docopt helps you to:

• define an interface for your command-line application, and
• automatically generate a parser for it.

R package docopt is an implementation of docopt in the R programming language. See my presentation on the useR! 2014 for more details.

For more information see docopt.org

To try docopt in your browser visit try.docopt.org

The easiest way to get docopt is to install from CRAN:

install.packages("docopt")
library(docopt)

The latest version of docopt can be installed from GitHub using devtools:

library(devtools)  # make sure to have devtools 1.4!
install_github("docopt/docopt.R")

It is tested against the tests defined for the reference implementation. It passes most tests. It currently fails tests that

• count arguments: my_prog.R -v -v should return list(v=2)

The tests can be run using devtools::test() and can be found in "tests" directory.

library(devtools)
devtools::test()

docopt uses the description of the command-line interface (i.e. help message docstring) to parse command-line arguments.

'Naval Fate.

Usage:
  naval_fate.R ship new <name>...
  naval_fate.R ship <name> move <x> <y> [--speed=<kn>]
  naval_fate.R ship shoot <x> <y>
  naval_fate.R mine (set|remove) <x> <y> [--moored | --drifting]
  naval_fate.R (-h | --help)
  naval_fate.R --version

Options:
  -h --help     Show this screen.
  --version     Show version.
  --speed=<kn>  Speed in knots [default: 10].
  --moored      Moored (anchored) mine.
  --drifting    Drifting mine.

' -> doc

library(docopt)
arguments <- docopt(doc, version = 'Naval Fate 2.0')
print(arguments)

The option parser is generated based on the docstring above that is passed to docopt function. docopt parses the usage pattern ("Usage: ...") and option descriptions (lines starting with dash "-") and ensures that the program invocation matches the usage pattern; it parses options, arguments and commands based on that. The basic idea is that a good help message has all necessary information in it to make a parser.

To execute your command-line application you need to provide path to command-line executable file (i.e. naval_fate.R in our case) and provide relevant command-line arguments/options/commands.

For example

• To print command-line application help message:

$ Rscript path/to/naval_fate.R --help
Naval Fate.

Usage:
  naval_fate.R ship new <name>...
  naval_fate.R ship <name> move <x> <y> [--speed=<kn>]
  naval_fate.R ship shoot <x> <y>
  naval_fate.R mine (set|remove) <x> <y> [--moored | --drifting]
  naval_fate.R (-h | --help)
  naval_fate.R --version

Options:
  -h --help     Show this screen.
  --version     Show version.
  --speed=<kn>  Speed in knots [default: 10].
  --moored      Moored (anchored) mine.
  --drifting    Drifting mine.

• To print command-line application version information:

$ Rscript path/to/naval_fate.R --version
Naval Fate 2.0

• docopt function returns a list of command-line parameters and their corresponding values that can be accessed via $ within your command-line application.

$ Rscript path/to/naval_fate.R ship Guardian move 10 50 --speed=20
List of 23
 $ --help    : logi FALSE
 $ --version : logi FALSE
 $ --speed   : chr "20"
 $ --moored  : logi FALSE
 $ --drifting: logi FALSE
 $ ship      : logi TRUE
 $ new       : logi FALSE
 $ <name>    : chr "Guardian"
 $ move      : logi TRUE
 $ <x>       : chr "10"
 $ <y>       : chr "50"
 $ shoot     : logi FALSE
 $ mine      : logi FALSE
 $ set       : logi FALSE
 $ remove    : logi FALSE
 $ help      : logi FALSE
 $ version   : logi FALSE
 $ speed     : chr "20"
 $ moored    : logi FALSE
 $ drifting  : logi FALSE
 $ name      : chr "Guardian"
 $ x         : chr "10"
 $ y         : chr "50"

• In case if provided command-line parameters are inconsistent with the "Usage: ..." pattern the error message will be printed along with usage pattern examples.

$ Rscript path/to/naval_fate.R ship mine
Error: 
 usage: naval_fate.R ship new <name>...
  
 usage: naval_fate.R ship <name> move <x> <y> [--speed=<kn>]
  
 usage: naval_fate.R ship shoot <x> <y>
  
 usage: naval_fate.R mine (set|remove) <x> <y> [--moored | --drifting]
  
 usage: naval_fate.R (-h | --help)
  
 usage: naval_fate.R --version
Execution halted