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.

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|
    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

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

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.

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!"
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

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.

Values from the environment

Sometimes you'll want to pass in option values from the environment:

option "--port", "PORT", "the port to listen on", :env => "PORT" do |val|
  val.to_i
end

The above means that in the absence of '--port' on the command line, Clamp will check for PORT in the environment. This lets you do:

% export PORT=8080 
% ./mycommand

Required options

While a 'required option' is a bit of an oxymoron, it is common to use options as named parameters to your program. Clamp lets you make an option required:

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

Special notes about required options:

  • You may not use :required and :default values in the same option ('require' doesn't make sense if you provide a default)
  • You may not use :required on :flag options. Since :flag options are boolean, a 'required flag' would mean "always true" ;)

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.

Parameters from the environment

Optional parameters can have values given from the environment the same way options can.

parameter "[EXAMPLE]", "This is an example", :env => "EXAMPLE"

The above means that in the absence of a value on the command line, Clamp will check for EXAMPLE in the environment. This lets you do:

% export EXAMPLE="hello" 
% ./mycommand

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|
  Integer(s)
end

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(":")
end

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
end

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
      # ...
    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

Default subcommand

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

class MainCommand < Clamp::Command

  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.

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
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

License

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.

Contributing to Clamp

Source-code for Clamp is on Github.