Rubish: A Unix shell written in pure Ruby

Rubish

A UNIX shell written in pure Ruby.

Shell syntax is parsed and compiled to Ruby code, then executed by the Ruby VM.

Concept

Fully Bash-compatible

Rubish supports all the features of bash, and the shell syntax is fully compatible. You can run your existing bash scripts without modification. If you found any bash script that doesn't work in rubish, we consider it a bug, so please report it!

Deep Ruby integration

Rubish is not just a shell implemented in Ruby, but a shell that deeply integrates Ruby. You can seamlessly mix shell commands and Ruby code, and even use Ruby's powerful features like blocks, iterators, and libraries in your shell scripts.

Installation

Homebrew (macOS)

brew tap amatsuda/rubish brew install --HEAD rubish

From source

git clone https://github.com/amatsuda/rubish.git cd rubish bundle install bundle exec exe/rubish

bin/rubish is a small bash launcher that finds a usable Ruby on its own (probes ~/.rbenv/shims/ruby, /opt/homebrew/bin/ruby, /usr/local/bin/ruby, system Ruby; honors $RUBY). Use it when bundler isn't around — for example as a login shell, from a .app bundle, or anywhere PATH may be minimal:

./bin/rubish RUBY=/opt/homebrew/opt/ruby@3.4/bin/ruby ./bin/rubish # explicit override

Usage

Start an interactive shell:

rubish

Run a single command:

rubish -c 'echo hello'

Run a script:

rubish script.sh

Or you can even use this as a login shell!

Set as login shell

echo "$(which rubish)" | sudo tee -a /etc/shells chsh -s "$(which rubish)"

Features beyond Bash

Ruby conditions

Use Ruby expressions as conditions in if, while, and until by wrapping them in { }. Shell variables are automatically bound as local variables in the Ruby expression:

COUNT=5 if { count.to_i > 3 } echo 'count is greater than 3' end while { count.to_i > 0 } echo $COUNT COUNT=$((COUNT - 1)) done

Ruby method call style

Commands can be invoked using Ruby method call syntax with parentheses, in addition to the traditional UNIX style with spaces:

# These are equivalent: ls -la ls('-la') # Arguments can be passed as method arguments: cat(file.txt) grep('pattern', file.txt)

Method chaining

Commands can be chained with Ruby methods using dot notation, forming a pipeline. The chain has to be opened by a parenthesized call, an array literal, or a block — once you're in chain context, subsequent methods can be bare:

# Equivalent to `ls | sort` ls().sort # Equivalent to `ls | sort | uniq` ls().sort.uniq # Equivalent to `cat file.txt | grep error` cat(file.txt).grep(/error/) # Chains can be combined with blocks (see "Ruby iterator blocks" below) ls.select { it.end_with?('.rb') }.each { |f| puts f.upcase }

The first segment needs the parens because bare cmd.method is ambiguous with paths and dotted filenames (./script.sh, file.tar.gz) — once () confirms a method-call form, the lexer knows it's safe to chain.

Ruby iterator blocks

Ruby iterator methods (.each, .map, .select, .detect) can take blocks to process command output line by line:

ls.each { |f| puts f.upcase } cat(file.txt).map { |line| line.strip } ls.select { it.end_with?('.rb') }

Inline Ruby evaluation

Any line starting with a capital letter is evaluated as Ruby code directly. This means you can use Ruby classes, methods, and expressions right from the shell prompt without any special syntax:

rubish$ Time.now => 2025-01-01 12:00:00 +0900 rubish$ Dir.glob('*.rb').sort => ["Gemfile", "Rakefile"] rubish$ ENV['HOME'] => "/Users/you"

Ruby array and regexp literals

Ruby array literals can be used directly in shell context. Rubish distinguishes them from glob patterns like [a-z] automatically:

rubish$ [1, 2, 3].map { |x| x * x } => [1, 4, 9]

Lambda expressions

You can execute any Ruby code by surrounding it with a lambda expression (-> { }):

rubish$ -> { 2 ** 10 } => 1024

Ruby-style function definitions

In addition to the standard shell function syntax, rubish supports Ruby-style def...end with named parameters and splat args:

def greet(name) echo "Hello, $name" end def log(level, *messages) echo "[$level] $messages" end greet world # => Hello, world

Custom Ruby prompts

Define your prompt as a Ruby function for full programmatic control. The function is called on every prompt render, so it can include dynamic content:

def rubish_prompt branch = `git branch --show-current 2>/dev/null`.strip dir = Dir.pwd.sub(ENV['HOME'], '~') "\e[36m#{dir}\e[0m \e[33m#{branch}\e[0m $ " end def rubish_right_prompt Time.now.strftime('%H:%M:%S') end

You can also use the traditional PS1/RPROMPT variables with bash (\X) or zsh (%X) escape codes.

Lazy loading

Slow shell initializations (e.g., rbenv init, nvm, pyenv) can be deferred to a background thread using lazy_load. The block runs immediately in the background, and its result (a string of shell code) is applied before the next prompt. This keeps shell startup instant:

# In ~/.rubishrc lazy_load { `rbenv init - --no-rehash bash` } lazy_load { `nodenv init - bash` }

Multiple lazy_load blocks run in parallel. By the time you type your first command, they're usually done.

Restricted mode

Running rubish -r disables all Ruby integration features (inline evaluation, lambdas, blocks, Ruby conditions, and array literals) for executing untrusted scripts safely. Only standard shell syntax is allowed.

Zsh compatibility

In addition to full Bash compatibility, rubish also supports zsh-style features:

setopt/unsetopt
compdef/compinit
autoload with fpath
%X prompt codes and RPROMPT/RPS1
Abbreviated path expansion: type a/c/a<Tab> and it expands to app/controllers/application_controller.rb

Configuration files

Login shells load (in order):

/etc/profile
~/.config/rubish/profile or ~/.rubish_profile (or ~/.bash_profile / ~/.bash_login / ~/.profile)

Interactive shells load:

~/.config/rubish/config or ~/.rubishrc (or ~/.bashrc)
./.rubishrc (project-local)

Logout:

~/.config/rubish/logout or ~/.rubish_logout (or ~/.bash_logout)

Embedding in a Ruby program

Rubish exposes a public API so other Ruby programs (terminal emulators, IDE plugins, GUI front-ends) can drive a rubish session in-process — no fork+exec, no JSON serialization, just method calls. The sibling Echoes terminal emulator uses this to render syntax-highlighted prompts and decide command-execution shape ahead of time.

require 'rubish' repl = Rubish::REPL.new(login_shell: true) # Run interactively (default). repl.run # Or drive it programmatically. repl.tokenize('ls | grep foo') # => Array of Rubish::Lexer::Token (each # with :type and :value) for syntax # highlighting; never raises. repl.try_parse('if true; then') # => :ok | :incomplete | :error # (use to decide PS2 vs. submit). repl.parse_ast('echo hi') # => AST root, or nil on parse failure. repl.complete_at(line: 'gi', point: 2) # => Array of completion candidates at # the cursor. repl.prompt_segments # => Array of styled-text segments # {text:, fg:, bg:, bold:, italic:, # underline:, inverse:}, ANSI codes # already parsed. repl.right_prompt_segments # => same shape for the right prompt, # or nil if unset.

Custom I/O backend

The default Rubish::Frontend::Tty wraps Reline + stdin/stdout. Hosts that own their own line editor can subclass Rubish::Frontend::Base and pass an instance into the REPL:

class MyFrontend < Rubish::Frontend::Base def read_line(prompt:, rprompt: nil) # ...feed input from your own UI here end end Rubish::REPL.new(frontend: MyFrontend.new).run

Child-process pre-exec hook

To run setup code in every forked child between fork() and exec() (e.g. to attach a per-command controlling tty so the line discipline can deliver Ctrl-C to the child):

Rubish::Command.child_pre_exec_hook = -> { Process.setsid # ...ioctls, signal handlers, etc. }

Builtins

Category	Commands
Directory	cd, pwd, pushd, popd, dirs
I/O	echo, printf, read, mapfile, readarray
Variables	export, declare, typeset, readonly, unset, local, shift, set
Process	exit, logout, exec, kill, wait, times
Job control	jobs, fg, bg, disown, suspend
Functions	function, return, caller
Aliases	alias, unalias
History	history, fc
Execution	eval, source, ., command, builtin
Testing	test, [, [[, (( )), let
Control	break, continue, trap
Completion	complete, compgen, compopt, bind
Config	shopt, setopt, unsetopt
Info	help, type, which, hash
Other	true, false, :, getopts, umask, ulimit, enable

Development

bundle install bundle exec rake test

Contributing

Bug reports and pull requests are welcome on GitHub at https://github.com/amatsuda/rubish.

License

MIT