Sum the numbers on standard in

Bash + coreutils, 16 bytes

xargs|tr \  +|bc

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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.

05AB1E, 2 bytes

|O

Explanation:

|   Get input as array
 O  Sum

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MATL, 2 bytes

Us

This expects the input in a text file called defin.

Gif or it didn't happen:

Or try it online! (thanks to Dennis for the set-up!)

Explanation

When a MATL program is run, if a file called defin is found (the name refers to "default input"), its contents are automatically loaded as text and pushed to the stack as a string before executing the code.

Function U evaluates the string to convert it to a column vector of numbers, and s computes the sum, which is implicitly displayed.