b47

Description

b47 is a high-performance, minimalist Unix shell engineered for advanced users demanding granular command-line control and extensibility. It features a sophisticated parsing engine capable of constructing and interpreting complex syntactic structures, including nested pipelines, conditional execution flows, and comprehensive I/O redirection semantics. Designed with modularity and robustness in mind, b47 implements a rich subset of POSIX-compliant shell behaviors, ensuring precise command evaluation and rigorous process management typically reserved for industrial-grade shell environments:

• Robust Command Parsing: Supports pipelines (|), command grouping (()), logical operators (&&, ||), sequential execution (;), and background processes (&).
• Masterful Redirection Handling: Fully implements input/output redirections (<, >, >>) and heredocs (<<), letting you manipulate data streams like a pro.
• Environment Variable Engine: Seamlessly manages environment variables with export/unset capabilities, ensuring your shell session is both dynamic and scriptable.
• Foreground/Background Job Control: Tracks, updates, and controls processes with detailed job management, allowing you to juggle multiple tasks effortlessly.
• Signal Savvy: Captures and handles UNIX signals to keep the shell responsive and stable, ignoring interruptions when needed but gracefully managing child processes.
• User-Friendly Command History: Leverages GNU Readline for smart history navigation and line editing.
• Advanced Expansion: Handles variable expansions and command preprocessing to let you build complex commands intuitively.

---

Project Structure

File	Purpose
main.c	The heart of b47: command input, shell loop, and initialization.
parser.c/h	Syntax analyzer: tokenizes input and builds the command AST.
process.c/h	Job control and process lifecycle management.
variable.c/h	Environment variables: set, unset, export, and manage vars.
shell.h	Core globals (like shell process group ID) shared across modules.
executor.c/h	Command executor handling built-in and external commands.
expand.c/h	Handles variable and tilde expansions inside commands.
builtins.c/h	Implements built-in commands like cd, exit, jobs, etc.
monitor.h	(Optional) System resource monitoring utilities.

---

Core Data Structures

Type	Description
node_t	Abstract syntax tree node for commands, pipelines, logical connectors, and redirections.
redir_t	Represents a command’s input/output redirections, including heredoc data.
process	Tracks running process metadata: PID, command string, and execution state.
var_entry	Linked list node for environment variables, including export flags.

---

Supported Syntax and Operators

Operator	Description
|	Pipe: Passes the standard output of one command as the standard input to the next.
&&	AND: Executes the following command only if the previous command exits successfully (exit status 0).
||	OR: Executes the following command only if the previous command fails (non-zero exit status).
;	Sequence: Executes commands sequentially, regardless of the previous command’s result.
&	Background execution: Runs the preceding command asynchronously, returning control immediately to the shell.
()	Grouping: Groups commands or pipelines into a subshell, enabling combined redirections or logical operations.
<, >, >>, <<	Redirections:
	- < : Redirects input from a file to the command.
	- > : Redirects output to a file, overwriting existing content.
	- >>: Redirects output to a file, appending to existing content.
	- <<: Heredoc: Takes multiline input until a delimiter line is found, feeding it to command input.
$VAR, ${VAR}	Variable expansion: Replaces variables with their current values in the environment.
~	Tilde expansion: Expands to the current user's home directory or another user’s home when followed by username.
" and '	Quoting:
	- " (Double quotes): Expands variables and allows command substitution inside.
	- ' (Single quotes): Treats everything literally, no expansion or substitution.
\ (Backslash)	Escape character: Escapes the next character, preventing its special interpretation.
$(...)	Command substitution: Executes the command inside and replaces the syntax with its output.
`...`	Alternative command substitution syntax (deprecated but supported).
jobs	Built-in command: Lists active background and stopped jobs.
fg, bg	Built-in commands: Resume stopped jobs in the foreground or background respectively.

---

Additional Notes:

• Subshell Execution: Using () creates a subshell environment, isolating variable changes and redirections inside the group.
• Command Chaining: Combining operators allows complex flow control, e.g., cmd1 && cmd2 || cmd3; cmd4 &.
• Signal Handling: The shell intercepts job control signals (e.g., SIGINT, SIGTSTP) and delegates them to child processes while preserving the shell's stability and responsiveness.
• History Expansion: Integrated with GNU Readline, allowing use of ! for recalling previous commands (if implemented).

---