Slop is a simple option parser with an easy to remember syntax and friendly API.

Version 4 of Slop is aimed at Ruby 2.0 or later. Please use Version 3 for Ruby 1.9 support.

Build Status


gem install slop


opts = Slop.parse do |o|
  o.string '-h', '--host', 'a hostname'
  o.integer '--port', 'custom port', default: 80
  o.bool '-v', '--verbose', 'enable verbose mode'
  o.bool '-q', '--quiet', 'suppress output (quiet mode)'
  o.on '--version', 'print the version' do
    puts Slop::VERSION

ARGV #=> -v --host

opts[:host]   #=>
opts.verbose? #=> true
opts.quiet?   #=> false

opts.to_hash  #=> { host: "", port: 80, verbose: true, quiet: false }

Option types

Built in Option types are as follows:

o.string  #=> Slop::StringOption, expects an argument
o.bool    #=> Slop::BoolOption, no argument, aliased to BooleanOption
o.integer #=> Slop::IntegerOption, expects an argument, aliased to IntOption
o.float   #=> Slop::FloatOption, expects an argument
o.array   #=> Slop::ArrayOption, expects an argument
o.null    #=> Slop::NullOption, no argument and ignored from `to_hash`
o.on      #=> alias for o.null

You can see all built in types in slop/types.rb. Suggestions or pull requests for more types are welcome.

Advanced Usage

This example is really just to describe how the underlying API works. It's not necessarily the best way to do it.

opts =
opts.banner = "usage: connect [options] ..."
opts.separator ""
opts.separator "Connection options:"
opts.string "-H", "--hostname", "a hostname" "-p", "--port", "a port", default: 80
opts.separator ""
opts.separator "Extra options:"
opts.array "--files", "a list of files to import"
opts.bool "-v", "--verbose", "enable verbose mode"

parser =
result = parser.parse(["--hostname", ""])

result.to_hash #=> { hostname: "", port: 80,
                 #     files: [], verbose: false }

puts opts # prints out help


It's common to want to retrieve an array of arguments that were not processed by the parser (i.e options or consumed arguments). You can do that with the Result#arguments method:

args = %w(connect --host GET)
opts = Slop.parse args do |o|
  o.string '--host'

p opts.arguments #=> ["connect", "GET"] # also aliased to `args`


Slop has a built in ArrayOption for handling array values:

opts = Slop.parse do |o|
  # the delimiter defaults to ','
  o.array '--files', 'a list of files', delimiter: ','

# both of these will return o[:files] as ["foo.txt", "bar.rb"]:
# --files foo.txt,bar.rb
# --files foo.txt --files bar.rb

Custom option types

Slop uses option type classes for every new option added. They default to the NullOption. When you type o.array Slop looks for an option called Slop::ArrayOption. This class must contain at least 1 method, call. This method is executed at parse time, and the return value of this method is used for the option value. We can use this to build custom option types:

module Slop
  class PathOption < Option
    def call(value)

opts = Slop.parse %w(--path ~/) do |o|
  o.path '--path', 'a custom path name'

p opts[:path] #=> #<Pathname:~/>

Custom options can also implement a finish method. This method by default does nothing, but it's executed once all options have been parsed. This allows us to go back and mutate state without having to rely on options being parsed in a particular order. Here's an example:

module Slop
  class FilesOption < ArrayOption
    def finish(opts)
      if opts.expand?
        self.value = { |f| File.expand_path(f) }

opts = Slop.parse %w(--files foo.txt,bar.rb -e) do |o|
  o.files '--files', 'an array of files'
  o.bool '-e', '--expand', 'if used, list of files will be expanded'

p opts[:files] #=> ["/full/path/foo.txt", "/full/path/bar.rb"]


Slop will raise errors for the following:

  • An option used without an argument when it expects one: Slop::MissingArgument
  • An option used that Slop doesn't know about: Slop::UnknownOption

These errors inherit from Slop::Error, so you can rescue them all. Alternatively you can suppress these errors with the suppress_errors config option:

opts = Slop.parse suppress_errors: true do
  o.string '-name'

# or per option:

opts = Slop.parse do
  o.string '-host', suppress_errors: true '-port'

Printing help

The return value of Slop.parse is a Slop::Result which provides a nice help string to display your options. Just puts opts or call opts.to_s:

opts = Slop.parse do |o|
  o.string '-h', '--host', 'hostname' '-p', '--port', 'port (default: 80)', default: 80
  o.string '--username'
  o.separator ''
  o.separator 'other options:'
  o.bool '--quiet', 'suppress output'
  o.on '-v', '--version' do
    puts "1.1.1"

puts opts


% ruby run.rb
usage: run.rb [options]
    -h, --host     hostname
    -p, --port     port (default: 80)

other options:
    --quiet        suppress output
    -v, --version

This method takes an optional prefix value, which defaults to " " * 4:

puts opts.to_s(prefix: "  ")

It'll deal with aligning your descriptions according to the longest option flag.

Here's an example of adding your own help option:

o.on '--help' do
  puts o


As of version 4, Slop does not have built in support for git-style subcommands. You can use version 3 of Slop (see v3 branch). I also expect there to be some external libraries released soon that wrap around Slop to provide support for this feature. I'll update this document when that happens.

Upgrading from version 3

Slop v4 is completely non-backwards compatible. The code has been rewritten from the group up. If you're already using version 3 you have have to update your code to use version 4. Here's an overview of the more fundamental changes:

No more instance_eval


Slop.parse do
  on 'v', 'version' do
    puts VERSION


Slop.parse do |o|
  o.on '-v', '--version' do
    puts VERSION

No more as for option types

Instead, the type is declared in the method call. Before:

on 'port=', as: Integer

After: '--port' # or integer

See the custom types section of the document.

No more trailing =

Instead, the "does this option expect an argument" question is answered by the option type (i.e on and bool options do not expect arguments, all others do. They handle type conversion, too.

Hyphens are required

This was a hard decision to make, but you must provide prefixed hyphens when declaring your flags. This makes the underlying code much nicer and much less ambiguous, which leads to less error prone code. It also means you can easily support single hyphen prefix for a long flag, i.e -hostname which you could not do before. It also provides a hidden feature, which is infinity flag aliases: o.string '-f', '-x', '--foo', '--bar', 'this is insane'

Strict is now on by default

v3 had a strict option. v4 has no such option, and to suppress errors you can instead provide the suppress_errors: true option to Slop.

No more parse!

Where v3 has both Slop.parse and Slop.parse!, v4 only has parse. The former was used to decide whether Slop should or should not mutate the original args (usually ARGV). This is almost never what you want, and it can lead to confusion. Instead, Slop::Result provides an arguments methods:

opts = Slop.parse do |o|
  o.string '--hostname', '...'

# ARGV is "hello --hostname foo bar"
p opts.arguments #=> ["hello", "bar"]