Sum the numbers on standard in
Bash + coreutils, 16 bytes

xargs|tr \  +|bc


Try it online!

There are two spaces after the \. This works for negative numbers as well.

Explanation:

xargs             # known trick to turn newlines into spaces, while adding a
                  #trailing newline when printing the result (needed for bc)
|tr \  +          # turn spaces into '+'s
|bc               # calculates the sum


You may wonder why tr \\n +|bc isn't better, since it turns newlines directly into '+'s. Well, that has 2 unforeseen errors:


if the input has a trailing newline, then it is converted to a trailing '+', hence there is no number after it to perform the addition
and the most weird issue is that bc requires a trailing newline after the input, but you just replaced all of the input newlines with '+'s.