slatel

#! /usr/bin/awk -f

# slatel - strip leading and trailing empty lines
# Copyright (C) 2019-2021  Erik Auerswald <auerswal@unix-ag.uni-kl.de>
#
# This program is free software: you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation, either version 3 of the License, or
# (at your option) any later version.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
# GNU General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with this program.  If not, see <http://www.gnu.org/licenses/>.

# This program is similar, but different, to Python's strip() method on
# strings in that empty lines, but not all whitespace, are removed from
# both the beginning and end of the input text file. Leading and trailing
# whitespace in both the first and last non-empty lines are preserved by
# this script.
#
# This script uses the following algorithm:
#
# 1) Do not output (or buffer) anything before reading a non-empty line.
# 2) Delay printing of empty lines until the next non-empty line is found.
#
# The script does not use the [:space:] character class for compatibility
# with ancient mawk versions as found in contemporary Debian and Ubuntu
# GNU/Linux distributions, where mawk 1.3.3 from 1996 is the default Awk.
# This is written in 2019. m(
 /^[ \t\r\f\v]*$/{if (found_non_empty) {empty_lines=empty_lines $0 "\n"}}
!/^[ \t\r\f\v]*$/{found_non_empty=1; print empty_lines $0; empty_lines=""}