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; and if there are no non-empty lines after them, at the end.)

Is this possible outside of a fully-featured scripting language like Perl or Ruby? I’d prefer to do this with sed or awk if possible. Basically, any light-weight and widely available UNIX-y tool would be fine, especially one I can learn more about quickly (Perl, thus, not included.)

Answer 1

From Useful one-line scripts for sed:

# Delete all leading blank lines at top of file (only).
sed '/./,$!d' file

# Delete all trailing blank lines at end of file (only).
sed -e :a -e '/^\n*$/{$d;N;};/\n$/ba' file

Therefore, to remove both leading and trailing blank lines from a file, you can combine the above commands into:

sed -e :a -e '/./,$!d;/^\n*$/{$d;N;};/\n$/ba' file

Answer 2

So I'm going to borrow part of @dogbane's answer for this, since that sed line for removing the leading blank lines is so short...

tac is part of coreutils, and reverses a file. So do it twice:

tac file | sed -e '/./,$!d' | tac | sed -e '/./,$!d'

It's certainly not the most efficient, but unless you need efficiency, I find it more readable than everything else so far.