Clamp

"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.

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. (Although, Clamp has a lot in common with Ara T. Howard's main.rb. Had I been aware of that project at the time, I might not have written Clamp.)

A typical Clamp script looks like this:

require 'clamp'

Clamp do

  option "--loud", :flag, "say it loud"
  option ["-n", "--iterations"], "N", "say it N times", default: 1 do |s|
    Integer(s)
  end

  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
    end
  end

end

Internally, Clamp models a command as a Ruby class (a subclass of Clamp::Command), and a command execution as an instance of that class. The example above is really just syntax-sugar for:

require 'clamp'

class SpeakCommand < Clamp::Command

  option "--loud", :flag, "say it loud"
  option ["-n", "--iterations"], "N", "say it N times", default: 1 do |s|
    Integer(s)
  end

  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
    end
  end

end

SpeakCommand.run

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.

There are more examples demonstrating various features of Clamp on Github.

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 inferred 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!"
end

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

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"

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.

Although "required option" is an oxymoron, Clamp lets you mark an option as required, and will verify that a value is provided:

option "--password", "PASSWORD", "the secret password", required: true

Note that it makes no sense to mark a :flag option, or one with a :default, as :required.

Declaring an option ":multivalued" allows it to be specified multiple times on the command line.

option "--format", "FORMAT", "output format", multivalued: true

The underlying attribute becomes an Array, and the suffix "_list" is appended to the default attribute name. In this case, an attribute called "format_list" would be generated (unless you override the default by specifying an :attribute_name).

Declaring an option ":hidden" will cause it to be hidden from --help output.

option "--some-option", "VALUE", "Just a little option", hidden: true

A common idiom is to have an option --version that outputs the command version and doesn't run any subcommands. This can be achieved by:

option "--version", :flag, "Show version" do
  puts MyGem::VERSION
  exit(0)
end

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.

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

parameter "[TARGET_DIR]", "target directory"

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", attribute_name: :files

Like multivalued options, greedy parameters are backed by an Array attribute (named with a "_list" suffix, by default).

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.

Both option and parameter accept an optional block. If present, the block will be called with the raw string argument, and is expected to validate it. The value returned by the block will be assigned to the underlying attribute, so it's also a good place to coerce the String to a different type, if appropriate.

For example:

option "--port", "PORT", "port to listen on" do |s|
  Integer(s)
end

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

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

For multivalued options and parameters, the validation block will be called for each value specified.

More complex validation, e.g. those involving multiple options/parameters, should be performed within the #execute method. Use #signal_usage_error to tell the user what they did wrong, e.g.

def execute
  if port < 1024 && user != 'root'
    signal_usage_error "port restricted for non-root users"
  end
  # ... carry on ...
end

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(":")
end

Default values can be specified for options, and optional parameters:

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

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
end

Options (and optional parameters) can also be associated with environment variables:

option "--port", "PORT", "the port to listen on", environment_variable: "MYAPP_PORT" do |val|
  val.to_i
end

parameter "[HOST]", "server address", environment_variable: "MYAPP_HOST"

Clamp will check the specified envariables in the absence of values supplied on the command line, before looking for a default value.

By default, Clamp only recognises options before positional parameters.

Some other option-parsing libraries - notably GNU getopt(3) - allow option and parameter arguments to appear in any order on the command-line, e.g.

foobar --foo=bar something --fnord=snuffle another-thing

If you want Clamp to allow options and parameters to be "interspersed" in this way, set:

Clamp.allow_options_after_parameters = true

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.

Clamp do

  subcommand "init", "Initialize the repository" do

    def execute
      # ...
    end

  end

end

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

end

class InitCommand < Clamp::Command

  def execute
    # ...
  end

end

Like options, subcommands may have aliases:

Clamp do

  subcommand ["initialize", "init"], "Initialize the repository" do
    # ...
  end

end

You can set a default subcommand, at the class level, as follows:

Clamp do

  self.default_subcommand = "status"

  subcommand "status", "Display current status" do

    def execute
      # ...
    end

  end

end

Then, if when no SUBCOMMAND argument is provided, the default will be selected.

Options are inheritable, so any options declared for a command are supported by it's sub-classes (e.g. those created using the block form of 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.

You can define a subcommand_missing method that is called when user tries to run an unknown subcommand:

Clamp do
  def subcommand_missing(name)
    if name == "foo"
      return Object.const_get(:FooPlugin) if Object.const_defined?(:FooPlugin)
      abort "Subcommand 'foo' requires plugin X"
    end
  end
end

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
Usage:
    speak [OPTIONS] WORDS ...

Arguments:
    WORDS ...                     the thing to say

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

Clamp comes with support for overriding strings with custom translations. You can use localization library of your choice and override the strings at startup.

Example usage:

require 'gettext'

Clamp.messages = {
  too_many_arguments:        _("too many arguments"),
  option_required:           _("option '%<option>s' is required"),
  option_or_env_required:    _("option '%<option>s' (or env %<env>s) is required"),
  option_argument_error:     _("option '%<switch>s': %<message>s")
  # ...
}

See messages.rb for full list of available messages.

Copyright (C) 2011 Mike Williams

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

Source-code for Clamp is on Github.