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 # Writing Commands

 ## Basics

 GCLI has opinions about how commands should be written, and it encourages you

 to do The Right Thing. The opinions are based on helping users convert their

 intentions to commands and commands to what's actually going to happen.

 - Related commands should be sub-commands of a parent command. One of the goals

   of GCLI is to support a large number of commands without things becoming

   confusing, this will require some sort of namespacing or there will be

   many people wanting to implement the ``add`` command. This style of

   writing commands has become common place in Unix as the number of commands

   has gone up.

   The ```context``` command allows users to focus on a parent command, promoting

   its sub-commands above others.

 - Each command should do exactly and only one thing. An example of a Unix

   command that breaks this principle is the ``tar`` command

         $ tar -zcf foo.tar.gz .

         $ tar -zxf foo.tar.gz .

   These 2 commands do exactly opposite things. Many a file has died as a result

   of a x/c typo. In GCLI this would be better expressed:

         $ tar create foo.tar.gz -z .

         $ tar extract foo.tar.gz -z .

   There may be commands (like tar) which have enough history behind them

   that we shouldn't force everyone to re-learn a new syntax. The can be achieved

   by having a single string parameter and parsing the input in the command)

 - Avoid errors. We try to avoid the user having to start again with a command

   due to some problem. The majority of problems are simple typos which we can

   catch using command metadata, but there are 2 things command authors can do

   to prevent breakage.

   - Where possible avoid the need to validate command line parameters in the

     exec function. This can be done by good parameter design (see 'do exactly

     and only one thing' above)

   - If there is an obvious fix for an unpredictable problem, offer the

     solution in the command output. So rather than use request.error (see

     Request Object below) output some HTML which contains a link to a fixed

     command line.

 Currently these concepts are not enforced at a code level, but they could be in

 the future.

 ## How commands work

 This is how to create a basic ``greet`` command:

     gcli.addCommand({

       name: 'greet',

       description: 'Show a greeting',

       params: [

         {

           name: 'name',

           type: 'string',

           description: 'The name to greet'

         }

       ],

       returnType: 'string',

       exec: function(args, context) {

         return 'Hello, ' + args.name;

       }

     });

 This command is used as follows:

     : greet Joe

     Hello, Joe

 Some terminology that isn't always obvious: a function has 'parameters', and

 when you call a function, you pass 'arguments' to it.

 ## Internationalization (i18n)

 There are several ways that GCLI commands can be localized. The best method

 depends on what context you are writing your command for.

 ### Firefox Embedding

 GCLI supports Mozilla style localization. To add a command that will only ever

 be used embedded in Firefox, this is the way to go. Your strings should be

 stored in ``browser/locales/en-US/chrome/browser/devtools/gclicommands.properties``,

 And you should access them using ``gcli.lookup(...)`` or ``gcli.lookupFormat()``

 For examples of existing commands, take a look in

 ``browser/devtools/webconsole/GcliCommands.jsm``, which contains most of the

 current GCLI commands. If you will be adding a number of new commands, then

 consider starting a new JSM.

 Your command will then look something like this:

     gcli.addCommand({

       name: 'greet',

       description: gcli.lookup("greetDesc")

       ...

     });

 ### Web Commands

 There are 2 ways to provide translated strings for web use. The first is to

 supply the translated strings in the description:

     gcli.addCommand({

       name: 'greet',

       description: {

         'root': 'Show a greeting',

         'fr-fr': 'Afficher un message d'accueil',

         'de-de': 'Zeige einen Gruß',

         'gk-gk': 'Εμφάνιση ένα χαιρετισμό',

         ...

       }

       ...

     });

 Each description should contain at least a 'root' entry which is the

 default if no better match is found. This method has the benefit of being

 compact and simple, however it has the significant drawback of being wasteful

 of memory and bandwidth to transmit and store a significant number of strings,

 the majority of which will never be used.

 More efficient is to supply a lookup key and ask GCLI to lookup the key from an

 appropriate localized strings file:

     gcli.addCommand({

       name: 'greet',

       description: { 'key': 'demoGreetingDesc' }

       ...

     });

 For web usage, the central store of localized strings is

 ``lib/gcli/nls/strings.js``. Other string files can be added using the

 ``l10n.registerStringsSource(...)`` function.

 This method can be used both in Firefox and on the Web (see the help command

 for an example). However this method has the drawback that it will not work

 with DryIce built files until we fix bug 683844.

 ## Default argument values

 The ``greet`` command requires the entry of the ``name`` parameter. This

 parameter can be made optional with the addition of a ``defaultValue`` to the

 parameter:

     gcli.addCommand({

       name: 'greet',

       description: 'Show a message to someone',

       params: [

         {

           name: 'name',

           type: 'string',

           description: 'The name to greet',

           defaultValue: 'World!'

         }

       ],

       returnType: 'string',

       exec: function(args, context) {

         return "Hello, " + args.name;

       }

     });

 Now we can also use the ``greet`` command as follows:

     : greet

     Hello, World!

 ## Positional vs. named arguments

 Arguments can be entered either positionally or as named arguments. Generally

 users will prefer to type the positional version, however the named alternative

 can be more self documenting.

 For example, we can also invoke the greet command as follows:

     : greet --name Joe

     Hello, Joe

 ## Short argument names

 GCLI allows you to specify a 'short' character for any parameter:

     gcli.addCommand({

       name: 'greet',

       params: [

         {

           name: 'name',

           short: 'n',

           type: 'string',

           ...

         }

       ],

       ...

     });

 This is used as follows:

     : greet -n Fred

     Hello, Fred

 Currently GCLI does not allow short parameter merging (i.e. ```ls -la```)

 however this is planned.

 ## Parameter types

 Initially the available types are:

 - string

 - boolean

 - number

 - selection

 - delegate

 - date

 - array

 - file

 - node

 - nodelist

 - resource

 - command

 - setting

 This list can be extended. See [Writing Types](writing-types.md) on types for

 more information.

 The following examples assume the following definition of the ```greet```

 command:

     gcli.addCommand({

       name: 'greet',

       params: [

         { name: 'name', type: 'string' },

         { name: 'repeat', type: 'number' }

       ],

       ...

     });

 Parameters can be specified either with named arguments:

     : greet --name Joe --repeat 2

 And sometimes positionally:

     : greet Joe 2

 Parameters can be specified positionally if they are considered 'important'.

 Unimportant parameters must be specified with a named argument.

 Named arguments can be specified anywhere on the command line (after the

 command itself) however positional arguments must be in order. So

 these examples are the same:

     : greet --name Joe --repeat 2

     : greet --repeat 2 --name Joe

 However (obviously) these are not the same:

     : greet Joe 2

     : greet 2 Joe

 (The second would be an error because 'Joe' is not a number).

 Named arguments are assigned first, then the remaining arguments are assigned

 to the remaining parameters. So the following is valid and unambiguous:

     : greet 2 --name Joe

 Positional parameters quickly become unwieldy with long parameter lists so we

 recommend only having 2 or 3 important parameters. GCLI provides hints for

 important parameters more obviously than unimportant ones.

 Parameters are 'important' if they are not in a parameter group. The easiest way

 to achieve this is to use the ```option: true``` property.

 For example, using:

     gcli.addCommand({

       name: 'greet',

       params: [

         { name: 'name', type: 'string' },

         { name: 'repeat', type: 'number', option: true, defaultValue: 1 }

       ],

       ...

     });

 Would mean that this is an error

     : greet Joe 2

 You would instead need to do the following:

     : greet Joe --repeat 2

 For more on parameter groups, see below.

 In addition to being 'important' and 'unimportant' parameters can also be

 optional. If is possible to be important and optional, but it is not possible

 to be unimportant and non-optional.

 Parameters are optional if they either:

 - Have a ```defaultValue``` property

 - Are of ```type=boolean``` (boolean arguments automatically default to being false)

 There is currently no way to make parameters mutually exclusive.

 ## Selection types

 Parameters can have a type of ``selection``. For example:

     gcli.addCommand({

       name: 'greet',

       params: [

         { name: 'name', ... },

         {

           name: 'lang',

           description: 'In which language should we greet',

           type: { name: 'selection', data: [ 'en', 'fr', 'de', 'es', 'gk' ] },

           defaultValue: 'en'

         }

       ],

       ...

     });

 GCLI will enforce that the value of ``arg.lang`` was one of the values

 specified. Alternatively ``data`` can be a function which returns an array of

 strings.

 The ``data`` property is useful when the underlying type is a string but it

 doesn't work when the underlying type is something else. For this use the

 ``lookup`` property as follows:

       type: {

         name: 'selection',

         lookup: {

           'en': Locale.EN,

           'fr': Locale.FR,

           ...

         }

       },

 Similarly, ``lookup`` can be a function returning the data of this type.

 ## Number types

 Number types are mostly self explanatory, they have one special property which

 is the ability to specify upper and lower bounds for the number:

     gcli.addCommand({

       name: 'volume',

       params: [

         {

           name: 'vol',

           description: 'How loud should we go',

           type: { name: 'number', min: 0, max: 11 }

         }

       ],

       ...

     });

 You can also specify a ``step`` property which specifies by what amount we

 should increment and decrement the values. The ``min``, ``max``, and ``step``

 properties are used by the command line when up and down are pressed and in

 the input type of a dialog generated from this command.

 ## Delegate types

 Delegate types are needed when the type of some parameter depends on the type

 of another parameter. For example:

     : set height 100

     : set name "Joe Walker"

 We can achieve this as follows:

     gcli.addCommand({

       name: 'set',

       params: [

         {

           name: 'setting',

           type: { name: 'selection', values: [ 'height', 'name' ] }

         },

         {

           name: 'value',

           type: {

             name: 'delegate',

             delegateType: function() { ... }

           }

         }

       ],

       ...

     });

 Several details are left out of this example, like how the delegateType()

 function knows what the current setting is. See the ``pref`` command for an

 example.

 ## Array types

 Parameters can have a type of ``array``. For example:

     gcli.addCommand({

       name: 'greet',

       params: [

         {

           name: 'names',

           type: { name: 'array', subtype: 'string' },

           description: 'The names to greet',

           defaultValue: [ 'World!' ]

         }

       ],

       ...

       exec: function(args, context) {

         return "Hello, " + args.names.join(', ') + '.';

       }

     });

 This would be used as follows:

     : greet Fred Jim Shiela

     Hello, Fred, Jim, Shiela.

 Or using named arguments:

     : greet --names Fred --names Jim --names Shiela

     Hello, Fred, Jim, Shiela.

 There can only be one ungrouped parameter with an array type, and it must be

 at the end of the list of parameters (i.e. just before any parameter groups).

 This avoids confusion as to which parameter an argument should be assigned.

 ## Sub-commands

 It is common for commands to be groups into those with similar functionality.

 Examples include virtually all VCS commands, ``apt-get``, etc. There are many

 examples of commands that should be structured as in a sub-command style -

 ``tar`` being the obvious example, but others include ``crontab``.

 Groups of commands are specified with the top level command not having an

 exec function:

     gcli.addCommand({

       name: 'tar',

       description: 'Commands to manipulate archives',

     });

     gcli.addCommand({

       name: 'tar create',

       description: 'Create a new archive',

       exec: function(args, context) { ... },

       ...

     });

     gcli.addCommand({

       name: 'tar extract',

       description: 'Extract from an archive',

       exec: function(args, context) { ... },

       ...

     });

 ## Parameter groups

 Parameters can be grouped into sections.

 There are 3 ways to assign a parameter to a group.

 The simplest uses ```option: true``` to put a parameter into the default

 'Options' group:

     gcli.addCommand({

       name: 'greet',

       params: [

         { name: 'repeat', type: 'number', option: true }

       ],

       ...

     });

 The ```option``` property can also take a string to use an alternative parameter

 group:

     gcli.addCommand({

       name: 'greet',

       params: [

         { name: 'repeat', type: 'number', option: 'Advanced' }

       ],

       ...

     });

 An example of how this can be useful is 'git' which categorizes parameters into

 'porcelain' and 'plumbing'.

 Finally, parameters can be grouped together as follows:

     gcli.addCommand({

       name: 'greet',

       params: [

         { name: 'name', type: 'string', description: 'The name to greet' },

         {

           group: 'Advanced Options',

           params: [

             { name: 'repeat', type: 'number', defaultValue: 1 },

             { name: 'debug', type: 'boolean' }

           ]

         }

       ],

       ...

     });

 This could be used as follows:

     : greet Joe --repeat 2 --debug

     About to send greeting

     Hello, Joe

     Hello, Joe

     Done!

 Parameter groups must come after non-grouped parameters because non-grouped

 parameters can be assigned positionally, so their index is important. We don't

 want 'holes' in the order caused by parameter groups.

 ## Command metadata

 Each command should have the following properties:

 - A string ``name``.

 - A short ``description`` string. Generally no more than 20 characters without

   a terminating period/fullstop.

 - A function to ``exec``ute. (Optional for the parent containing sub-commands)

   See below for more details.

 And optionally the following extra properties:

 - A declaration of the accepted ``params``.

 - A ``hidden`` property to stop the command showing up in requests for help.

 - A ``context`` property which defines the scope of the function that we're

   calling. Rather than simply call ``exec()``, we do ``exec.call(context)``.

 - A ``manual`` property which allows a fuller description of the purpose of the

   command.

 - A ``returnType`` specifying how we should handle the value returned from the

   exec function.

 The ``params`` property is an array of objects, one for each parameter. Each

 parameter object should have the following 3 properties:

 - A string ``name``.

 - A short string ``description`` as for the command.

 - A ``type`` which refers to an existing Type (see Writing Types).

 Optionally each parameter can have these properties:

 - A ``defaultValue`` (which should be in the type specified in ``type``).

   The defaultValue will be used when there is no argument supplied for this

   parameter on the command line.

   If the parameter has a ``defaultValue``, other than ``undefined`` then the

   parameter is optional, and if unspecified on the command line, the matching

   argument will have this value when the function is called.

   If ``defaultValue`` is missing, or if it is set to ``undefined``, then the

   system will ensure that a value is provided before anything is executed.

   There are 2 special cases:

   - If the type is ``selection``, then defaultValue must not be undefined.

     The defaultValue must either be ``null`` (meaning that a value must be

     supplied by the user) or one of the selection values.

   - If the type is ``boolean``, then ``defaultValue:false`` is implied and

     can't be changed. Boolean toggles are assumed to be off by default, and

     should be named to match.

 - A ``manual`` property for parameters is exactly analogous to the ``manual``

   property for commands - descriptive text that is longer than than 20

   characters.

 ## The Command Function (exec)

 The parameters to the exec function are designed to be useful when you have a

 large number of parameters, and to give direct access to the environment (if

 used).

     gcli.addCommand({

       name: 'echo',

       description: 'The message to display.',

       params: [

         {

           name: 'message',

           type: 'string',

           description: 'The message to display.'

         }

       ],

       returnType: 'string',

       exec: function(args, context) {

         return args.message;

       }

     });

 The ``args`` object contains the values specified on the params section and

 provided on the command line. In this example it would contain the message for

 display as ``args.message``.

 The ``context`` object has the following signature:

     {

       environment: ..., // environment object passed to createTerminal()

       exec: ...,        // function to execute a command

       update: ...,      // function to alter the text of the input area

       createView: ...,  // function to help creating rich output

       defer: ...,       // function to create a deferred promise

     }

 The ``environment`` object is opaque to GCLI. It can be used for providing

 arbitrary data to your commands about their environment. It is most useful

 when more than one command line exists on a page with similar commands in both

 which should act in their own ways.

 An example use for ``environment`` would be a page with several tabs, each

 containing an editor with a command line. Commands executed in those editors

 should apply to the relevant editor.

 The ``environment`` object is passed to GCLI at startup (probably in the

 ``createTerminal()`` function).

 The ``document`` object is also passed to GCLI at startup. In some environments

 (e.g. embedded in Firefox) there is no global ``document``. This object

 provides a way to create DOM nodes.

 ``defer()`` allows commands to execute asynchronously.

 ## Returning data

 The command meta-data specifies the type of data returned by the command using

 the ``returnValue`` setting.

 ``returnValue`` processing is currently functioning, but incomplete, and being

 tracked in [Bug 657595](http://bugzil.la/657595). Currently you should specify

 a ``returnType`` of ``string`` or ``html``. If using HTML, you can return

 either an HTML string or a DOM node.

 In the future, JSON will be strongly encouraged as the return type, with some

 formatting functions to convert the JSON to HTML.

 Asynchronous output is achieved using a promise created from the ``context``

 parameter: ``context.defer()``.

 Some examples of this is practice:

     { returnType: "string" }

     ...

     return "example";

 GCLI interprets the output as a plain string. It will be escaped before display

 and available as input to other commands as a plain string.

     { returnType: "html" }

     ...

     return "<p>Hello</p>";

 GCLI will interpret this as HTML, and parse it for display.

     { returnType: "dom" }

     ...

     return util.createElement(context.document, 'div');

 ``util.createElement`` is a utility to ensure use of the XHTML namespace in XUL

 and other XML documents. In an HTML document it's functionally equivalent to

 ``context.document.createElement('div')``. If your command is likely to be used

 in Firefox or another XML environment, you should use it. You can import it

 with ``var util = require('util/util');``.

 GCLI will use the returned HTML element as returned. See notes on ``context``

 above.

     { returnType: "number" }

     ...

     return 42;

 GCLI will display the element in a similar way to a string, but it the value

 will be available to future commands as a number.

     { returnType: "date" }

     ...

     return new Date();

     { returnType: "file" }

     ...

     return new File();

 Both these examples return data as a given type, for which a converter will

 be required before the value can be displayed. The type system is likely to

 change before this is finalized. Please contact the author for more

 information.

     { returnType: "string" }

     ...

     var deferred = context.defer();

     setTimeout(function() {

       deferred.resolve("hello");

     }, 500);

     return deferred.promise;

 Errors can be signaled by throwing an exception. GCLI will display the message

 property (or the toString() value if there is no message property). (However

 see *3 principles for writing commands* above for ways to avoid doing this).

 ## Specifying Types

 Types are generally specified by a simple string, e.g. ``'string'``. For most

 types this is enough detail. There are a number of exceptions:

 * Array types. We declare a parameter to be an array of things using ``[]``,

   for example: ``number[]``.

 * Selection types. There are 3 ways to specify the options in a selection:

   * Using a lookup map

             type: {

               name: 'selection',

               lookup: { one:1, two:2, three:3 }

             }

     (The boolean type is effectively just a selection that uses

     ``lookup:{ 'true': true, 'false': false }``)

   * Using given strings

             type: {

               name: 'selection',

               data: [ 'left', 'center', 'right' ]

             }

   * Using named objects, (objects with a ``name`` property)

             type: {

               name: 'selection',

               data: [

                 { name: 'Google', url: 'http://www.google.com/' },

                 { name: 'Microsoft', url: 'http://www.microsoft.com/' },

                 { name: 'Yahoo', url: 'http://www.yahoo.com/' }

               ]

             }

 * Delegate type. It is generally best to inherit from Delegate in order to

   provide a customization of this type. See settingValue for an example.

 See below for more information.