argparse Command Line Argument Parsing

In argparse: Command Line Optional and Positional Argument Parser

argparse is a command line argument parser inspired by Python's "argparse" library. Use this with Rscript to write "#!"-shebang scripts that accept short and long flags/options and positional arguments, generate a usage statement, and set default values for options that are not specified on the command line.

library("knitr")
rscript_executable <- paste(file.path(R.home(), "bin", "Rscript"), "--vanilla")
opts_knit$set(root.dir = system.file("exec", package = "argparse")) # to access the "Rscript files"
opts_chunk$set(comment = NA, echo = FALSE)
list_file_command <- "ls"
chmod_command <- "chmod ug+x display_file.R example.R"
path_command <- "export PATH=$PATH:`pwd`"
run_command <- function(string) suppressWarnings(cat(system(string, intern = TRUE), sep = "\n"))

In our working directory we have two example R scripts, named "example.R" and "display_file.R" illustrating the use of the argparse package.

r paste("bash$", list_file_command)

run_command(sprintf("%s", list_file_command))
command <- "display_file.R example.R" # to show file

In order for a *nix system to recognize a "#!"-shebang line you need to mark the file executable with the chmod command, it also helps to add the directory containing your Rscripts to your path:

r paste("bash$", chmod_command)

r paste("bash$", command)

Here is what "example.R" contains:

r paste("bash$", command)

run_command(sprintf("%s %s 2>&1", rscript_executable, command))
command <- "example.R --help" # same as system("Rscript example.R -h")

By default argparse will generate a help message if it encounters --help or -h on the command line. Note how %(default)s in the example program was replaced by the actual default values in the help statement that argparse generated.

r paste("bash$", command)

run_command(sprintf("%s %s 2>&1", rscript_executable, command))
command <- "example.R" # rely only on defaults

If you specify default values when creating your ArgumentParser then argparse will use them as expected.

r paste("bash$", command)

run_command(sprintf("%s %s 2>&1", rscript_executable, command))
command <- "example.R --mean=10 --sd=10 --count=3"

Or you can specify your own values.

r paste("bash$", command)

run_command(sprintf("%s %s 2>&1", rscript_executable, command))
command <- "example.R --quiet -c 4 --generator=\"runif\"" #  same as above but "quiet"

If you remember from the example program that --quiet had action="store_false" and dest="verbose". This means that --quiet is a switch that turns the verbose option from its default value of TRUE to FALSE. Note how the verbose and quiet options store their value in the exact same variable.

r paste("bash$", command)

run_command(sprintf("%s %s 2>&1", rscript_executable, command))
command <- "example.R --silent -m 5" #  same as above but "quiet"

If you specify an illegal flag then \emph{argparse} will print out a usage message and an error message and quit.

r paste("bash$", command)

run_command(sprintf("%s %s 2>&1", rscript_executable, command))
command <- "example.R -c 100 -c 2 -c 1000 -c 7" #  same as above but "quiet"

If you specify the same option multiple times then \emph{argparse} will use the value of the last option specified.

r paste("bash$", command)

run_command(sprintf("%s %s 2>&1", rscript_executable, command))
command <- "display_file.R --help"

argparse can also parse positional arguments. Below we give an example program display_file.R, which is a program that prints out the contents of a single file (the required positional argument, not an optional argument) and which accepts the normal help option as well as an option to add line numbers to the output.

r paste("bash$", command)

run_command(sprintf("%s %s 2>&1", rscript_executable, command))
command <- "display_file.R --add_numbers display_file.R"

r paste("bash$", command)

run_command(sprintf("%s %s 2>&1", rscript_executable, command))
command <- "display_file.R non_existent_file.txt"

r paste("bash$", command)

run_command(sprintf("%s %s 2>&1", rscript_executable, command))
command <- "display_file.R"

r paste("bash$", command)

run_command(sprintf("%s %s 2>&1", rscript_executable, command))

argparse documentation built on Feb. 16, 2023, 8:23 p.m.