#!/bin/sh

#
# This give the diff between two runs of commands.
#
# SYNOPSIS
#
#     diffthis [COMMAND]
#     diffthat [COMMAND]
#
# DESCRIPTION
#
#     This give the diff between two runs of commands. First run a command and
#     cache the output in tmp. Later the user may request the diff between the
#     cached output and the output of a new command. This basically does little
#     more than what you might do manually:
#
#         ls -latr /dev >/tmp/temp_data_for_diff
#         ls -latr /dev | diff -u /tmp/temp_data_for_diff -
#
#    Note that the '''diffthat''' is not a separate script. It should just be a
#    symlink to '''diffthis'''.
#        ln -s diffthis diffthat
#
#    Stderr output is not currently handled; although, the script has
#    preliminary code handling stderr to support this in the future. By default
#    stderr is mixed into stdout and becomes part of the diff. If you with to
#    ignore stderr then pass the '-E' option and stderr will be sent to
#    /dev/null. If you wish to preserve stderr, but diff it separately from
#    stdout then pass the '-e' option and stderr will be sent to a separate log
#    file and diffed separately.
#
#    Beware of shell aliases and functions. You may have 'ls' aliased to 'ls
#    ${LS_OPTIONS}' which will cause it to run differently than the default.
#    This tool does not resolve aliases, functions, that may shadow default
#    behavior of a command.
#
#
# EXAMPLES
#
#	diffthis ls -latr /dev diffthat ls -latr /dev
#
#	diffthis ls /proc diffthat ls /proc
#
# EXIT STATUS
#
#     This exits with status 0 on success or non-zero on error.
#
# AUTHOR
#
#     Noah Spurrier noah@noah.org
#
# LICENSE
#
#     This license is OSI and FSF approved as GPL-compatible. This license
#     identical to the ISC License and is registered with and approved by the
#     Open Source Initiative. For more information vist:
#     http://opensource.org/licenses/isc-license.txt
#
#     Copyright (c)2015 Noah Spurrier noah@noah.org
#
#     Permission to use, copy, modify, and/or distribute this software for any
#     purpose with or without fee is hereby granted, provided that the above
#     copyright notice and this permission notice appear in all copies.
#
#     THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
#     WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
#     MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
#     ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
#     WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
#     ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
#     OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
#
# VERSION
#
#     Version 1
#
######################################################################

function get_realpath() {
        if [ -d "$1" ]; then
                cd "$1"
        else
                if [ "${1}" != "${1%/*}" ]; then
                        cd "${1%/*}"
                fi
        fi
        realpath="$(pwd -P)"
        cd - >/dev/null
        if [ -d "$1" ]; then
                echo "${realpath}"
        else
                echo "${realpath}/${1##*/}"
        fi
        return 0
}

# This sets variables which record where the script was started from and
# where the script itself is installed.
PWD="$(pwd)"
START_DIRECTORY="${PWD}"
if [ -x "$0" ]; then
        COMMAND="$(get_realpath "$0")"
else
        COMMAND="$(get_realpath "${PWD}/$0")"
fi
INSTALLDIR="$(dirname "${COMMAND}")"

FILENAME_DIFFTHIS="/tmp/diffthis-$(id -u)$(tty|tr / -)"
if [ "$1" = "-e" ]; then
	FILENAME_DIFFTHIS_STDERR="/tmp/diffthis-stderr-$(id -u)$(tty|tr / -)"
	shift
elif [ "$1" = "-E" ]; then
	FILENAME_DIFFTHIS_STDERR="/dev/null"
	shift
fi
echo $@

case "$0" in
	*that*)
		echo "# Behave as diffthat (compete the diff)."
		FILENAME_DIFFTHIS="/tmp/diffthis-$(id -u)$(tty|tr / -)"
		$@ | diff -us "${FILENAME_DIFFTHIS}" -
		exit 0
		;;
esac

if [ -n "${FILENAME_DIFFTHIS_STDERR}" ]; then
	$@ >"${FILENAME_DIFFTHIS}" 2>"${FILENAME_DIFFTHIS_STDERR}"
else
	$@ >"${FILENAME_DIFFTHIS}" 2>&1
fi
if [ -n "${FILENAME_DIFFTHIS_STDERR}" ]; then
	echo "Output saved to ${FILENAME_DIFFTHIS} and ${FILENAME_DIFFTHIS_STDERR}."
	echo "Run 'diffthat' to complete the diff."
else
	echo "Output saved to ${FILENAME_DIFFTHIS}."
	echo "Run 'diffthat' to complete the diff."
fi