flippar.sh : reverse the order of paragraph blocks where such blocks are delimited by two or more newlines.

flippar.sh

#!/usr/bin/env bash
#              bash 4.1.5(1)     Linux Ubuntu 10.04           Date : 2011-12-10
#
# _______________|  flippar : reverse order of paragraph blocks
#                             (delimited by two or more newlines;
#                              last block is exempt from this rule).
#
#           Usage:  flippar  [filename (pipe acceptable)]
#
#    Dependencies:  tac (from package 'coreutils' in main)


#  CHANGE LOG  get LATEST version from https://bitbucket.org/rsvp/gists/src
#
#  2011-12-10  More comments to clarify.
#  2011-12-09  Revise solution by peth given in 
#                 http://superuser.com/questions/276103/
#                 Reverse order of multiline text sections separated by newlines.

#           _____ Prelims
set -u
#   ^ unbound (i.e. unassigned) variables shall be errors.
#           Example of default assignment:    arg1=${1:-'foo'}
set -e
#   ^ error checking :: Highly Recommended (caveat: you can't check $? later).  
#
# _______________     ::  BEGIN  Script ::::::::::::::::::::::::::::::::::::::::

arg1=${1:-'-'}
#         ^default is stdin for pipe usage.


#                         tac is cat in reverse...
( cat "$arg1" ; echo ) |  tac --separator=$'\n\n' |  sed '$d'
#                             ^PARAGRAPH block DELIMITED by TWO or MORE NEWLINES 
#                              (using --regex flag produces unintended result).
#                              Specifying that separator is actually very tricky.
#
#              ^echo insures that the last block satisfies delimiter requirement
#               (since most often it may not :-)
#
#        To truly maintain an inverse operation, i.e. for "flippar foo | flippar" 
#        to return exactly foo, we need sed to remove the last line.


exit 0
# _______________ EOS ::  END of Script ::::::::::::::::::::::::::::::::::::::::


#  #     2011-12-10  UNTESTED Python one-liner by BigChief, ibid.
#  open('output.txt', 'w').write('\n\n'.join(reversed(open('input.txt', 'r').read().split('\n\n'))))
#  #     Change \n to \r\n for Windows-formatted text files.
#  #     If it works, one could easily create a cross-platform script.


#  vim: set fileencoding=utf-8 ff=unix tw=78 ai syn=sh :