[R] [FORGED] file.exists() on device files

[Rolf Turner]

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

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Technical Editor ANZJS
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University of Auckland
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