A POSIX-compliant shell parser, formatter, and interpreter implemented in Rust.
Flash is a high-performance, extensible toolkit for processing POSIX-style shell scripts. The system comprises three primary components: a lexical analyzer, a syntax parser, and an execution interpreter, all implemented from the ground up in Rust. Flash provides comprehensive support for real-world shell syntax and offers structured Abstract Syntax Tree (AST) access for static analysis, code transformation, and tooling development.
The project draws inspiration from mvdan/sh while prioritizing performance optimization and architectural extensibility through modern systems programming practices.
Development Status: This project is currently under active development and should be considered experimental for production use cases.
The following table provides a comprehensive overview of POSIX Shell and Bash feature support within the Flash implementation. This matrix serves as both a development roadmap and compatibility reference.
Legend:
- Fully Supported: Complete implementation with full functionality
- Parser Only: Syntax recognition and AST generation without execution support
- Not Supported: Feature not currently implemented
| Category | Functionality / Feature | POSIX Shell | Bash | Flash | Implementation Notes |
|---|---|---|---|---|---|
| Basic Syntax | Variable assignment | Fully Supported | Fully Supported | Fully Supported | VAR=value syntax |
| Command substitution | Fully Supported | Fully Supported | Fully Supported | Both $(cmd) and `cmd` forms | |
| Arithmetic substitution | Not Supported | Fully Supported | Fully Supported | $((expr)) evaluation | |
Comments (#) | Fully Supported | Fully Supported | Fully Supported | Standard comment syntax | |
Quoting (', "", \) | Fully Supported | Fully Supported | Fully Supported | All quoting mechanisms | |
Globbing (*, ?, [...]) | Fully Supported | Fully Supported | Fully Supported | Pattern matching | |
| Control Structures | if / else / elif | Fully Supported | Fully Supported | Fully Supported | Conditional execution |
case / esac | Fully Supported | Fully Supported | Fully Supported | Pattern matching constructs | |
for loops | Fully Supported | Fully Supported | Fully Supported | Iteration constructs | |
while, until loops | Fully Supported | Fully Supported | Fully Supported | Loop constructs | |
select loop | Not Supported | Fully Supported | Fully Supported | Interactive selection | |
[[ ... ]] test command | Not Supported | Fully Supported | Fully Supported | Extended test expressions | |
| Functions | Function definition (name() {}) | Fully Supported | Fully Supported | Fully Supported | Standard function syntax |
function keyword | Not Supported | Fully Supported | Fully Supported | Bash-specific syntax | |
| I/O Redirection | Output/input redirection (>, <, >>) | Fully Supported | Fully Supported | Fully Supported | Standard redirection |
Here documents (<<, <<-) | Fully Supported | Fully Supported | Parser Only | Partial implementation | |
Here strings (<<<) | Not Supported | Fully Supported | Parser Only | Partial implementation | |
File descriptor duplication (>&, <&) | Fully Supported | Fully Supported | Parser Only | Partial implementation | |
| Job Control | Background execution (&) | Fully Supported | Fully Supported | Fully Supported | Process backgrounding |
Job control commands (fg, bg, jobs) | Fully Supported | Fully Supported | Fully Supported | Interactive mode only | |
Process substitution (<(...), >(...)) | Not Supported | Fully Supported | Parser Only | Basic <(cmd) support | |
| Arrays | Indexed arrays | Not Supported | Fully Supported | Fully Supported | arr=(a b c) syntax |
| Associative arrays | Not Supported | Fully Supported | Not Supported | declare -A requirement | |
| Parameter Expansion | ${var} basic expansion | Fully Supported | Fully Supported | Fully Supported | Variable expansion framework |
${var:-default}, ${var:=default} | Fully Supported | Fully Supported | Fully Supported | Default value expansion | |
${#var}, ${var#pattern} | Fully Supported | Fully Supported | Fully Supported | Length and pattern operations | |
${!var} indirect expansion | Not Supported | Fully Supported | Fully Supported | Variable indirection | |
${var[@]} / ${var[*]} array expansion | Not Supported | Fully Supported | Not Supported | Array element expansion | |
| Command Execution | Pipelines | Fully Supported | Fully Supported | Fully Supported | Command chaining |
Logical AND / OR (&&, ` | `) | Fully Supported | Fully Supported | ||
Grouping (( ), { }) | Fully Supported | Fully Supported | Fully Supported | Command grouping | |
Subshell (( )) | Fully Supported | Fully Supported | Fully Supported | Isolated execution context | |
Coprocesses (coproc) | Not Supported | Fully Supported | Not Supported | Bidirectional pipes | |
| Builtins | cd, echo, test, read, eval, etc. | Fully Supported | Fully Supported | Fully Supported | Core built-in commands |
shopt, declare, typeset | Not Supported | Fully Supported | Not Supported | Bash-specific builtins | |
let, local, export | Fully Supported | Fully Supported | Fully Supported | Variable management | |
| Debugging | set -x, set -e, trap | Fully Supported | Fully Supported | Parser Only | Partial debugging support |
BASH_SOURCE, FUNCNAME arrays | Not Supported | Fully Supported | Not Supported | Runtime introspection | |
| Miscellaneous | Brace expansion ({1..5}) | Not Supported | Fully Supported | Fully Supported | Sequence generation |
Extended globbing (extglob) | Not Supported | Fully Supported | Not Supported | Requires shopt configuration | |
Version variables ($BASH_VERSION) | Not Supported | Fully Supported | Fully Supported | $FLASH_VERSION in Flash | |
Script sourcing (. or source) | Fully Supported | Fully Supported | Fully Supported | External script inclusion |
A shell fundamentally operates as a macro processor that executes commands, where macro processing refers to the expansion of text and symbols into more complex expressions. The Unix shell paradigm encompasses dual functionality: serving as both a command interpreter and a programming language environment.
As a command interpreter, the shell provides the primary user interface to the comprehensive suite of Unix utilities and system commands. The programming language capabilities enable the composition and combination of these utilities into more sophisticated operations. Shell scripts, containing sequences of commands, achieve the same execution status as system binaries located in standard directories such as /bin, enabling users and organizations to establish customized automation environments.
Shell execution operates in two primary modes: interactive and non-interactive. Interactive mode processes user input from keyboard interfaces in real-time, while non-interactive mode executes command sequences from script files.
Flash maintains substantial compatibility with both POSIX shell (sh) and Bash specifications, implementing the core language features and execution semantics expected by existing shell scripts.
Production Readiness: Flash is currently in active development and should be evaluated carefully before deployment in production environments.
cargo install flashgit clone https://github.com/raphamorim/flash.git
cd flash && cargo install --path .git clone https://github.com/raphamorim/flash.git
cd flash
cargo build --release
# Linux systems
sudo cp target/release/flash /bin/
# macOS/BSD systems
sudo cp target/release/flash /usr/local/bin/
# Verify installation
flashTo configure Flash as the default system shell:
# Add Flash binary path to system shells registry
vim /etc/shells
# Linux systems
chsh -s /bin/flash
# macOS/BSD systems
chsh -s /usr/local/bin/flashFlash implements a configuration system through the .flashrc initialization file located in the user's home directory. This file executes during shell startup, enabling environment customization and initialization script execution.
The shell prompt can be customized through the PROMPT environment variable within the .flashrc configuration file:
# Minimal prompt configurationexport PROMPT="flash> "# Directory-aware promptexport PROMPT='flash:$PWD$ '# Full context prompt with user and hostnameexport PROMPT='$USER@$HOSTNAME:$PWD$ 'The PROMPT variable supports full variable expansion, allowing integration of any available environment variables into the prompt display.
# Prompt configurationexport PROMPT='flash:$PWD$ '# Standard environment variablesexport EDITOR=vim
export PAGER=less
# Future alias support (planned feature)# alias ll="ls -la"# alias grep="grep --color=auto"Flash provides comprehensive library functionality for integration into Rust applications, supporting multiple use cases including testing frameworks, shell script parsing, custom shell backend development, code formatting, and static analysis tooling.
A WebAssembly version with interactive documentation and examples is available at raphamorim.io/flash.
The Flash interpreter can be embedded directly into Rust applications:
use flash::interpreter::Interpreter;use std::io;fnmain() -> io::Result<()>{letmut interpreter = Interpreter::new();
interpreter.run_interactive()?;Ok(())}The run_interactive method utilizes Flash's default evaluation engine:
// Default interactive shell implementationpubfnrun_interactive(&mutself) -> io::Result<()>{let default_evaluator = DefaultEvaluator;self.run_interactive_with_evaluator(default_evaluator)}Flash supports custom evaluation logic through the Evaluator trait, enabling specialized shell behavior:
// Evaluation trait for custom implementationspubtraitEvaluator{fnevaluate(&mutself,node:&Node,interpreter:&mutInterpreter) -> Result<i32, io::Error>;}// Standard shell behavior implementationpubstructDefaultEvaluator;implEvaluatorforDefaultEvaluator{fnevaluate(&mutself,node:&Node,interpreter:&mutInterpreter) -> Result<i32, io::Error>{match node {Node::Command{ name, args, redirects } => {self.evaluate_command(name, args, redirects, interpreter)}Node::Pipeline{ commands } => {self.evaluate_pipeline(commands, interpreter)}Node::List{ statements, operators } => {self.evaluate_list(statements, operators, interpreter)}Node::Assignment{ name, value } => {self.evaluate_assignment(name, value, interpreter)}// Additional node types...
_ => Err(io::Error::other("Unsupported node type")),}}}The DefaultEvaluator implements comprehensive shell semantics including built-in command handling, pipeline execution, variable assignment, and external command invocation with proper environment variable propagation and I/O redirection support.
Flash provides direct access to its lexical analyzer for token-level processing:
fntest_tokens(input:&str,expected_tokens:Vec<TokenKind>){letmut lexer = Lexer::new(input);for expected in expected_tokens {let token = lexer.next_token();assert_eq!(
token.kind, expected,"Expected {:?} but got {:?} for input: {}",
expected, token.kind, input
);}// Verify complete token consumptionlet final_token = lexer.next_token();assert_eq!(final_token.kind,TokenKind::EOF);}#[test]fntest_function_declaration(){let input = "function greet() { echo hello; }";let expected = vec![TokenKind::Function,TokenKind::Word("greet".to_string()),TokenKind::LParen,TokenKind::RParen,TokenKind::LBrace,TokenKind::Word("echo".to_string()),TokenKind::Word("hello".to_string()),TokenKind::Semicolon,TokenKind::RBrace,];test_tokens(input, expected);}The parser component transforms token streams into structured Abstract Syntax Trees:
use flash::lexer::Lexer;use flash::parser::Parser;#[test]fntest_simple_command(){let input = "echo hello world";let lexer = Lexer::new(input);letmut parser = Parser::new(lexer);let result = parser.parse_script();match result {Node::List{ statements, operators } => {assert_eq!(statements.len(),1);assert_eq!(operators.len(),0);match&statements[0]{Node::Command{ name, args, redirects } => {assert_eq!(name,"echo");assert_eq!(args,&["hello","world"]);assert_eq!(redirects.len(),0);}
_ => panic!("Expected Command node"),}}
_ => panic!("Expected List node"),}}Flash includes a comprehensive formatter for shell script standardization:
// String-based formattingassert_eq!(Formatter::format_str(" # This is a comment"),"# This is a comment");// AST-based formattingletmut formatter = Formatter::new();let node = Node::Comment(" This is a comment".to_string());assert_eq!(formatter.format(&node),"# This is a comment");This project is licensed under the GPL-3.0 License. Copyright © Raphael Amorim.