Vert.x Core provides an API for parsing command line arguments passed to programs. It's also able to print help
messages detailing the options available for a command line tool. Even if such features are far from
the Vert.x core topics, this API is used in the {@link io.vertx.core.Launcher} class that you can use in _fat-jar_
and in the `vertx` command line tools. In addition, it's polyglot (can be used from any supported language) and is
used in Vert.x Shell.

Vert.x CLI provides a model to describe your command line interface, but also a parser. This parser supports
different types of syntax:

* POSIX like options (ie. `tar -zxvf foo.tar.gz`)
* GNU like long options (ie. `du --human-readable --max-depth=1`)
* Java like properties (ie. `java -Djava.awt.headless=true -Djava.net.useSystemProxies=true Foo`)
* Short options with value attached (ie. `gcc -O2 foo.c`)
* Long options with single hyphen (ie. `ant -projecthelp`)

Using the CLI api is a 3-steps process:

1. The definition of the command line interface
2. The parsing of the user command line
3. The query / interrogation

=== Definition Stage

Each command line interface must define the set of options and arguments that will be used. It also requires a
name. The CLI API uses the {@link io.vertx.core.cli.Option} and {@link io.vertx.core.cli.Argument} classes to
describe options and arguments:

[source,$lang]
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{@link examples.cli.CLIExamples#example1}
----

As you can see, you can create a new {@link io.vertx.core.cli.CLI} using
{@link io.vertx.core.cli.CLI#create(java.lang.String)}. The passed string is the name of the CLI. Once created you
can set the summary and description. The summary is intended to be short (one line), while the description can
contain more details. Each option and argument are also added on the `CLI` object using the
{@link io.vertx.core.cli.CLI#addArgument(io.vertx.core.cli.Argument)} and
{@link io.vertx.core.cli.CLI#addOption(io.vertx.core.cli.Option)} methods.

==== Options

An {@link io.vertx.core.cli.Option} is a command line parameter identified by a _key_ present in the user command
line. Options must have at least a long name or a short name. Long name are generally used using a `--` prefix,
while short names are used with a single `-`. Options can get a description displayed in the usage (see below).
Options can receive 0, 1 or several values. An option receiving 0 values is a `flag`, and must be declared using
{@link io.vertx.core.cli.Option#setFlag(boolean)}. By default, options receive a single value, however, you can
configure the option to receive several values using {@link io.vertx.core.cli.Option#setMultiValued(boolean)}:

[source,$lang]
----
{@link examples.cli.CLIExamples#example2}
----

Options can be marked as mandatory. A mandatory option not set in the user command line throws an exception during
the parsing:

[source,$lang]
----
{@link examples.cli.CLIExamples#example3}
----

Non-mandatory options can have a _default value_. This value would be used if the user does not set the option in
the command line:

[source,$lang]
----
{@link examples.cli.CLIExamples#example4}
----

An option can be _hidden_ using the {@link io.vertx.core.cli.Option#setHidden(boolean)} method. Hidden option are
not listed in the usage, but can still be used in the user command line (for power-users).

If the option value is contrained to a fixed set, you can set the different acceptable choices:

[source,$lang]
----
{@link examples.cli.CLIExamples#example41}
----

Options can also be instantiated from their JSON form.

==== Arguments

Unlike options, arguments do not have a _key_ and are identified by their _index_. For example, in
`java com.acme.Foo`, `com.acme.Foo` is an argument.

Arguments do not have a name, there are identified using a 0-based index. The first parameter has the
index `0`:

[source,$lang]
----
{@link examples.cli.CLIExamples#example5}
----

If you don't set the argument indexes, it computes it automatically by using the declaration order.

[source,$lang]
----
{@link examples.cli.CLIExamples#example51}
----

The `argName` is optional and used in the usage message.

As options, {@link io.vertx.core.cli.Argument} can:

* be hidden using {@link io.vertx.core.cli.Argument#setHidden(boolean)}
* be mandatory using {@link io.vertx.core.cli.Argument#setRequired(boolean)}
* have a default value using {@link io.vertx.core.cli.Argument#setDefaultValue(java.lang.String)}
* receive several values using {@link io.vertx.core.cli.Argument#setMultiValued(boolean)} - only the last argument
can be multi-valued.

Arguments can also be instantiated from their JSON form.

==== Usage generation

Once your {@link io.vertx.core.cli.CLI} instance is configured, you can generate the _usage_ message:

[source,$lang]
----
{@link examples.cli.CLIExamples#example6}
----

It generates an usage message like this one:

[source]
----
Usage: copy [-R] source target

A command line interface to copy files.

  -R,--directory   enables directory support
----

If you need to tune the usage message, check the {@link io.vertx.core.cli.UsageMessageFormatter} class.

=== Parsing Stage

Once your {@link io.vertx.core.cli.CLI} instance is configured, you can parse the user command line to evaluate
each option and argument:

[source,$lang]
----
{@link examples.cli.CLIExamples#example7}
----

The {@link io.vertx.core.cli.CLI#parse(java.util.List)} method returns a {@link io.vertx.core.cli.CommandLine}
object containing the values. By default, it validates the user command line and checks that each mandatory options
and arguments have been set as well as the number of values received by each option. You can disable the
validation by passing `false` as second parameter of {@link io.vertx.core.cli.CLI#parse(java.util.List,boolean)}.
This is useful if you want to check an argument or option is present even if the parsed command line is invalid.

You can check whether or not the
{@link io.vertx.core.cli.CommandLine} is valid using {@link io.vertx.core.cli.CommandLine#isValid()}.

=== Query / Interrogation Stage

Once parsed, you can retrieve the values of the options and arguments from the
{@link io.vertx.core.cli.CommandLine} object returned by the {@link io.vertx.core.cli.CLI#parse(java.util.List)}
method:

[source,$lang]
----
{@link examples.cli.CLIExamples#example8}
----

One of your option can have been marked as "help". If a user command line enabled a "help" option, the validation
won't failed, but give you the opportunity to check if the user asks for help:

[source,$lang]
----
{@link examples.cli.CLIExamples#example9}
----

[language,java]
----
include::cli-for-java.adoc[]
----
