# Clamp

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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.  (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.)

[optparse]: http://ruby-doc.org/stdlib/libdoc/optparse/rdoc/index.html
[Thor]: http://github.com/wycats/thor
[Clip]: http://clip.rubyforge.org/
[main.rb]: https://github.com/ahoward/main

## Quick Start

A typical Clamp script looks like this:

```ruby
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:

```ruby
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][examples].

[examples]: https://github.com/mdub/clamp/tree/master/examples

## 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:

```ruby
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.

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

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

```ruby
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:

```ruby
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:

```ruby
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:

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

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

### Required options

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

```ruby
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`.

### Multivalued options

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

```ruby
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`).

### Hidden options

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

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

### Version option

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:

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

## Declaring parameters

Positional parameters can be declared using `parameter`, specifying

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

For example:

```ruby
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.

```ruby
parameter "[TARGET_DIR]", "target directory"
```

### Multivalued (aka "greedy") parameters

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

```ruby
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).

## 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

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:

```ruby
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:

```ruby
!!!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.

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

### 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.

```ruby
parameter "SERVER", "location of server"

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

### Default values

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

```ruby
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}`":

```ruby
option "--http-port", "PORT", "web-server port", default:  9000

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

def default_admin_port
   http_port + 1
end
```

### Environment variable support

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

```ruby
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.

### Allowing options after parameters

By default, Clamp only recognises options _before_ positional parameters.

Some other option-parsing libraries - notably [GNU `getopt(3)`](https://www.gnu.org/software/libc/manual/html_node/Using-Getopt.html) - 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:

```ruby
Clamp.allow_options_after_parameters = true
```

## 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.

```ruby
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:

```ruby
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:

```ruby
Clamp do

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

end
```

### Default subcommand

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

```ruby
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.

### Subcommand options and parameters

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:

```ruby
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
```

## 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`.

```sh
$ 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
```

## Localization

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:

```ruby
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](https://github.com/mdub/clamp/blob/master/lib/clamp/messages.rb) for full list of available messages.

## License

Copyright (C) 2011 [Mike Williams](mailto:mdub@dogbiscuit.org)

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](https://github.com/mdub/clamp).
