shell will look like this:

1. Startup the shell
2. Wait for user input
3. Parse user input
4. Execute the command and return the result
5. Go back to 2

The shell is the parent process. This is the main process in our program which is waiting for user input. However, we cannot execute the command in the parent process itself, because of the following reasons:

1. A command with error will cause the shell to stop working. We want to avoid this.

2. Independent commands should have their own process blocks. This is known as the isolation principle.

Required Features

Here are the features you should support in your shell:

1. The shell should indicate the current working directory using pwd.

2. cd <path> should change the working directory

3. Create a file by the command cat > <filename> or nano <filename>.

4. cat <filename>. View the contents of the file.

5. wc [options/flags] filename. The wc command should support the following flags wc -l <filename>, wc -c <filename>, wc -w <filename>.

6. cp <file1> <file2>. Copy the file to file

7. The shell has to support pipe operator "|" between processes.

8. whoami. Display the username of the current user when this command is invoked.

9. grep. Return lines with a specific word/string/pattern in a file. grep [options/flags] [Pattern] <filename>. Should support in the following options(flags): grep -c [pattern] <filename>

10. man [command name]. A man command can have one argument (command name) and then it will display a manual only for the commands that you have to implement in your own shell; man without arguments will display a manual for all the commands we requested to execute in this assignment.

11. ls command lists directory contents of files and directories. ls -l show long listing information about the file/directory.

12. tree print all files. It is used to display the contents of any desired directory of your computer system in the form of a tree structure. By using this tree structure, you can easily find out the paths of your files and directories.