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

Paste typically merges lines of multiple files, but it can also be used to convert individual lines of a file into a single line. The delimiter flag allows you to pass a x+x type equation to bc.

paste -s -d+ infile | bc

Alternatively, when piping from stdin,

<commands> | paste -s -d+ - | bc

The one-liner version in Python:

$ python -c "import sys; print(sum(int(l) for l in sys.stdin))"