[FORGED] file.exists() on device files
On 11/01/17 23:12, Benjamin Tyner wrote:
Hi, On my linux machine (Ubuntu, and also tested on RHEL), I am curious to know what might be causing file.exists (and also normalizePath) to not see the final device file here:
> list.files("/dev/fd", full.names = TRUE)
[1] "/dev/fd/0" "/dev/fd/1" "/dev/fd/2" "/dev/fd/3"
> file.exists(list.files("/dev/fd", full.names = TRUE))
[1] TRUE TRUE TRUE FALSE
> normalizePath(list.files("/dev/fd", full.names = TRUE))
[1] "/dev/pts/2" "/dev/pts/2" "/dev/pts/2" "/dev/fd/3"
Warning message:
In normalizePath(list.files("/dev/fd", full.names = TRUE)) :
path[4]="/dev/fd/3": No such file or directory
(a) Exactly the same thing happens to me (I am also running Ubuntu 16.04). (b) Things get a bit confused because /dev/fd is actually a symbolic link:
$ ls -l /dev/fd lrwxrwxrwx 1 root root 13 Jan 6 20:16 /dev/fd -> /proc/self/fd/
(c) But then doing
file.exists(list.files("/proc/self/fd", full.names = TRUE))
Gives the same result as before:
[1] TRUE TRUE TRUE FALSE
(d) It turns out that the four "files" in /proc/self/fd are again symbolic links:
$ ls -l /proc/self/fd total 0 lrwx------ 1 rolf rolf 64 Jan 12 12:32 0 -> /dev/pts/3 lrwx------ 1 rolf rolf 64 Jan 12 12:32 1 -> /dev/pts/3 lrwx------ 1 rolf rolf 64 Jan 12 12:32 2 -> /dev/pts/3 lr-x------ 1 rolf rolf 64 Jan 12 12:32 3 -> /proc/7150/fd/
(e) But now do it again!!!
$ ls -l /proc/self/fd total 0 lrwx------ 1 rolf rolf 64 Jan 12 12:32 0 -> /dev/pts/3 lrwx------ 1 rolf rolf 64 Jan 12 12:32 1 -> /dev/pts/3 lrwx------ 1 rolf rolf 64 Jan 12 12:32 2 -> /dev/pts/3 lr-x------ 1 rolf rolf 64 Jan 12 12:32 3 -> /proc/7154/fd/
Different number; 7154 rather than 7150. (f) The name "/proc" would seem to imply that this has something to do with processes; the directories "7150", "7154" etc. are being created and removed on the fly, as a result of some process (presumably the "ls" process) starting and finishing. I have no insight into what is being effected here, or what is really going on "deep down", but the foregoing is some sort of "explanation". By the time file.exists() is invoked, the ls process called by list.files() has finished and the associated directory (e.g. "7150", "7154", ...) has ceased to be. What you do with this "explanation" is up to you. My advice would be to forget about it and go to the pub! :-) cheers, Rolf Turner
Technical Editor ANZJS Department of Statistics University of Auckland Phone: +64-9-373-7599 ext. 88276