Rapid Unix SHell (rush)

Introduction

rush (Rapid Unix SHell) is a simple, efficient Unix shell implemented as a command line interpreter (CLI). It repeatedly prints a prompt rush> , parses the input, executes the specified command, and waits for the command to finish. This cycle continues until the user types exit.

Features

• Basic shell functionality: prompt, parse input, execute commands.
• Path handling to locate executables.
• Built-in commands: exit, cd, and path.
• Output redirection.
• Parallel command execution.
• Robust error handling and whitespace management.

Program Specification

Basic Shell

The shell operates in a loop, performing the following steps:

1. Print the prompt rush> .
2. Read the input command.
3. Parse the command and its arguments.
4. Execute the command.
5. Wait for the command to complete.
6. Repeat until the user types exit.

Execution of Commands

Commands are executed by creating a new process using fork() and execv(). If fork() fails, an error is reported. The shell waits for the command to finish using wait() or waitpid().

Paths

The shell searches for executables in directories specified by the path variable. Initially, the path contains one directory: /bin. The shell checks if a command is executable using the access() system call. If a command is not found in any specified directory, an error is reported.

Built-in Commands

• exit: Exits the shell. No arguments are allowed.
• cd <directory>: Changes the current directory to <directory>. If cd fails, an error is reported. It must take exactly one argument.
• path [<dir1> <dir2> ...]: Sets the search path for executables. If no arguments are provided, the path is cleared.

Redirection

The shell supports output redirection using the > symbol. For example, ls -la /tmp > output redirects the output of ls to the file output, overwriting its content if it exists. Errors in opening the file are reported.

Parallel Commands

The shell can execute multiple commands in parallel using the & symbol. For example, cmd1 & cmd2 args1 args2 & cmd3 args1 runs cmd1, cmd2, and cmd3 in parallel. The shell waits for all commands to complete before returning control to the user.

Error Handling

The shell prints a single error message An error has occurred\n to stderr whenever an error is encountered. The shell continues processing after most errors, except when invoked with arguments, in which case it exits with status 1.

Whitespace Management

The shell handles various kinds of whitespace, including spaces and tabs, ensuring that commands and arguments are correctly parsed regardless of whitespace variations.

Usage

1. Compile the program:

   make
   

2. Run the compiled program:

   ./rush