Add up a column of numbers at the Unix shell

Question

Given a list of files in files.txt, I can get a list of their sizes like this:

cat files.txt | xargs ls -l | cut -c 23-30

which pr

Answer 1

... | paste -sd+ - | bc

is the shortest one I've found (from the UNIX Command Line blog).

Edit: added the - argument for portability, thanks @Dogbert and @Owen.