fread to read top n rows from a large file

Another workaround is to fetch the first 500 lines with shell command:

rdata<- fread(
    cmd = paste('head -n 500', csvfile),
    sep= "|", header=FALSE, col.names= colsinfile,
    select= colstoselect, key = "keycolname", na.strings= c("", "NA")
)

I don't known why nrows doesn't work, though.