How to get a user input in command prompt and pass it to R

Question

I am trying to use

var <- as.numeric(readline(prompt="Enter a number: "))

and later use this in a calculation.

It works fine when running in RStudio but I need to be able to pass this input from the command line in Windows 10 I am using a batch file with a single line

Rscript.exe "C:\My Files\R_scripts\my_script.R"

When it gets to the user input part it freezes and it doesn't provide expected output.

Does this answer your question? [How can I read command line parameters from an R script?](https://stackoverflow.com/questions/2151212/how-can-i-read-command-line-parameters-from-an-r-script) — Peace Wang, Dec 29 '21 at 15:43
`readline` only waits for user input if run on its own. You are running a full script so it will run until the end of the script and assign `NA` to `var`. — Rui Barradas, Dec 29 '21 at 15:44
I imagine because executing via Rscript doesn't give you an interactive environment - as automated scripts will never proceed. See the the help on `?readline` example. — , Dec 29 '21 at 15:46

jpiversen · Accepted Answer · 2021-12-30T15:40:20.140

From the documentation of readline():

This can only be used in an interactive session. [...] In non-interactive use the result is as if the response was RETURN and the value is "".

For non-interactive use - when calling R from the command line - I think you've got two options:

Use readLines(con = "stdin", n = 1) to read user input from the terminal.
Use commandArgs(trailingOnly = TRUE) to supply the input as an argument from the command line when calling the script instead.

Under is more information.

1. Using `readLines()`

readLines() looks very similar to readline() which you're using, but is meant to read files line by line. If we instead of a file points it to the standard input (con = "stdin") it will read user input from the terminal. We set n = 1 so that it stops reading from the command line when you press Enter (that is, it only read one line).

Example

Use readLines() in a R-script:

# some-r-file.R

# This is our prompt, since readLines doesn't provide one
cat("Please write something: ")
args <- readLines(con = "stdin", n = 1)

writeLines(args[[1]], "output.txt")

Call the script:

Rscript.exe "some-r-file.R"

It will now ask you for your input. Here is a screen capture from PowerShell, where I supplied "Any text!".

Then the output.txt will contain:

Any text!

2. Using`commandArgs()`

When calling an Rscript.exe from the terminal, you can add extra arguments. With commandArgs() you can capture these arguments and use them in your code.

Example:

Use commandArgs() in a R-script:

# some-r-file.R
args <- commandArgs(trailingOnly = TRUE)

writeLines(args[[1]], "output.txt")