[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.
FYI, the /proc is there because Unix has something called the "proc
filesystem (procfs; https://en.wikipedia.org/wiki/Procfs) is a special
filesystem in Unix-like operating systems that presents information
about processes and other system information in a hierarchical
file-like structure".  For instance, you can query the uptime of the
machine by reading from /proc/uptime:

$ cat /proc/uptime
332826.96 661438.10

$ cat /proc/uptime
332871.40 661568.50

You can get all IDs (PIDs) of all processes currently running:

$ ls /proc/ | grep -E '^[0-9]+$'

and for each process you there are multiple attributes mapped as
files, e.g. if I start R as:

$ R --args -e "message('hello there')"

then I can query that process as:

$ pid=$(pidof R)
$ echo $pid
26323

$ cat /proc/26323/cmdline
/usr/lib/R/bin/exec/R--args-emessage('hello there')

Unix is neat

/Henrik
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

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[FORGED] file.exists() on device files

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