Problem Sets 3

ABSTRACT

This problem set focuses on the interaction between a user-space program and the Operating System kernel. You will practice the “Fork-Exec-Wait” pattern, manage file I/O, and implement the string parsing logic required to build a functional command-line interface.

1. Process Management & System Calls

The core mechanics of how a shell launches and manages programs.

• Fork and Wait: Mastering the creation of child processes and parent synchronization.
• Running ls Command: Using the exec family to replace a process image with an external binary.
• Run CD: Implementing “Built-in” commands that modify the shell’s own state (like the current directory).
• Run Exit: Handling clean termination of a session.
• Test Perror: Using perror to translate system error codes into human-readable strings.

2. Command Line Parsing

Turning raw user input into actionable data for system calls.

• Tokenize Args & Parse Args: Converting a single command string into an array of pointers (char** argv).
• Parse Flags: Identifying and separating command options (e.g., -l, -a).
• Split Space: General utility for breaking strings based on whitespace.
• Print First Word Until Exit: Basic loop structure for a Read-Eval-Print Loop (REPL).

3. File I/O and String Utilities

Interacting with the file system and cleaning up data for display.

• Find In File: Searching for patterns within a file stream.
• Write With Line Numbers: Formatting output while writing to a file.
• Get Line: Safely reading a line of arbitrary length from a stream.
• Read History & Write History: Persistent storage for shell command history.
• Trim Trailing Spaces from a String: Sanitizing input before processing.
• Count Occurrences of a Character in a String: Utility for analyzing command syntax or delimiter frequency.

4. Advanced Formatting

• Print Parts Tabs: Managing tab-separated values.
• Print Space-Separated Parts of a String: Breaking down complex output into readable sections.
• Extract Second: Targeted extraction of specific arguments or tokens.

Implementation Mapping: The Shell Architecture