compare files line-by-line, showing differences within each line by colour-highlighting (or visual symbols)

hairOfTheDiff.sh

#!/bin/bash

same='-' #unchanged
up='△' #exists in first line, but not in second 
down='▽' #exists in second line, but not in first
reset=''

reset=$'\e[0m'
same=$reset
up=$reset$'\e[1m\e[7m'
down=$reset$'\e[1m\e[7m\e[31m'

timeout=1


if [[ "$1" != '' ]]
then
	paste -d'\n' "$1" "$2" | "$0"
	exit
fi

function demo {
	"$0" <<EOF
Paris in the spring 
Paris in the the spring
A cat on a hot tin roof.
a cant on a hot in roof
the quikc brown box jupps ober the laze dogs 
The quickbrown fox jumps over the lazy dogs
EOF
}

# Change \x20 to \x02 to simplify parsing diff's output,
#+   then change \x02 back to \x20 for the final output. 
# Change \x09 to \x01 to simplify parsing diff's output, 
#+   then change \x01 into → U+1F143 (Squared Latin Capital Letter T)
function input {
	sed \
		-e "s/\x09/\x01/g" \
		-e "s/\x20/\x02/g" \
		-e "s/\(.\)/\1\n/g"
}
function output {
	sed -n \
		-e "s/\x01/→/g" \
		-e "s/\x02/ /g" \
		-e "s/^\(.\) *\x3C$/\1 \x3C  /g" \
		-e "s/\(.\) *\(.\) \(.\)$/\1\2\3/p"
}

ifs="$IFS"
IFS=$'\n'
demo=true

while IFS= read -t "$timeout" -r a
do
	demo=false
	IFS= read -t "$timeout" -r b
	if [[ $? -ne 0 ]]
	then
		echo 'No corresponding line to compare with' > /dev/stderr
		exit 1
	fi

	diff --text -yt -W 19  \
		<(echo "$a" | input) \
		<(echo "$b" | input) \
	| \
	output | \
	{
		type=''
		buf=''
		while read -r line
		do
			if [[ "${line:1:1}" != "$type" ]]
			then
				if [[ "$type" = '|' ]]
				then
					type='>'
					echo -n "$down$buf"
					buf=''
				fi

				if [[ "${line:1:1}" != "$type" ]]
				then
					type="${line:1:1}"

					echo -n "$type" \
						| sed \
							-e "s/[<|]/$up/" \
							-e "s/>/$down/" \
							-e "s/ /$same/"
				fi
			fi

			case "$type" in
			'|')
				buf="$buf${line:2:1}"
				echo -n "${line:0:1}"
				;;
			'>')
				echo -n "${line:2:1}"
				;;
			*)
				echo -n "${line:0:1}"
				;;
			esac
		done

		if [[ "$type" = '|' ]]
		then
			echo -n "$down$buf"
		fi
	}

	echo -e "$reset"
done

IFS="$ifs"

if $demo
then
	demo
fi

• It only prints every line once, using colour to show you the differences.
• It doesn't write any temp files, piping everything instead.
• You can provide two filenames and it'll compare the corresponding lines in each file. ./hairOfTheDiff.sh file1.txt file2.txt
• Otherwise, if you use the original format (a single file with every second line needing to be compared to the one before) you may now simply pipe it in, no file needs to exist to be read. Take a look at demo in the source; this may open the door to fancy piping in order to not need files for two separate inputs too, using paste and multiple file-descriptors.

No highlight means the character was in both lines, highlight means it was in the first, and red means it was in the second.

The colours are changeable through variables at the top of the script and you can even forego colours entirely by using normal characters to express differences.