Rapp

Rapp (short for "R application") makes it fun to write and share command line applications in R.

It is an alternative front end to R, a drop-in replacement for Rscript that automatically parses command line arguments into R values. The goal is to make it easy to build a polished CLI application from a simple R script.

It aims to provide a seamless transition from interactive repl-driven development at the R console to non-interactive execution at the command line.

Here is an example Rapp, a simple R script named flip-coin.R:

#!/usr/bin/env Rapp
#| description: Flip a coin.

#| description: Number of coin flips
n <- 1L

cat(sample(c("heads", "tails"), n, TRUE), fill = TRUE)

You can invoke it from the command line:

$ flip-coin
tails

$ flip-coin --n 3
tails heads tails

$ flip-coin --help
Usage: flip-coin [OPTIONS]

Flip a coin.

Options:
  --n <FLIPS>    Number of coin flips
                 [default: 1] [type: integer]

$ flip-coin --help-yaml
name: flip-coin
description: Flip a coin.
options:
  'n':
    default: 1
    val_type: integer
    arg_type: option
    action: replace
    description: Number of coin flips

Defining the CLI

Rapp recognizes a small set of expression patterns at the top level of your script and converts them into command-line options, flags, positional arguments, and commands. The sections below cover the supported patterns.

Help

All Rapps comes with built-in flags for help.

• --help shows usage, description, and options for the app (and for subcommands when used after a command, e.g., todo list --help).
• --help-yaml prints machine-readable metadata for the app as YAML.

Options

Simple assignments of scalar (length-1) literals at the top level of the R script are automatically treated as command line options.

n <- 1

becomes an option at the command line:

flip-coin --n 1

Non-string option values passed from the command line are parsed as YAML/JSON, and then coerced to the original R type. Values can be supplied after the option flag, or as part of the option flag string with =. The following two usages are the same:

flip-coin --n=1
flip-coin --n 1

Bool options, (that is, assignments of TRUE or FALSE in an R app) are a little different. They support usage as switches or toggles at the command line. For example in an R script:

echo <- TRUE

means that at the command line the following are supported:

my-app --echo       # TRUE
my-app --echo=yes   # TRUE
my-app --echo=true  # TRUE
my-app --echo=1     # TRUE

my-app --no-echo     # FALSE
my-app --echo=no     # FALSE
my-app --echo=false  # FALSE
my-app --echo=0      # FALSE

Assigning c() or list() declares an option that can be supplied multiple times. Use c() when you want to keep the exact strings provided on the command line, and list() when you want Rapp to attempt to parse the supplied strings as YAML/JSON values and convert them into R objects. For example, a repeatable filter option that keeps raw strings:

#| description: File name patterns to include (repeatable).
pattern <- c()

can be invoked as:

list-files --pattern '*.csv' --pattern 'sales-*'

Or, to collect numeric limits and have them parsed into integers:

#| description: Score thresholds (parsed as numbers, repeatable).
threshold <- list()

which lets callers supply structured values:

report --threshold 5 --threshold '[10, 20, 30]'

Positional arguments

Assigning NULL to a symbol declares a positional argument. If the symbol has a ... suffix or prefix, it becomes a collector for a variable number of positional arguments. Positional arguments always come into the R app as character strings, and they are required by default unless you mark them as required: false via annotations.

For example, this small greet app declares a required <NAME> positional argument and prints it:

#!/usr/bin/env Rapp
#| name: greet
#| description: Greet someone.

#| description: Name to greet.
name <- NULL

cat("Hello ", name, "!\n", sep = "")

Running it shows how positional arguments appear in --help:

$ greet --help
Greet someone.

Usage: greet <NAME>

Arguments:
  <NAME>  Name to greet.

To make the positional argument optional, add an annotation above the assignment:

#| required: false
name <- NULL

This changes the usage to Usage: greet [<NAME>] (with brackets).

Commands

Use a switch() statement whose first argument is either a character scalar or an assignment (for example switch("") or switch(command <- "", ...)) to declare commands. The corresponding branch runs when the matching command is supplied on the command line. Declare command specific options and positional arguments with the same rules inside the branch.

#!/usr/bin/env Rapp
#| name: todo
#| title: Todo manager
#| description: Manage a simple todo list.

#| description: Path to the todo list file.
#| short: s
store <- ".todo.yml"

switch(
  "",

  #| title: Display the todos
  #| description: Print the contents of the todo list.
  list = {
    limit <- 30L
    ...
  },

  #| title: Add a new todo
  add = {
    task <- NULL
    ...
  },

  #| title: Mark a task as completed
  done = {
    index <- 1L
    ...
  }
)

The command shown above exposes a todo app with list, add, and done commands. Each command can declare its own options (limit, index) or positional arguments (task), and command metadata can be documented with the same hash-pipe annotations used for options.

Command-line help reflects the available commands, and each command has its own help page:

$ todo --help
Todo manager

Usage: todo [OPTIONS] <COMMAND>

Manage a simple todo list.

Commands:
  list  Display the todos
  add   Add a new todo
  done  Mark a task as completed

$ todo list --help
Display the todos

Usage: todo list [OPTIONS]

Print the contents of the todo list.

Options:
  --limit <LIMIT>  Maximum number of entries to display (-1 for all).
                   [default: 30] [type: integer]

Global options:
  -s, --store <STORE>  Path to the todo list file.
                       [default: ".todo.yml"] [type: string]

Commands can be nested by including additional switch() blocks inside a command branch; each level adds its own command-specific options, help, and positional arguments.

Help output automatically includes any parent and global options for nested commands.

Annotations

You can add YAML hash-pipe annotations in the script front matter or right above individual options. YAML annotations are primarily used to adjust help output. The entries you'll most commonly use are title and description. Another YAML annotation you can provide is short, a short option name. For example:

#!/usr/bin/env Rapp
#| description: Flip a coin.

#| description: Number of coin flips
#| short: n
n_flips <- 1L

cat(sample(c("heads", "tails"), n_flips, TRUE))

then lets you supply the alias -n or the full option name --n-flips at the command line (note also the automatic mapping of snake case n_flips to kebab-case --n-flips).

$ flip-coin --help
Usage: flip-coin [OPTIONS]

Flip a coin.

Options:
  -n, --n-flips <N-FLIPS>  Number of coin flips
                           [default: 1] [type: integer]

$ flip-coin -n 3
tails heads heads

Other YAML fields you can supply to change the behavior of Rapp

• val_type: expected value type (string, integer, float, bool; any).
• arg_type: how the input appears on the CLI (option, switch, positional).
• action: whether values replace or accumulate (replace vs append for repeatable options and collectors).

Summary

Here is a summary table of different R expressions that Rapp treats as command line arguments.

R Expression	CLI surface
Assignment of scalar literal foo <- ""	Option APP --foo value
Assignment of NULL foo <- NULL	Positional Arg APP foo-value
Assignment of TRUE or FALSE foo <- TRUE	Boolean switch APP --foo or APP --no-foo
Assignment of c() or list() foo <- c()	Repeatable option APP --foo val1 --foo val2
Assignment of NULL to name with ... args... <- NULL	Positional Arg Collector APP foo bar baz
Switch with string literal switch("", cmd1 = {}, cmd2 = {})	Commands APP cmd1 --help APP cmd2 --help

Running interactively

While developing, you can drive the app directly from R:

Rapp::run("path/to/app.R", c("--help"))
Rapp::run("path/to/app.R", c("--myopt", "my-opt-val"))

Pass a character vector of arguments exactly as you would supply them on the command line. Inside the app you can drop browser() statements to pause execution and inspect state while Rapp::run() executes.

Installation

# Install the package
install.packages("Rapp")

# Add `Rapp` to your PATH
Rapp::install_pkg_cli_apps("Rapp")

Alternatively, install the development version:

# pak::pak("r-lib/Rapp")
# remotes::install_github("r-lib/Rapp")

On macOS and Linux, make your Rapp script executable (chmod +x flip-coin.R) and run them directly. On Windows, or if you prefer, call the front end explicitly with Rapp flip-coin.R --n 3.

Installing launchers

Rapp::install_pkg_cli_apps("Rapp") installs Rapp on the PATH.

If you are shipping Rapps via an R package, you can call Rapp::install_pkg_cli_apps("mypackage") to install lightweight launchers for every Rapp (and Rscript shebang) in the package's exec/ directory.

Rapp::install_pkg_cli_apps("mypackage")

You can either include the command in install instructions, or export your own thin wrapper:

#' Install `mypackage` cli applications.
#'
#' @inheritDotParams Rapp::install_pkg_cli_apps -package -lib.loc
#' @export
install_mypackage_cli <- function(...) {
  Rapp::install_pkg_cli_apps(package = "mypackage", lib.loc = NULL, ...)
}

App launchers are written to destdir, which defaults to the first available location from RAPP_INSTALL_DIR, XDG_BIN_HOME, XDG_DATA_HOME/../bin, or the default location, ~/.local/bin on macOS and Linux and %LOCALAPPDATA%\Programs\R\Rapp\bin on Windows. On Windows the directory is automatically added to PATH; on macOS and Linux the directory generally is already present on PATH (you may need to restart your shell if the Rapp installer created the directory). Use the destdir argument if you prefer an alternate location. If you are working with a standalone .R file on Windows, call the launcher explicitly (Rapp path\to\flip-coin.R --n 3) because native shebangs are not supported.

Using package exec/ directories directly

Launchers are optional. You can add Rapp and a package's exec/ directory to the PATH and run the apps directly from the package's installed directory. For example, after installing {Rapp}, you can place something like this in a shell startup script like .bashrc:

export PATH="$(Rscript -e 'cat(normalizePath(system.file("exec", package = "Rapp")))'):$PATH"
export PATH="$(Rscript -e 'cat(normalizePath(system.file("exec", package = "mypackage")))'):$PATH"

On Windows, run Rscript -e "cat(normalizePath(system.file('exec', package = 'Rapp')))" to print the directory and add it to PATH via System Properties → Environment Variables.

Shipping an Rapp as part of an R package

You can easily share your R app command line executable as part of an R package.

• Add {Rapp} as a dependency in your DESCRIPTION.

• Place your app in the exec folder in your package (for example exec/myapp). Apps are automatically installed as executables.

• Encourage users to run Rapp::install_pkg_cli_apps("mypackage") or your own exported wrapper mypackage::install_mypackage_cli() after installing your package so the launchers land in a directory on their PATH. install_pkg_cli_apps() keeps existing launchers up to date and also deletes old launchers for apps that have been removed from your package.

• If rig is already on the PATH, you can also use rig to run a script in a package's exec directory (Stay tuned, improved rig support is in flight):

  rig run <pkg>::<script>

Windows

Rapp works on Windows. Running install_pkg_cli_apps() creates .bat wrappers for each app and installs a top-level Rapp.bat, adding their location to PATH. After that, you can invoke apps from R packages just like on other platforms:

flip-coin --n 3

Because Windows does not natively support shebangs, to invoke an Rapp developed outside an R package, you'll need to invoke the Rapp front-end directly (after calling Rapp::install_pkg_cli_apps("Rapp")):

Rapp path/to/flip-coin.R --n 3

More examples

See the inst/examples folder for more example R apps.

Other Approaches

This package is just one set of ideas for how to build command line apps in R. Some other packages in this space:

• littler (typically paired with one of the below)
• docopt
• optparse
• argparse
• argparser

Also, some interesting examples of other approaches to exporting cli interfaces from R packages:

• renv
• bspm