Lush

A Unix shell with developer-first design.

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What is Lush?

Lush is a Unix shell built entirely from scratch. It combines POSIX compliance with carefully chosen extensions from Bash and Zsh, plus capabilities found nowhere else—most notably an integrated debugger for shell scripts. The native line editor (LLE) provides syntax highlighting and context-aware completions without relying on GNU Readline or any external library. Lush is a single binary with zero runtime dependencies.

Current status: Under heavy development. Not yet suitable for daily use or production environments. Many features work well; others remain incomplete.

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Core Components

LLE (Lush Line Editor)

A native line editor built specifically for lush:

• Real-time syntax highlighting with 45 token types
• Context-aware tab completions for all 50 shell builtins
• Emacs keybindings (44 actions) with kill ring and undo
• Multi-line editing with automatic continuation

Inspired by the line editors in Zsh (ZLE) and Fish.

Multi-Mode Architecture

Run scripts with different compatibility levels:

set -o posix   # Strict POSIX sh compliance
set -o bash    # Bash compatibility features
set -o zsh     # Zsh compatibility features  
set -o lush  # Default mode - curated feature set

Integrated Debugger

Debug shell scripts interactively - breakpoints, stepping, variable inspection. Not just set -x tracing.

debug on                         # Enable debugging
debug break add script.sh 15     # Set breakpoint
debug vars                       # Inspect variables
debug step                       # Step to next line

Unified Configuration (v1.5.0)

TOML-based configuration with XDG Base Directory compliance:

# ~/.config/lush/lushrc.toml
[shell]
mode = "lush"

[display]
syntax_highlighting = true

[history]
size = 10000

The setopt/unsetopt commands provide Zsh-style option control. A central config registry keeps runtime state and configuration files synchronized.

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Extended Syntax

Lush implements extended shell features beyond POSIX:

• Brace expansion - {a,b,c} and {1..10} sequence expansion
• Arrays - Indexed arrays with negative index support (${arr[-1]}) and append syntax (arr+=(x y))
• Associative arrays - Full support including literal syntax declare -A map=([key]=value)
• Extended tests - [[ ]] with pattern matching, regex, and file comparison (-nt, -ot, -ef)
• Process substitution - <(cmd) and >(cmd)
• Parameter expansion - Case modification, substitution, slicing, transformations (@Q, @E, @P, @a)
• Extended globbing - ?(pat), *(pat), +(pat), @(pat), !(pat)
• Advanced redirections - Compound command redirections ({ cmd; } > file, while ...; done < input)
• Hook functions - precmd, preexec, chpwd, periodic

Context-Aware Error System (v1.5.0)

Rust-style error reporting with source locations and suggestions:

error[E1001]: expected 'THEN', got 'FI'
  --> script.sh:5:10
   |
 5 | if true; fi
   |          ^~
   = while: parsing if statement
   = help: 'if' requires 'then' before 'fi'

Command-not-found errors include "did you mean?" suggestions using Unicode-aware fuzzy matching:

error[E1101]: gti: command not found
  --> <stdin>:1:1
   = help: did you mean 'git', 'gtail', or 'gtr'?

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Building

Requirements

• C11 compiler (GCC 7+ or Clang 5+)
• Meson build system
• Ninja

Build

git clone https://github.com/lush/lush.git
cd lush
meson setup build
ninja -C build
./build/lush

Test

meson test -C build

57 tests, zero memory leaks (verified with valgrind).

Platforms

Linux (primary), macOS, BSD.

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Development Status

Component	Status
Core shell / POSIX builtins	Working
LLE - Emacs mode	Complete
LLE - Vi mode	Framework only
Extended tests [[ ]]	Complete
Brace expansion {a,b} {1..10}	Complete
Extended globbing ?(pat) *(pat)	Complete
Parameter transformations @Q @P @a	Complete
Negative array indices ${arr[-1]}	Complete
Shell modes	Working
Debugger	Working
Configuration system	Complete
Context-aware error system	Complete
Associative arrays	Complete
Advanced redirections	Complete
Arithmetic expansion	Complete
User extensibility / plugins	Not yet implemented

The shell is functional for many use cases. 82 extended syntax tests pass. Some edge cases remain.

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Documentation

• User Guide - Feature reference
• LLE Guide - Line editor
• Config System - Configuration
• Debugger Guide - Debugging
• Builtin Commands - All 50 builtins
• Changelog - Version history

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License

MIT License. See LICENSE.

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Lush is a real shell, built from scratch, doing things differently.

It's not finished. But it's not vaporware either - it's 50 builtins, 57 tests, zero leaks, and years of development.

If you're curious about what a shell could be, lush is worth watching.