Removing trailing / starting newlines with sed, awk, tr, and friends

Question

I would like to remove all of the empty lines from a file, but only when they are at the end/start of a file (that is, if there are no non-empty lines before them, at the start;

Answer 1

As mentioned in another answer, tac is part of coreutils, and reverses a file. Combining the idea of doing it twice with the fact that command substitution will strip trailing new lines, we get

echo "$(echo "$(tac "$filename")" | tac)"

which doesn't depend on sed. You can use echo -n to strip the remaining trailing newline off.