Using existing envars in Renviron on friendly Windows

Dear colleagues,

I would be grateful if somebody could explain and perhaps help work
around the following.

I have .Renviron with, say:

AVAR=${APPDATA}/foo/bar

Which is a documented way of referring to existing environment
variables. Now, with that in R I'm getting:

I think in your example, AVAR would be set using

AVAR=C:\Users\mbojanowski\AppData\Roaming/foo/bar

When I export that value in a bash shell (on a Mac, not Windows), I get 
the same thing as you saw:

$ printenv AVAR
C:UsersmbojanowskiAppDataRoaming/foo/bar

Here R was not involved at all, this is the shell eating the backslashes.

So I suppose R is following the same rules as bash (or maybe getting 
bash or sh to handle .Renviron).  Those rules are that the single 
backslashes are treated as escapes, and so they are dropped and the 
following character is preserved: 
https://www.gnu.org/software/bash/manual/html_node/Escape-Character.html .

I think you don't have a lot of choice here:  if you don't have control 
over how an environment variable is being set, then don't try to use it 
in an expansion in .Renviron.  If you do have control, then avoid using 
backslashes.

So this would be fine:

APPDATA=C:/Users/mbojanowski/AppData/Roaming
AVAR=${APPDATA}/foo/bar

but your APPDATA setting needs to be handled in some other way, e.g. in 
.Rprofile instead of .Renviron.

Duncan Murdoch

Sys.getenv("APPDATA")    # That works OK
[1] "C:\\Users\\mbojanowski\\AppData\\Roaming"

so OK, but:

Sys.getenv("AVAR")
[1] "C:UsersmbojanowskiAppDataRoaming/foo/bar"

So all the (back)slashes are gone from APPDATA.

Does processing ${} removes the backslashes? I could not find anything
on that in R Windows FAQ nor on the web.
Thanks in advance!

Michal

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