How to read input from the terminal using /dev/std

问题:

I'm using:

R version 3.0.0 (2013-04-03) -- "Masked Marvel"
Platform: x86_64-pc-linux-gnu (64-bit)

I try to use read.csv to input a little CSV data snippet + header, directly from the terminal.

I'm encountering a problem that may be related to R skips lines from /dev/stdin and read.csv, header on first line, skip second line but is different enough (the answers there don't explain what I see here) to warrant a separate question.

R seems to skip the header line and treat the second (data) line as header:

R> d <- read.csv(file='/dev/stdin', header=TRUE) 
a,b
1,2
3,4
# hit CTRL-D twice here to end the input
# (this is also unexpected:
#  when reading a few lines interactively in bash, one CTRL-D suffices.
#  Why is doing it twice necessary in R?)

R> d
  X1 X2
1  3  4

R> colnames(d)
[1] "X1" "X2"

I found a workaround: since by default read.csv has blank.lines.skip = TRUE, I prefix the input with some blank lines. 5 empty lines before starting the input, seem to be the minimum required to get this to work as expected. BTW: a single line with 5 spaces works just as well, hinting at some 5 byte (or more) required whitespace padding:

R> d <- read.csv(file='/dev/stdin', header=TRUE)





a,b
1,2
3,4
# Enter CTRL-D twice here to mark the end of terminal input

R> d
  a b
1 1 2
2 3 4

R> colnames(d)
[1] "a" "b"

Questions:

• Why isn't the 1st example working as expected?
• Why are 5 blank lines or spaces needed (even 4 aren't enough) to make it work?
• Is there a better way to reading a short csv snippet directly from the terminal? (I know about scan and readLines, but my data is in csv format already, so I want to make it as simple to read/parse/assign as possible)

回答1:

I think the answer in the first link you posted may actually be applicable. R appears to create a 4 byte buffer on /dev/stdin. Also, as mentioned in the comment, you can use stdin instead, and it appears to work fine. (Although I still don't get why you have to hit Ctrl+D twice).

d <- read.csv(file='stdin', header=TRUE)
a,b
1,2
3,4
# Hit Control+D twice.
> d
  a b
1 1 2
2 3 4