Apache Commons CLI Overview

The Apacahe Commons CLI component parses command-line arguments for your application.

Commons CLI parses command-line arguments using a descriptor of valid options (long and short), potentially with arguments.

Command-line arguments may be of the typical {@code String[]} form, but also may be a {@code java.util.List}. Indexes allow for parsing only a portion of the command-line. Also, functionality for parsing the command-line in phases is built in, allowing for 'cvs-style' command-lines, where some global options are specified before a 'command' argument, and command-specific options are specified after the command argument:

            myApp -p <port> command -p <printer>

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# Introducing Apache Commons CLI

There are three stages to command line processing. They are the definition, parsing and interrogation stages. The following sections discuss each of these stages in turn, and show how to implement them with CLI.

## Defining the CLI

Each command line must define the set of options that will be used to define the interface to the application.

CLI uses the Options class, as a container for Option instances. There are two ways to create `Option`s in CLI. One of them is via the constructors, the other way is via the factory methods defined in `Options`.

The Usage Scenarios document provides examples how to create an `Options` object and also provides some real world examples.

The result of the definition stage is an `Options` instance.

## Parsing the CLI

The parsing stage is where the text passed into the application via the command line is processed. The text is processed according to the rules defined by the parser implementation.

The `parse` method defined on CommandLineParser takes an `Options` instance and a `String[]` of arguments and returns a CommandLine.

The result of the parsing stage is a `CommandLine` instance.

## Interrogating the CLI

The interrogation stage is where the application queries the `CommandLine` to decide what execution branch to take depending on boolean options and uses the option values to provide the application data.

This stage is implemented in the user code. The accessor methods on `CommandLine` provide the interrogation capability to the user code.

The result of the interrogation stage is that the user code is fully informed of all the text that was supplied on the command line and processed according to the parser and `Options` rules.

# Using Apache Commons CLI

The following sections describe some example scenarios on how to use CLI in applications.

## Using a boolean option

A boolean option is represented on a command line by the presence of the option, i.e. if the option is found then the option value is `true`, otherwise the value is `false`.

The `DateApp` utility prints the current date to standard output. If the `-t` option is present the current time is also printed.

## Creating the Options

An Options object must be created and the `Option` must be added to it.

    // create Options object
    Options options = new Options();

    // add t option
    options.addOption("t", false, "display current time");

The `addOption` method has three parameters. The first parameter is a `java.lang.String` that represents the option. The second parameter is a `boolean` that specifies whether the option requires an argument or not. In the case of a boolean option (sometimes referred to as a flag) an argument value is not present so `false` is passed. The third parameter is the description of the option. This description will be used in the usage text of the application.

## Parsing the command line arguments

The `parse` methods of `CommandLineParser` are used to parse the command line arguments. There may be several implementations of the `CommandLineParser` interface, the recommended one is the `DefaultParser`.

`CommandLineParser parser = new DefaultParser(); CommandLine cmd = parser.parse(options, args);`

Now we need to check if the `t` option is present. To do this we will interrogate the CommandLine object. The `hasOption` method takes a `java.lang.String` parameter and returns `true` if the option represented by the `java.lang.String` is present, otherwise it returns `false`.

    if(cmd.hasOption("t")) {
        // print the date and time
    }
    else {
        // print the date
    }

Note

As of version 1.5, the `DefaultParser`'s constructor now has an override with the signature `DefaultParser(final boolean allowPartialMatching)`. Given the following code: `final Options options = new Options(); options.addOption(new Option("d", "debug", false, "Turn on debug.")); options.addOption(new Option("e", "extract", false, "Turn on extract.")); options.addOption(new Option("o", "option", true, "Turn on option with argument."));` we define "partial matching" as `-de` only matching the `"debug"` option. We can consequently, now, turn this off and have `-de` match both the `debug` option as well as the `extract` option.

## International Time

The `InternationalDateApp` utility extends the `DateApp` utility by providing the ability to print the date and time in any country in the world. To facilitate this a new command line option, `c`, has been introduced.

    // add c option
    options.addOption("c", true, "country code");

The second parameter is `true` this time. This specifies that the `c` option requires an argument value. If the required option argument value is specified on the command line it is returned, otherwise `null` is returned.

## Retrieving the argument value

The `getOptionValue` methods of `CommandLine` are used to retrieve the argument values of options.

    // get c option value
    String countryCode = cmd.getOptionValue("c");

    if(countryCode == null) {
        // print default date
    }
    else {
        // print date for country specified by countryCode
    }

# Using Ant as an Example

Ant will be used here to illustrate how to create the `Options` required. The following is the help output for Ant.

    ant [options] [target [target2 [target3] ...]]
      Options:
      -help                  print this message
      -projecthelp           print project help information
      -version               print the version information and exit
      -quiet                 be extra quiet
      -verbose               be extra verbose
      -debug                 print debugging information
      -emacs                 produce logging information without adornments
      -logfile <file>        use given file for log
      -logger <classname>    the class which is to perform logging
      -listener <classname>  add an instance of class as a project listener
      -buildfile <file>      use given buildfile
      -D<property>=<value>   use value for given property
      -find <file>           search for buildfile towards the root of the
                             filesystem and use it

## Defining Boolean Options

Lets create the boolean options for the application as they are the easiest to create. For clarity the constructors for `Option` are used here.

    Option help = new Option("help", "print this message");
    Option projecthelp = new Option("projecthelp", "print project help information");
    Option version = new Option("version", "print the version information and exit");
    Option quiet = new Option("quiet", "be extra quiet");
    Option verbose = new Option("verbose", "be extra verbose");
    Option debug = new Option("debug", "print debugging information");
    Option emacs = new Option("emacs",
                               "produce logging information without adornments");

## Defining Argument Options

The argument options are created using the `Option#Builder`.

    Option logFile = Option.builder("logfile")
                             .argName("file")
                             .hasArg()
                             .desc("use given file for log")
                             .build();

    Option logger = Option.builder("logger")
                             .argName("classname")
                             .hasArg()
                             .desc("the class which it to perform logging")
                             .build();

    Option listener = Option.builder("listener")
                             .argName("classname")
                             .hasArg()
                             .desc("add an instance of class as "
                                    + "a project listener")
                             .build();

    Option buildFile = Option.builder("buildfile")
                             .argName("file")
                             .hasArg()
                             .desc("use given buildfile")
                             .build();

    Option find = Option.builder("find")
                             .argName("file")
                             .hasArg()
                             .desc("search for buildfile towards the "
                                    + "root of the filesystem and use it")
                             .build();

## Defining Java Property Option

The last option to create is the Java property, and it is also created using the Option class' Builder.

`Option property = Option property = Option.builder("D") .hasArgs() .valueSeparator('=') .build();`

The map of properties specified by this option can later be retrieved by calling `getOptionProperties("D")` on the `CommandLine`.

## Creating the Options

Now that we have created each Option we need to create the Options instance. This is achieved using the addOption method of `Options`.

    Options options = new Options();

    options.addOption(help);
    options.addOption(projecthelp);
    options.addOption(version);
    options.addOption(quiet);
    options.addOption(verbose);
    options.addOption(debug);
    options.addOption(emacs);
    options.addOption(logfile);
    options.addOption(logger);
    options.addOption(listener);
    options.addOption(buildfile);
    options.addOption(find);
    options.addOption(property);

All the preparation is now complete, and we are now ready to parse the command line arguments.

## Creating the Parser

We now need to create a `CommandLineParser`. This will parse the command line arguments, using the rules specified by the `Options` and return an instance of CommandLine.

    public static void main(String[] args) {
        // create the parser
        CommandLineParser parser = new DefaultParser();
        try {
            // parse the command line arguments
            CommandLine line = parser.parse(options, args);
        }
        catch (ParseException exp) {
            // oops, something went wrong
            System.err.println("Parsing failed.  Reason: " + exp.getMessage());
        }
    }

## Querying the commandline

To see if an option has been passed the `hasOption` method is used. The argument value can be retrieved using the `getOptionValue` method.

    // has the buildfile argument been passed?
    if (line.hasOption("buildfile")) {
        // initialize the member variable
        this.buildfile = line.getOptionValue("buildfile");
    }

## Displaying Usage and Help

CLI also provides the means to automatically generate usage and help information. This is achieved with the HelpFormatter class.

    // automatically generate the help statement
    HelpFormatter formatter = new HelpFormatter();
    formatter.printHelp("ant", options);

When executed the following output is produced:

    usage: ant
    -D <property=value>     use value for given property
    -buildfile <file>       use given buildfile
    -debug                  print debugging information
    -emacs                  produce logging information without adornments
    -file <file>            search for buildfile towards the root of the
                            filesystem and use it
    -help                   print this message
    -listener <classname>   add an instance of class as a project listener
    -logger <classname>     the class which it to perform logging
    -projecthelp            print project help information
    -quiet                  be extra quiet
    -verbose                be extra verbose
    -version                print the version information and exit

If you also require to have a usage statement printed then calling `formatter.printHelp("ant", options, true)` will generate a usage statement as well as the help information.

# Creating an ls Example

One of the most widely used command line applications in the *nix world is `ls`. Due to the large number of options required for `ls` this example will only cover a small proportion of the options. The following is a section of the help output.

    Usage: ls [OPTION]... [FILE]...
    List information about the FILEs (the current directory by default).
    Sort entries alphabetically if none of -cftuSUX nor --sort.

    -a, --all                  do not hide entries starting with .
    -A, --almost-all           do not list implied . and ..
    -b, --escape               print octal escapes for non-graphic characters
        --block-size=SIZE      use SIZE-byte blocks
    -B, --ignore-backups       do not list implied entries ending with ~
    -c                         with -lt: sort by, and show, ctime (time of last
                               modification of file status information)
                               with -l: show ctime and sort by name
                               otherwise: sort by ctime
    -C                         list entries by columns

The following is the code that is used to create the Options for this example.

    // create the command line parser
    CommandLineParser parser = new DefaultParser();

    // create the Options
    Options options = new Options();
    options.addOption("a", "all", false, "do not hide entries starting with .");
    options.addOption("A", "almost-all", false, "do not list implied . and ..");
    options.addOption("b", "escape", false, "print octal escapes for non-graphic "
                                             + "characters");
    options.addOption(Option.builder("SIZE").longOpt("block-size")
                                    .desc("use SIZE-byte blocks")
                                    .hasArg()
                                    .build());
    options.addOption("B", "ignore-backups", false, "do not list implied entries "
                                                     + "ending with ~");
    options.addOption("c", false, "with -lt: sort by, and show, ctime (time of last "
                                   + "modification of file status information) with "
                                   + "-l:show ctime and sort by name otherwise: sort "
                                   + "by ctime");
    options.addOption("C", false, "list entries by columns");

    String[] args = new String[]{ "--block-size=10" };

    try {
        // parse the command line arguments
        CommandLine line = parser.parse(options, args);

        // validate that block-size has been set
        if (line.hasOption("block-size")) {
            // print the value of block-size
            System.out.println(line.getOptionValue("block-size"));
        }
    }
    catch (ParseException exp) {
        System.out.println("Unexpected exception:" + exp.getMessage());
    }

# Converting (Parsing) Option Values

By in most cases the values on the command line are retrieved as Strings via the `commandLine.getOptionValue(key)` command. However, it is possible for the CLI library to convert the string into a different object. For example to specify that the "count" option should return an Integer the following code could be used:

    public static void main(String[] args) {
        Option count = Option.builder("count")
                      .hasArg()
                      .desc("the number of things")
                      .type(Integer.class)
                      .build();
        Options options = new Options().addOption(count);
        // create the parser
        CommandLineParser parser = new DefaultParser();
        try {
            // parse the command line arguments
            CommandLine line = parser.parse(options, args);
        } catch (ParseException exp) {
            // oops, something went wrong
            System.err.println("Parsing failed.  Reason: " + exp.getMessage());
        }

        try {
            Integer value = line.getParsedOptionValue(count);
            System.out.format("The value is %s%n", value );
        } catch (ParseException e) {
            e.printStackTrace();
        }
    }

The value types natively supported by commons-cli are:

- Object.class - The string value must be the name of a class with a no argument constructor
- Class.class - The string value must be the name of a class
- Date.class - The string value must be a date parsable by `new SimpleDateFormat("EEE MMM dd HH:mm:ss zzz yyyy")`
- File.class - The string value is the name of the file.
- Path.class - The string value is the name of a Path.
- Number.class - The string value is a number representation can can be converted into an Integer or a Double.
- URL.class - The string value is the textual representation of a URL
- FileInputStream.class - The string value is passed to `new FileInputStream(s)`.
- Long.class - The string value is a valid argument to `Long.parseLong()`.
- Integer.class - The string value is a valid argument to `Integer.parseInt()`.
- Short.class - The string value is a valid argument to `Short.parseShort()`.
- Byte.class - The string value is a valid argument to `Byte.parseByte()`.
- Character.class - The string value is either a UTF-8 encoding for a character (e.g. "\\u0124") or the first character from the String."
- Double.class - The string value is a valid argument to `Double.parseDouble()`.
- Float.class - The string value is a valid argument to `Float.parseFloat()`.
- BigInteger.class - The string value is a valid argument to `new BigInteger(s)`.
- BigDecimal.class - The string value is a valid argument to `new BigDecimal(s)`.

Additional types may be added to the automatic parsing system by calling `TypeHandler.register(Class<T> clazz, Converter<T> converter)`. The `Class<T>` can be any defined class. The converter is a function that takes a `String` argument and returns an instance of the class. Any exception thrown by the constructor will be caught and reported as a `ParseException`

Conversions can be specified without using the `TypeHandler` class by specifying the converter directly during the option build. For example:

      Option fooOpt = Option.builder("foo")
              .hasArg()
              .desc("the foo arg")
              .converter(Foo::new)
              .build();

The above will create an option that passes the string value to the Foo constructor when `commandLine.getParsedOptionValue(fooOpt)` is called.

Conversions that are added to the TypeHandler or that are specified directly will not deserialize if the option is serialized unless the type is registered with the TypeHandler before deserialization begins.

# Deprecating Options

Options may be marked as deprecated using ghe `Option.builder.deprecated()` method. Additional information may be specified by passing a `DeprecatedAttributes` instance to the `deprecated` method. Usage of the deprecated option is announced when the presence of the option is checked or the value of the option is retrieved. By default, the announcement printed to Standard out. The HelpFormatter output will by default show the description prefixed by "[Deprecated]"

The examples below will implement `doSomething` in the following code block.

      public static void main(String[] args) {
          Option n = Option.builder("n")
              .deprecated(DeprecatedAttributes.builder()
                  .setDescription("Use '-count' instead")
                  .setForRemoval(true)
                  .setSince("now").get())
              .hasArg()
              .desc("the number of things")
              .type(Integer.class)
              .build();
          Option count = Option.builder("count")
              .hasArg()
              .desc("the number of things")
              .type(Integer.class)
              .build();
          Options options = new Options().addOption(n).addOption(count);

          doSomething(options);
      }

## Changing Usage Announcement

The usage announcement may be changed by providing a `Consumer<Option>` to the `CommandLine.Builder.deprecatedHandler` method. This is commonly used to log usage of deprecated options rather than printing them on the standard output.

for example if `doSomething` is implemented as:

      void doSomething(Options options) {
          CommandLineParser parser = new DefaultParser();
          CommandLine line;
          try {
              // parse the command line arguments
              line = parser.parse(options, new String[] {"-n", "5"});
              System.out.println("n=" + line.getParsedOptionValue("n"));
          } catch (ParseException exp) {
              // oops, something went wrong
              System.err.println("Parsing failed. Reason: " + exp.getMessage());
          }
      }

The output of the run would be.

      Option 'n': Deprecated for removal since now: Use '-count' instead
      n=5

for example if `doSomething` is implemented as:

      void doSomething(Options options) {
          Consumer<Option> deprecatedUsageAnnouncement = o -> {
              final StringBuilder buf = new StringBuilder()
                  .append("'")
                  .append(o.getOpt())
                  .append("'");
              if (o.getLongOpt() != null) {
                  buf.append("'").append(o.getLongOpt()).append("'");
              }
              System.err.printf("ERROR: Option %s: %s%n", buf, o.getDeprecated());
          };
          DefaultParser parser = DefaultParser.builder().setDeprecatedHandler(deprecatedUsageAnnouncement).build();
          CommandLine line;
          try {
              // parse the command line arguments
              line = parser.parse(options, new String[] {"-n", "5"});
              System.out.println("n=" + line.getParsedOptionValue("n"));
          } catch (ParseException exp) {
            // oops, something went wrong
            System.err.println("Parsing failed. Reason: " + exp.getMessage());
          }
      }

The output of the run would be.

`ERROR: Option 'n': Deprecated for removal since now: Use '-count' instead n=5`

However, the first line would be printed on system err instead of system out.

## Changing help format

By default the help formater prints "[Deprecated]" in front of the description for the option. This can be changed to display any values desired.

If `doSomething` is implemented as:

      void doSomething(Options options) {
        HelpFormatter formatter = HelpFormatter.builder().get();
        formatter.printHelp("Command line syntax:", options);
      }

To output is

      usage: Command line syntax:
      -count <arg>   the number of things
      -n <arg>       [Deprecated] the number of things

The display of deprecated options may be changed through the use of the `HelpFormatter.Builder.setShowDeprecated()` method.

- Calling `HelpFormatter.Builder.setShowDeprecated(false)` will disable the "[Deprecated]" tag.
- Calling `HelpFormatter.Builder.setShowDeprecated` with a `Function<Option, String>` will use the output of the function as the description for the option.

As an example of the second case above, changing the implementation of `doSomething` to

      void doSomething(Options options) {
          Function<Option, String> disp = option -> String.format("%s. %s", HelpFormatter.getDescription(option),
                      option.getDeprecated().toString());
          HelpFormatter formatter = HelpFormatter.builder().setShowDeprecated(disp).get();
          formatter.printHelp("Command line syntax:", options);
      }

changes the output to

      usage: Command line syntax:
      -count <arg>   the number of things
      -n <arg>       the number of things. Deprecated for removal since now:
      Use '-count' instead

# Defining Option Properties

The following are the properties that each Option has. All of these can be set using the accessors or using the methods defined in the Option.Builder.

Option Properties
Name | Type | Description
--- | --- | ---
arg | boolean | A flag to say whether the option takes an argument.
args | boolean | A flag to say whether the option takes more than one argument.
argName | java.lang.String | The name of the argument value for the usage statement.
converter | org.apache.commons.cli.Converter<T, E extends Throwable> | A FunctionalInterface that converts a String to type T and may throw an exception E. When CommandLine.getParsedValue() is called this FunctionalInterface will perform the conversion from the command line argument to the parsed type. This is used when a desired type is not registered, or to provide a custom type implementation. The 'type' property is not required to use the 'converter' property.
deprecated | org.apache.commons.cli.DeprecatedAttributes | Marks the option as deprecated and has the deprecation properties.
description | java.lang.String | A description of the function of the option.
longOpt | java.lang.String | An alias and more descriptive identification string. May be null or not specified if 'opt' is provided.
opt | java.lang.String | The identification string of the Option. May be null or not specified if 'longOpt' is provided.
optionalArg | boolean | A flag to say whether the option's argument is optional.
required | boolean | A flag to say whether the option must appear on the command line.
type | java.lang.Class<?> | The class of the object returned from getParsedValue(). The class must be registered in TypeHandler instance. See also 'converter' property.
value | java.lang.String | The value of the option.
values | java.lang.String[] | The values of the option.
valueSeparator | char | The character value used to split the argument string, that is used in conjunction with multipleArgs e.g. if the separator is ',' and the argument string is 'a,b,c' then there are three argument values, 'a', 'b' and 'c'.