find /backup -type f -mtime -14 -print0 | xargs -0 du -csh | tail -n1

$ find -type f -exec du -s {} \; | awk 'BEGIN{ s=0; }; {s+=$1}; END {print s}'

awk '{$sum+=$1} END{print $sum}'

# find /backup -type f -mtime -14 -print0 | xargs -0 du -csh | tail -n1
71G	total

$ find /backup -type f -mtime -14 | xargs du -csh | tail -n 1

 find /backup -type f -mtime -14 -printf "%s\n" | awk '{a+=$1} END {print a}' 

find . -type f -printf '%s\n' | perl -MMath::BigInt -e '$s=Math::BigInt->new();map{$s->badd($_)}grep(/\d/,split(/\D+/s,<STDIN>));print qq/$s\n/;'

$ (find . -type f -printf '%s + ' ; echo 0) | bc -l
10117567898

find . -type f -printf '%s\n' | perl6 -e 'say [+] $*IN.lines;'