Flash

Flash is a Unix-style shell written in Rust to explore how real shells work under the hood. The project focuses on process creation, pipes, redirection, job control, and parsing command syntax into an executable AST.

It is intentionally built with low-level system calls (fork, execvp, waitpid, pipe, dup2, sigaction, setpgid, tcsetpgrp) so the execution model stays explicit and close to the OS.

What Flash Supports

• External command execution via fork + execvp
• Pipelines: cmd1 | cmd2
• Redirection: <, >, >>
• Logical operators: &&, ||
• Command sequencing: ;
• Background execution: &
• Built-ins: cd, jobs, exit
• Quoted string literals and escaped characters in the lexer

Architecture

Flash is split into clear stages:

1. Lexer (src/lexer/mod.rs)

   • Converts raw input into shell tokens.
   • Handles operators (|, &&, ||, redirects, ;, &), words, and string literals.

2. Parser (src/parser/mod.rs)

   • Uses recursive descent to build an AST.
   • Produces nodes for simple commands, pipes, redirects, conditionals, sequences, and background jobs.

3. Executor (src/executor/mod.rs)

   • Walks the AST and executes nodes recursively.
   • Manages file descriptors and anonymous pipes for stream routing.
   • Handles foreground/background process behavior and child reaping.

4. Runtime and Built-ins (src/main.rs, src/utils/mod.rs)

   • Runs the shell loop.
   • Tracks execution context (job table, shell PGID, exit status).
   • Installs/ignores terminal-related signals in the shell process.

Build and Run

Prerequisites

• Rust toolchain (stable)
• Linux or Unix-like environment

Commands

cargo build
cargo run

You will get an interactive prompt:

$

Example Commands

# basic execution
ls -la
# pipelines
cat Cargo.toml | grep edition
# redirectionecho hello > out.txt
cat < out.txt
echo world >> out.txt
# conditionals
mkdir test&&cdtest
cat missing_file ||echo"fallback"# sequencingpwd; ls
# background jobs
sleep 10 &jobs

Notes and Current Limitations

• This project is educational and under active iteration; behavior may differ from bash/zsh in edge cases.
• No glob expansion (*), environment variable expansion ($VAR), command substitution, or scripting support yet.
• Parsing/execution precedence is implemented for supported operators, but shell grammar coverage is intentionally minimal.

Why This Project

Flash was built to gain hands-on systems experience in:

• Linux process and signal semantics
• IPC and file descriptor lifecycle management
• Recursive command execution over ASTs
• Building language frontends (tokenizer + parser) for a real runtime

Roadmap

• Improve POSIX behavior compatibility
• Add variable expansion and quoting edge-case handling
• Add tests for lexer/parser/executor invariants
• Improve job control and terminal state transitions