Using Sys.chmod to toggle world-readability in R

Question

How can I use the Sys.chmod function in R to toggle readability by "other", i.e. the equivalent of:

chmod o+r myfile
chmod o-r myfile

I specifically want to leave the other parts of the mode alone.

Answer 1

chmod o+r:

mod <- file.info('tmp.txt')$mode
Sys.chmod('tmp.txt', mod|4)

chmod o-r:

mod <- file.info('tmp.txt')$mode
Sys.chmod('tmp.txt', mod&!as.octmode(4))