a Ruby command-line application framework
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"Clamp" is a minimal framework for command-line utilities.

It handles boring stuff like parsing the command-line, and generating help, so you can get on with making your command actually do stuff.

Not another one!

Yeah, sorry. There are a bunch of existing command-line parsing libraries out there, and Clamp draws inspiration from a variety of sources, including Thor, optparse, and Clip. In the end, though, I wanted a slightly rounder wheel.

Quick Start

Clamp models a command as a Ruby class; a subclass of Clamp::Command. They look something like this:

class SpeakCommand < Clamp::Command

  option "--loud", :flag, "say it loud"
  option ["-n", "--iterations"], "N", "say it N times", :default => 1 do |s|

  parameter "WORDS ...", "the thing to say", :attribute_name => :words

  def execute
    the_truth = words.join(" ")
    the_truth.upcase! if loud?
    iterations.times do
      puts the_truth


Calling run on a command class creates an instance of it, then invokes it using command-line arguments (from ARGV, by default).


Class-level methods like option and parameter declare attributes (in a similar way to attr_accessor), and arrange for them to be populated automatically based on command-line arguments. They are also used to generate help documentation.

Declaring options

Options are declared using the option method. The three required arguments are:

  1. the option switch (or switches),
  2. an option argument name
  3. a short description

For example:

option "--flavour", "FLAVOUR", "ice-cream flavour"

It works a little like attr_accessor, defining reader and writer methods on the command class. The attribute name is derived from the switch (in this case, "flavour"). When you pass options to your command, Clamp will populate the attributes, which are then available for use in your #execute method.

def execute
  puts "You chose #{flavour}.  Excellent choice!"

If you don't like the inferred attribute name, you can override it:

option "--type", "TYPE", "type of widget", :attribute_name => :widget_type
                                           # to avoid clobbering Object#type

Short/long option switches

The first argument to option can be an array, rather than a single string, in which case all the switches are treated as aliases:

option ["-s", "--subject"], "SUBJECT", "email subject line"

Flag options

Some options are just boolean flags. Pass ":flag" as the second parameter to tell Clamp not to expect an option argument:

option "--verbose", :flag, "be chatty"

For flag options, Clamp appends "?" to the generated reader method; ie. you get a method called "#verbose?", rather than just "#verbose".

Negatable flags are easy to generate, too:

option "--[no-]force", :flag, "be forceful (or not)"

Clamp will handle both "--force" and "--no-force" options, setting the value of "#force?" appropriately.

Declaring parameters

Positional parameters can be declared using parameter, specifying

  1. the parameter name, and
  2. a short description

For example:

parameter "SRC", "source file"

Like options, parameters are implemented as attributes of the command, with the default attribute name derived from the parameter name (in this case, "src"). By convention, parameter names are specified in uppercase, to make them obvious in usage help.

Optional parameters

Wrapping a parameter name in square brackets indicates that it's optional, e.g.

parameter "[TARGET_DIR]", "target directory"

Greedy parameters

Three dots at the end of a parameter name makes it "greedy" - it will consume all remaining command-line arguments. For example:

parameter "FILE ...", "input files"

The suffix "_list" is appended to the default attribute name for greedy parameters; in this case, an attribute called "file_list" would be generated.

Parsing and validation of options and parameters

When you #run a command, it will first attempt to #parse command-line arguments, and map them onto the declared options and parameters, before invoking your #execute method.

Clamp will verify that all required (ie. non-optional) parameters are present, and signal a error if they aren't.

Validation block

Both option and parameter accept an optional block. If present, the block will be called with the raw string option argument, and is expected to coerce it to the correct type, e.g.

option "--port", "PORT", "port to listen on" do |s|

If the block raises an ArgumentError, Clamp will catch it, and report that the value was bad:

ERROR: option '--port': invalid value for Integer: "blah"

Advanced option/parameter handling

While Clamp provides an attribute-writer method for each declared option or parameter, you always have the option of overriding it to provide custom argument-handling logic, e.g.

parameter "SERVER", "location of server"

def server=(server)
  @server_address, @server_port = server.split(":")

Default values

Default values can be specified for options:

option "--flavour", "FLAVOUR", "ice-cream flavour", :default => "chocolate"

and also for optional parameters

parameter "[HOST]", "server host", :default => "localhost"

For more advanced cases, you can also specify default values by defining a method called "default_#{attribute_name}":

option "--http-port", "PORT", "web-server port", :default => 9000

option "--admin-port", "PORT", "admin port"

def default_admin_port
   http_port + 1

Declaring Subcommands

Subcommand support helps you wrap a number of related commands into a single script (ala tools like "git"). Clamp will inspect the first command-line argument (after options are parsed), and delegate to the named subcommand.

Unsuprisingly, subcommands are declared using the subcommand method. e.g.

class MainCommand < Clamp::Command

  subcommand "init", "Initialize the repository" do

    def execute
      # ...


Clamp generates an anonymous subclass of the current class, to represent the subcommand. Alternatively, you can provide an explicit subcommand class:

class MainCommand < Clamp::Command

  subcommand "init", "Initialize the repository", InitCommand

class InitCommand < Clamp::Command

  def execute
    # ...


Default subcommand

You can mark a subcommand as "default" by using default_subcommand to declare it, rather than subcommand. Usually the SUBCOMMAND paramater is mandatory, but if a default subcommand is declared, it becomes optional.

class MainCommand < Clamp::Command

  default_subcommand "status", "Display current status" do

    def execute
      # ...


Subcommand options and parameters

Options are inheritable, so any options declared for a command are supported for it's sub-classes (e.g. those created using subcommand). Parameters, on the other hand, are not inherited - each subcommand must declare it's own parameter list.

Note that, if a subcommand accepts options, they must be specified on the command-line after the subcommand name.

Getting help

All Clamp commands support a "--help" option, which outputs brief usage documentation, based on those seemingly useless extra parameters that you had to pass to option and parameter.

$ speak --help
    speak [OPTIONS] WORDS ...

    WORDS ...                     the thing to say

    --loud                        say it loud
    -n, --iterations N            say it N times (default: 1)
    --help                        print help

Contributing to Clamp

Source-code for Clamp is on Github.