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Simple Lightweight Option Parsing - ✨ new contributors welcome ✨


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

Build Status


gem install slop


opts = Slop.parse do |o|
  o.string '-h', '--host', 'a hostname'
  o.integer '--port', 'custom port', default: 80
  o.string '-l', '--login', required: true
  o.symbol '-m', '--method', default: :get
  o.bool '-v', '--verbose', 'enable verbose mode'
  o.bool '-q', '--quiet', 'suppress output (quiet mode)'
  o.bool '-c', '--check-ssl-certificate', 'check SSL certificate for host'
  o.bool '-k', '--use-keychain', 'store passphrase in OS keychain'
  o.on '--version', 'print the version' do
    puts Slop::VERSION

ARGV #=> -v --login alice --host -m post --check-ssl-certificate --use-keychain false

opts[:host]         #=>
opts[:login]        #=> alice
opts[:method]       #=> :post
opts[:use_keychain] #=> false

# We can also check if a flag was passed (this has no bearing on the options default value):
opts.use_keychain?          #=> true
opts.verbose?               #=> true
opts.quiet?                 #=> false
opts.check_ssl_certificate? #=> true

opts.to_hash  #=> { host: "", port: 80, login: "alice", method: :post, verbose: true, quiet: false, check_ssl_certificate: true }

Note that the block we've added to the --version flag will be executed during parse time. Therefore these blocks should be reserved for immediately reacting to the presence of a flag. If you want to access other options or mutate values, check out the "Custom option types" section below and implement the #finish method.

Option types

Built in Option types are as follows:

o.string  #=> Slop::StringOption, expects an argument
o.bool    #=> Slop::BoolOption, argument optional, 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.regexp  #=> Slop::RegexpOption, expects an argument
o.symbol  #=> Slop::SymbolOption, 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", default: true

parser =
result = parser.parse(["--hostname", "", "--no-verbose"])

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`

This is particularly useful when writing scripts with ARGF:

opts = Slop.parse do |blah|
  # ...

# make sure sloptions aren't consumed by ARGF
ARGV.replace opts.arguments

ARGF.each { |line|
  # ...


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
# This will return o[:files] as []:
# --files ""

If you want to disable the built-in string-splitting, set the delimiter to nil.

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
  • An option marked as required when not provided: Slop::MissingRequiredOption
  • An option marked as validate_types, with an argument that does not match its type (i.e. bla for integer): Slop::InvalidOptionValue

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'

Validating Types

By default, Slop does not validate whether an argument is a valid value for a given option; instead, if the option has a default value, it will be used over the invalid argument provided. In order to have types (such as integer and float) validate and indicate that the provided value is invalid, an extra option can be either provided to the argument itself, or its option set:

opts = "-p", "--port", "a port", default: 80, validate_types: true

parser =
result = parser.parse(["--port", "bla"])
# invalid value for -p, --port (Slop::InvalidOptionValue)

# Or to the option set...
opts = true) "-p", "--port", "a port", default: 80

parser =
result = parser.parse(["--port", "bla"])
# invalid value for -p, --port (Slop::InvalidOptionValue)

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


Slop no longer has built in support for git-style subcommands.