cmdfun

A simple framework for building shell interfaces

The purpose of cmdfun is to significantly reduce the overhead involved in wrapping shell programs in R. The tools are intended to be intuitive and lightweight enough to use for data scientists trying to get things done quickly, but robust and full-fledged enough for developers to extend them to more advanced use cases.

Installation

Install the development version of cmdfun with:

if (!requireNamespace("remotes"))
  install.packages("remotes")

remotes::install_github("snystrom/cmdfun")

Quick Overview

The cmdfun framework provides mechanisms for capturing function arguments:

cmd_args_dots() captures all arguments passed to ...
cmd_args_named() captures all keyword arguments defined by the user
cmd_args_all() captures both named + dot arguments

library(cmdfun)

myFunction <- function(input, ...){
  cmd_args_all()
}

(argsList <- myFunction(input = "test", boolean_flag = TRUE))

## $input
## [1] "test"
## 
## $boolean_flag
## [1] TRUE

cmd_list_interp converts the captured argument list to a parsed list of flag/value pairs.

(flagsList <- cmd_list_interp(argsList))

## $input
## [1] "test"
## 
## $boolean_flag
## [1] ""

cmd_list_to_flags converts a list to a vector of commandline-style flags using the list names as flag names and the list values as the flag values (empty values return only the flag). This output can be directly fed to system2 or processx.

cmd_list_to_flags(flagsList)

## [1] "-input"        "test"          "-boolean_flag"

cmd_path_search() allows package builders to search default locations for installed tools.

bin_path <- cmd_path_search(default_path = "/bin", utils = c("ls", "cut"))

bin_path(util = "ls")

## [1] "//bin/ls"

Introduction

cmdfun attempts to solve the problem of wrapping external software in R. Calling external software is done with system2 or processx.

For example, calling ls -l *.md using system2.

system2("ls", "-l *.md", stdout = TRUE)

## [1] "-rw-r--r-- 1 snystrom its_employee_psx 1077 Aug 20 19:20 LICENSE.md"      
## [2] "-rw-r--r-- 1 snystrom its_employee_psx  837 Aug 26 21:51 NEWS.md"         
## [3] "-rw-r--r-- 1 snystrom its_employee_psx 7718 Aug 26 20:14 README.md"       
## [4] "-rw-r--r-- 1 snystrom its_employee_psx  744 Aug 26 21:35 cran-comments.md"

However, when using multiple commandline flags each flag should be passed as a member of a character vector as follows:

When calling ls -l -i

system2("ls", c("-l", "-i", "*.md"), stdout = TRUE)

## [1] "1163031755 -rw-r--r-- 1 snystrom its_employee_psx 1077 Aug 20 19:20 LICENSE.md"      
## [2] "1163031757 -rw-r--r-- 1 snystrom its_employee_psx  837 Aug 26 21:51 NEWS.md"         
## [3] "1163031758 -rw-r--r-- 1 snystrom its_employee_psx 7718 Aug 26 20:14 README.md"       
## [4] "1163031762 -rw-r--r-- 1 snystrom its_employee_psx  744 Aug 26 21:35 cran-comments.md"

This becomes even more difficult if trying to support user input, as a significant amount of overhead is required to parse user inputs and optional flags into these vectors.

cmdfun provides utilities for converting function arguments into lists which can easily convert to character vectors suitable for use with system2 or processx.

library(cmdfun)

myFunction <- function(input, option1){
  # Grabs named arguments as key/value pairs
  cmd_args_named()
}

(argsList <- myFunction("myInput.txt", "foo"))

## $input
## [1] "myInput.txt"
## 
## $option1
## [1] "foo"

# Converts list to character vector of flags & values
cmd_list_to_flags(argsList)

## [1] "-input"      "myInput.txt" "-option1"    "foo"

Wrapping `ls` with cmdfun

These tools can be used to easily wrap ls

library(magrittr)

shell_ls <- function(dir = ".", ...){
  # grab arguments passed to "..." in a list
  flags <- cmd_args_dots() %>% 
    # prepare list for conversion to vector
    cmd_list_interp() %>% 
    # Convert the list to a flag vector
    cmd_list_to_flags()
  
  # Run ls shell command
  system2("ls", c(flags, dir), stdout = TRUE)
}

shell_ls("*.md")

## [1] "LICENSE.md"       "NEWS.md"          "README.md"        "cran-comments.md"

Boolean flags are passed as bool operators

ls -l can be mimicked by passing l = TRUE to ‘…’.

shell_ls("*.md", l = TRUE)

## [1] "-rw-r--r-- 1 snystrom its_employee_psx 1077 Aug 20 19:20 LICENSE.md"      
## [2] "-rw-r--r-- 1 snystrom its_employee_psx  837 Aug 26 21:51 NEWS.md"         
## [3] "-rw-r--r-- 1 snystrom its_employee_psx 7718 Aug 26 20:14 README.md"       
## [4] "-rw-r--r-- 1 snystrom its_employee_psx  744 Aug 26 21:35 cran-comments.md"

Named vectors can be used to provide user-friendly aliases for single-letter flags

Commandline tools can have hundreds of arguments, many with uninformative, often single-letter, names. To prevent developers from having to write aliased function arguments for all, often conflicting flags, cmd_list_interp can additionally use a lookup table to allow developers to provide informative function argument names for unintuitive flags.

For example, allowing long to act as -l in ls.

shell_ls_alias <- function(dir = ".", ...){
  
  # Named vector acts as lookup table
  # name = function argument
  # value = flag name
  names_arg_to_flag <- c("long" = "l")
  
  flags <- cmd_args_dots() %>% 
    # Use lookup table to manage renames
    cmd_list_interp(names_arg_to_flag) %>% 
    cmd_list_to_flags()
  
  system2("ls", c(flags, dir), stdout = TRUE)
}

shell_ls_alias("*.md", long = TRUE)

## [1] "-rw-r--r-- 1 snystrom its_employee_psx 1077 Aug 20 19:20 LICENSE.md"      
## [2] "-rw-r--r-- 1 snystrom its_employee_psx  837 Aug 26 21:51 NEWS.md"         
## [3] "-rw-r--r-- 1 snystrom its_employee_psx 7718 Aug 26 20:14 README.md"       
## [4] "-rw-r--r-- 1 snystrom its_employee_psx  744 Aug 26 21:35 cran-comments.md"

Wrapping `cut` with cmdfun

Here is another example wrapping cut which separates text on a delimiter (set with -d) and returns selected fields (set with -f) from the separation.

shell_cut <- function(text, ...){

  names_arg_to_flag <- c("sep" = "d",
                         "fields" = "f")
    
    flags <- cmd_args_dots() %>%
        cmd_list_interp(names_arg_to_flag) %>% 
      cmd_list_to_flags()

    system2("cut", flags, stdout = T, input = text)
}

shell_cut("hello_world", fields = 2, sep = "_")

## [1] "world"

Multiple values are passed as vectors

shell_cut("hello_world_hello", fields = c(1,3), sep = "_")

## [1] "hello_hello"

Additionally, cmdfun provides utilites for searching & checking valid tool installs, expecting system behavior, and helpful error handling to allow simple construction of external tool wrappers (see vignette for details).