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

Here's an awk version that removes trailing blank lines (both empty lines and lines consisting of nothing but white space).

It is memory efficient; it does not read the entire file into memory.

awk '/^[[:space:]]*$/ {b=b $0 "\n"; next;} {printf "%s",b; b=""; print;}'

The b variable buffers up the blank lines; they get printed when a non-blank line is encountered. When EOF is encountered, they don't get printed. That's how it works.

If using gnu awk, [[:space:]] can be replaced with \s. (See full list of gawk-specific Regexp Operators.)

If you want to remove only those trailing lines that are empty, see @AndyMortimer's answer.