R check if `stdin()` is empty

Question

I believe this question has been asked for almost all languages except for R.

Essentially I'd like to check if stdin() is empty but I do not know how to do it.

This is because if you perform the following bit of code your session will hang (probably until something is sent to the relevant R process).

input <- file("stdin", "r")
result <- readLines(input, 1) # Session hangs here

Ideally, I could check the status of input before I try to read anything from it.

~~WARNING~~

If you run the above script then you will likely need to kill the running process wherever it lives (either interactively in some IDE or in an Rscript).

EDIT

I found a helpful SO post that shed some light on the situation. In summary you can use CTRL + D to signal end of file condition to the input (so far working for readLines(input, -1).

My ultimate goal is to write an Rscript that could accept input from a stdin connection via a pipe. However as I have discovered, the R interpreter will wait until it receives an end of file condition before returning. In the event nothing is actually piped into stdin, is there a way to programatically call this signal?

I can't reproduce the issue running R from a shell. Or maybe I don't understand the issue? After calling `readLines`, I can type into `stdin`. Hitting Return closes the connection and `readLines` returns the typed characters as a string. What behaviour are you expecting? — Mikael Jagan, Mar 03 '22 at 05:51
If your R process hangs when running R from a GUI, then maybe you should be doing `readLines(stdin(), 1)`, _not_ `readLines(file("stdin"), 1)`. The semantics are slightly different, as noted in `?stdin`. — Mikael Jagan, Mar 03 '22 at 06:04

score 1 · Accepted Answer · answered Mar 03 '22 at 15:36

1

You can use blocking = FALSE. The following script will check input from stdin every half second and terminate when a full line is received.

#!/usr/bin/Rscript
inp <- file("stdin", "r", blocking = FALSE)
i <- 0
while (TRUE) {
    line <- readLines(inp, n = 1)
    if (length(line) > 0) {
        cat(sprintf("read a line: %s\n", line))
        break;
    }
    Sys.sleep(0.5)
    cat(sprintf("%d - nothing\n", i))
    i <- i + 1
}

answered Mar 03 '22 at 15:36

Stefano Barbi

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I think this is probably the best solution for right now. My only modifications is that I do a pushback in the first `if` statement before `break`. This way I can just use `readLines(input, -1L)` to get everything later. And also after the `Sys.sleep` call I put a timeout condition on `i`, so that it wont wait forever. – Justin Landis Mar 03 '22 at 16:15

R check if `stdin()` is empty

1 Answers1