sum all numbers from "du"

In AWK:

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

So

du -b /tmp/* | awk '{ sum += $1 } END { print sum }'

Note that the result won’t be correct if the directories under /tmp have subdirectories themselves, because du produces running totals on directories and their children.

du -s will calculate the sum for you correctly (on all subdirectories and files in /tmp, including hidden ones):

du -sb /tmp

and du -c will calculate the sum of the listed directories and files, correctly too:

du -cb /tmp/*

You can also produce a total sum of selected files with du -c. This works even if an argument of du is not a directory, what is not the case of du -s:

$ du -sb file1 file2
17  file1
18  file2

$ du -cb file1 file2
17  file1
18  file2
35  total

BTW, for interactive use I recommend adding -h option instead of -b or any other multiplier of block-size. This will print the size in human readable unit format.

$ du -ch file1 file2
4.0K    file1
4.0K    file2
8.0K    total

It is simple you can use:

 du -b /tmp/* | awk 'BEGIN{i=0} {i=i+$1} END{print i}'

If you are not using wildcard, if you are using directory name like /tmp, then you need to avoid the last entry because output of du -b /tmp is like:

size1 file1
size2 file2
size_total .

So now you should avoid this last entry, so use:

du -b /tmp | awk 'BEGIN{i=0} {if( $2 != "." ){i=i+$1}} END{print i}'

However you can also use -s option, it will calculate the summary for you then you don't need to add the values, just print the last one, i.e.:

du -s directory