Danger title image

Rubya project from orta therox, Juanito Fatas and and others.

#Aims of this Guide

My assumption is that you are a programmer, with some experience. You should be familiar with the terminal. This guide aims to cover enough Ruby for you to feel somewhat productive. You don’t need to know all of the details of the language, but you should probably have some familiarity with a few of these major topics.

Ruby has a repl, the best one by far is pry. So, if you want to have a play with the language. Run gem install pry then run pry. If the gem does not install, you should follow the instructions here to do a sudo-less install.


The Dangerfile is a scripting environment, this means that you can totally get by with some simple hacking. Inside the Dangerfile, you only need to understand how to manipulate the data exposed via the API. You can see the full extent of the DSL in the reference and from the plugins overview in the homepage.

Realistically, the DSL is small. To be productive in your Dangerfile you will need to use Ruby to put the pieces together. You don’t need to be a pro at Ruby though, as you’ll mostly be doing string and array manipulation, file accessing, and executing shell commands. Which is what we’ll cover here.

#Variables and Keyword Syntax

Ruby does not have a keyword for variables, so you can write path = "./the/path" and you can re-use it later on.

For methods, parentheses are optional. You can write do_something and do_something(). Method parameters are fine with this too: do_something "OK", 3. Danger tries to use named parameters a lot, so you might end up writing do_something "OK", times: 3.

If statements are cool in Ruby, here’s some interesting cases:

Plain old if statement:

if three == 3

For single actions, you can move it before the if, this is succinct, and matches how English reads:

do_something if three == 3

If you need to negate something, you can use unless instead of a !:

unless three == 3


You can use both double and single quotes to create a string, though single quotes don’t do interpolation. This means you should nearly always use double quotes.

If a function has a ! in it, that generally means it mutates the object. Otherwise you get a change if there’s an = in the LOC.

name = "orta"                 # orta
name += " therox"             # orta therox
name.start_with? "orta"       # true
name.end_with? "orta"         # false
name.gsub "orta", "Orta"      # Orta therox
name.gsub /therox/, 'Therox'  # orta Therox
# Note the !, this will mutate `name`.
name.gsub! /therox/, 'Therox' # orta Therox
name.include? "ta thero"      # false
name.include? "Thero"         # true
name[0..3]                    # orta

There are multiple ways to make a string, I recommend using double quotes " like above. If you want to do string interpolation with it, you can use #{} and run code inside the brackets. "5 = ${ 2+3 }".

That should be enough to play around with your code review data.

#Arrays and Closures

Arrays are also mostly immutable, it’s common to iterate with array.each instead of for thing in array. Closures for functions like each, map, select, flat_map, reject and the like, can be executed with a single line with braces:

paths.map { |path| File.read(path) }.select { |content| content.include? "orta" }

For longer closures, it’s common to use do and end:

paths.each do |path|
  path.name += "Dangerfile"
  do_something path

Finally, the simplest “closure”, that only works if you want to call a function on the object in your collection is this interesting pattern: lowercase_path = paths.map(&:downcase) - where downcase is a function on the strings in the array.

Note: In Danger, for lists of files (e.g.git.modified_files) we use a class called a FileList which is an array subclass. This adds one extra method: include?. This lets you use path globbing to determine if a string is in the list:

paths = ["path1/file_name.txt", "path1/file_name1.txt", "path2/subfolder/example.json"]
filelist = Danger::FileList.new(paths)

filelist.include? "path1/file_name.txt"   # true
filelist.include? "path2/*.json"          # true
filelist.include? "path1/*.json"          # false


You can read a file with File.read("path/to/file") this returns a string, or nil. Paths within Danger are represented as strings.

#Terminal commands

There are three ways to interact with commands to do something.

The humble backtick ` - Takes whatever shell command you want to run, and returns a string of the output, and a final newline. You don’t see any output by default.

plugin_json = `bundle exec danger plugin json`

system - Outputs the command you run in the shell, it will return true if the process succeeded.

send_congrats if system("gem install pry")

exec - Replaces the current process with another, you probably won’t be needing this for Danger.

If you want to know more about any of these, check out this Stack Overflow.

#JSON Parsing

To work with JSON data, you may have to require the JSON library first. Note some systems are case-sensitive, so always do require "json". I keep getting this wrong, it’s always in lowercase. :D

For example grabbing some JSON data, parsing it, then pulling something out:

path = "path/to/file.json"
contents = File.read path

require "json"
json = JSON.parse contents

thing = json["things"][0]

#Running a Dangerfile inside IRB

There’s a great section in the troubleshooting on letting you dig around inside Danger via pry.

Got improvements? Help improve this document via sending MRs.