Shell command to sum integers, one per line?

Bit of awk should do it?

awk '{s+=$1} END {print s}' mydatafile

Note: some versions of awk have some odd behaviours if you are going to be adding anything exceeding 2^31 (2147483647). See comments for more background. One suggestion is to use printf rather than print:

awk '{s+=$1} END {printf "%.0f", s}' mydatafile