MSH (mini shell) - project analysis

Overview
- This project implements a small interactive Unix-like shell in pure C.
- It is built around the C standard library, POSIX system calls, GNU Readline, and a local libft utility library.
- The final executable name is `minishell`.

High-level execution flow
1. `msh.c` initializes shell state (environment copy and shell path).
2. It enters a REPL loop using `readline("> ")`.
3. User input is tokenized (`src/tokenizer`), parsed into an AST (`src/parser`), and executed (`src/exec`).
4. Command status is stored/retrieved through utilities in `src/utils/status.c`.

Main components
- `src/tokenizer/`
  - Splits input into tokens (words, pipes, redirections, heredoc operators).
  - Handles quote-aware word extraction and basic syntax checks for unclosed quotes.

- `src/parser/`
  - Converts token lists into command/pipe AST nodes.
  - Builds command nodes with arguments and redirection metadata.
  - Provides AST analysis and cleanup helpers.

- `src/expander/`
  - Expands environment variables.
  - Handles quote processing and text transformation before execution.

- `src/heredoc/`
  - Implements heredoc collection and related signal behavior.

- `src/exec/`
  - Executes command ASTs, including pipelines.
  - Applies redirections and runs external commands via an execvp-style wrapper.
  - Manages process creation (`fork`, `pipe`, `dup2`, `wait`) and exit status propagation.

- `src/builtins/`
  - Built-in commands: `cd`, `export`, `env`, `unset`, `pwd`, `echo`, `exit`.
  - Distinguishes builtins that must run in the parent shell process (state-changing operations) from child-process execution.

- `src/utils/`
  - Signal setup for interactive behavior.
  - Environment list helpers and path handling.
  - Global status getter/setter.

- `libc/`
  - Local libft-style helper library (string/list/memory helpers) built as `libft.a`.

Build and run
- Build:
  - `make`
- Run:
  - `./minishell`
- Clean:
  - `make clean`
  - `make fclean`
  - `make re`

Dependencies
- C compiler (`cc`)
- GNU Readline development library (`-lreadline`)
- POSIX environment (Linux/macOS-like system calls)

Notes
- Input history is enabled through Readline (`add_history` in parser entry).
- The shell includes custom signal handling for interactive usage and heredoc mode.
- The Makefile includes a `valgrind` target for memory/debug checks.