Arguments - The Basics

In JNode's command line interface, the Argument types are the Command programmers main tool for interacting with the user. The Argument provides support to the syntax mechanism to accept parameters, or reject malformed parameters, issuing a useful error message. The argument also supplies completion support, allowing a command to provide specific completions on specific domains.


At the moment, Arguments are mostly grouped into the shell project under the package. For the time being they will remain here. There is an effort being made to 'untangle' the syntax/argument APIs so this designation is subject to change in the future.

New arguments that are created should be placed into the cli project under the org.jnode.command.argument package if their only use is by the commands under the cli project.

How it works

Every command that accepts an option will require Arguments to capture the options and their associated values. The syntax parser makes use of an argument 'label' to map a syntax node to a specific argument. The parser then asks the Argument to 'accept' the given value. The argument may reject the token if it doesn't not satisfy it's requirements, and provide a suitable error message as to why. If it accepts the token, then it will be captured by the argument for later use by it's command.

Arguments also provide the ability to 'complete' a partial token. In some situations completions are not possible or do not make sense, but in many situations completions can be very helpful and save on typing, reduce errors, and even provide a little help if there are alot of options. The more characters there are in the token, the narrower the list of completions becomes. If the argument supplies only a single completion, this completion will be filled in for the user. This is a very powerful capability that can be used to great effect!

Using arguments

Before writing a command, it is important to consult the various specifications that many commands may have. Once you have an idea of the arguments you will need for the command, and you have a syntax put together, you can begin by adding your arguments to the command.

Along with the label that was discussed earlier, commands also take a set of flags. A set of flags is supplied by the Argument class, but individual Argument types may also supply their own specific flags. At the end of this document will be a list of known flags and their purpose, but for now we will discuss the common Argument flags.

By default arguments are 'SINGLE'. This means that the argument may only contain one value. In order to change this, and allow the argument to capture multiple values, you must set the MULTIPLE flag.
By default arguments are 'OPTIONAL'. This means that if the argument is not populated with any values, it will not be considered an error. In order to have the parser fail if the argument is not populated with at least one value, set the MANDATORY flag.
These flags are not used by all arguments and may be used to alter the behavior of 'accept' and 'complete' depending on the argument, and their values. As an example, the FileArgument by default will accept most any string that denotes a legal file name. In order to force it to only accept tokens that denote an existing file on the file system than set the EXISTING flag. In order to force it to only accept tokens that denote a file that does not exist already, set the NONEXISTENT flag.

Most arguments have overloaded constructors that allow you to not set any flags. If no such constructor exists, then feel free to create one! Optionally, it is safe to provide '0'(zero) for the flags parameter to mean no flags.

Once you have created the arguments that your command will need, you need to 'register' the arguments. This needs to be done in the Command constructor. Each argument needs to be passed to the registerArguments(Argument...) method. Once this is done, your arguments are ready to be populated the syntax parser.

(Note: Arguments that have been registered, but do not have a matching syntax node with its label will not cause an error at runtime. But they do make trouble for the 'help' command. For this reason it is recommended to not register arguments that have not yet been mapped in the syntax.)

Using arguments

When your command enters at the execute() method, the arguments will be populated with any values that were capture from the command line. For the most part, you will only need to be concerned with three methods supplied by Argument.

public boolean isSet()
If the argument has accepted and captured a token, then this method will return true. Commands should always check this method before querying for the captured values. If you query an argument for its values when it has none, the behavior is undefined and the return value (or possible exception) is unspecified and subject to change without notice. (The one case where this is not totally true is when an argument has the MANDATORY flag, as in this case this will _always_ return true. Though it is still considered 'good practice' to check this method before querying for values)
public V getValue()
This method returns the single value of an argument that was registered as SINGLE. If the argument has the MULTIPLE flag set, this method should not be used as it will throw an exception if there is more than one value captured by the argument. If there are no values captured, this currently returns null, but as noted earlier, this may not always be the case, and should not be relied upon.
public V[] getValues()
This method returns the captured values as an array. Calling this method when there the SINGLE flag is set is perfectly acceptable. Though it is usually more convenient to use the getValue() method.

Thats about it for arguments. Simple huh? Arguments are designed to allow for rapid development of commands and as such provide a nice simple interface for using arguments 'out of the box' so to speak. But the real power of arguments are their ability to be extended and manipulated in many ways so as to provide a more feature filled command line interface.

Basic argument types

Here are a list of the more common argument types, along with a short description on their purpose, features and usage.

An argument that accepts an 'alias'. It provides completion against those aliases that have been registered via plugin descriptors, and may also include the bjorne-style aliases if the bjorne interpreter is in use. (I'm not sure if it currently does, if not it should!)
An argument that accepts a As used in an example above, this argument is affected by the EXISTING and NONEXISTENT flags of Argument. FileArgument also currently provides two of its own flags that may be set. ALLOW_DODGY_NAMES is used to override the 'accept' and 'complete' features to allow filenames that begin with a '-'(hyphen). Normally FileArgument would consider such a filename to be an error, and reject such a token. There is also the HYPHEN_IS_SPECIAL flag, which allows a single '-' to be accepted. The purpose for this is to allow '-' to exist amongst a list of files, denoting stardnard input or output should be used instead. This feature is subject to change (pending some 'better way' of handling this).
This is likely to be the most used argument of all. It is used to denote a option that has no associated argument. This argument does not actually capture a token, instead it holds a single value of true if it has been found. This means that you can use isSet() to map its value to a local/instance boolean value. In some cases, a command may wish to allow a flag to be used multiple times to add advanced meaning. The command can use getValues().length in such a case to determine the number of times it has been specified.
IntegerArgument / LongArgument / DecimalArgument(TODO)
Allows an integer value to be captured on the command line. These arguments do not provide very helpful completion, as their domain of completions is generally too large. Their main purpose is to parse valid integers, rejecting those that are malformed.
This is one of the most 'accepting' arguments, as it will accept any token that is given to it. It also provides no completion. If your command really needs an unbounded String, then this is the right argument to use. This argument should be extended for cases where you want to accept a string, but the domain of acceptable strings is limited, and you wish to reject those tokens not within that domain and also possibly provide completion for the argument.
Similar in some respects to FileArgument, the URLArgument accepts valid tokens that represent a URL. This argument also respects the EXISTING and NONEXISTENT flags. Completion for parts of a url (the scheme for example), may be able to complete, but actual URL completion of domain names and the like may be nearly impossible. It should also be noted that the EXISTING and NONEXISTENT flags will likely cause a DNS lookup to be performed,