Why isn't the R function sink() writing a summary output to my results file?

Edit: This problem doesn't seem to be reproducible at this point, but I've updated this question to a more concise example that illustrates what the behavior was, in case anyone encounters a similar issue.

sink("res4.txt")
    cat("Here are my results:\n")
    summary(mtcars)
sink()

The sink("~/R/res4.txt") function in the last line will store the "Here are my results" line, but not the summary(res4) line in the .txt file.

Typing summary(mtcars) produces the correct data set, and I don't understand why the output of summary(mtcars) isn't included.

Solution

There are as many summary functions as there are regression procedures and many of them use cat with would not get into a value returned. My suggestion is to use cat and capture.output both of which have a file destination parameter and an append option:

cat("Here are my results:\n", file="~/R/res4.txt")
capture.output( summary(res4), file"~/R/res4.txt", append=TRUE)