Language cheatsheet

Ruby 3 Cheatsheet

Run Ruby code, manage gems with Bundler, and use blocks, collections, classes, pattern matching and error handling.

Command line and gems

Check the Ruby version — Print the installed Ruby version.
Run a script — Run a Ruby file from the command line.
Run a one-line command — Evaluate code from the command line.
Open the interactive shell — Start IRB, the interactive Ruby shell.
Install a gem — Install a library from RubyGems.
Start a Gemfile — Create a Gemfile for a project's dependencies.
Install the bundle — Install the gems listed in the Gemfile and record exact versions in Gemfile.lock.
Run a command with the bundle — Run a command using the gem versions in Gemfile.lock.
Lint the code — Check style and common mistakes with RuboCop. Add -a to fix what is safe.

Print and interpolate — Print a line, inserting expressions with #{}. Double quotes interpolate; single quotes don't.
Use hashes and symbols — Hashes map keys to values. Symbols like :name are cheap, immutable names.
Transform arrays — Arrays have rich methods, and most take blocks.
Loop with an index — Iterate with each_with_index, or each_with_object to build a result.
Branch — Use if, unless and the ternary operator. Only nil and false are falsy.
Define a method with keyword arguments — Keyword arguments make calls self-documenting. The last expression is returned.
Handle nil safely — &. calls a method only if the receiver is not nil.
Common string methods — Strings have many helpers.

Write a method that takes a block — yield runs the block the caller supplied. Pass data to the block as arguments.
Define a class — A class with an initializer, readers and a method.
Share behavior with a module — Include a module to add its methods to a class. Including Comparable gives you < > == from <=>.
Define simple data classes — Data.define creates immutable value objects (Ruby 3.2+); Struct is the mutable version.
Match patterns — case/in destructures arrays and hashes and binds variables.
Create a lambda — A lambda is a callable object that checks its arguments.

Rescue exceptions — Handle errors with begin/rescue/ensure. Rescue specific classes, not Exception.
Raise a custom error — Subclass StandardError for your own exceptions, then raise and rescue it.
Read a file — Read a whole file, or iterate lines without loading it all.
Write a file — Write text, with the file closed automatically at the end of the block.
Work with JSON — Convert between Ruby hashes and JSON text.
Use a Set — A collection of unique values with fast membership tests.
Write a test — A Minitest test, which ships with Ruby.

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