How do I log an R session to a file?

Do you know about sink() in base R ?

There are also some logging packages on CRAN: logging, log4r and possibly more.

Lastly, Emacs user have ESS and its transcript mode. You can save your session as a log, and in general, the 'work from file and execute from the file' approach builds a (partial, commands-only) log as you work.

There's savehistory(file) which will write the entire history as plaintext, or, if you're trying to log output, use sink(file, split = TRUE).

For the sake of completing the answer using sink

# copy the log to a text file
sink("./logofcode.txt")
Your R code(s) goes here
you can use a stored R code as well using source()
source("./XS_SPEC_CF.R",echo=T, max.deparse.length=1e3)
sink()

In addition to the sink function you might also look at the txtStart and related functions in the TeachingDemos package or the similar functionality in the R2HTML package (which came first).