monitor changes to a file in R (similar to tail "follow") - r

Question

I'm wondering if there's a way to monitor the contents of a file from within R, similar to the behavior of tail -f (details here) in the Linux terminal.
Specifically, I want a function that you could pass a file path and it would
print the last n lines of the file to the console
hold the console
continue printing any new lines, as they are added
There are outstanding questions like "what if previously printed lines in the file get modified?" and honestly I'm not sure how tail -f handles that, but I'm interested in streaming a log file to the console, so it's kind of beside the point for my current usage.
I was looking around in the ?readLines and ?file docs and I feel like I'm getting close, but I can't quite figure it out. Plus, I can't imagine I'm the first one to want to do this, so maybe there's an established best practice (or even an existing function). Any help is greatly appreciated.
Thanks!

Answer 1

I made progress on this using the processx package. I created an R script which I named fswatch.R:
library(processx)
monitor <- function(fpath = "test.csv", wait_monitor = 1000 * 60 * 2){
system(paste0("touch ", fpath))
print_last <- function(fpath){
con <- file(fpath, "r", blocking = FALSE)
lines <- readLines(con)
print(lines[length(lines)])
close(con)
}
if(file.exists(fpath)){
print_last(fpath)
}
p <- process$new("fswatch", fpath, stdin = "|", stdout = "|", stderr = "|")
while(
# TRUE
p$is_alive() &
file.exists(fpath)
){
p$poll_io(wait_monitor)
p$read_output()
print_last(fpath)
# call poll_io twice otherwise endless loop :shrug:
p$poll_io(wait_monitor)
p$read_output()
}
p$kill()
}
monitor()
Then I ran the script as a "job" in RStudio.
Every time I wrote to test.csv the job printed the last line. I stopped monitoring by deleting the log file:
log_path <- "test.csv"
write.table('1', log_path, sep = ",", col.names = FALSE,
append = TRUE, row.names = FALSE)
write.table("2", log_path, sep = ",", col.names = FALSE,
append = TRUE, row.names = FALSE)
unlink(log_path)