diff showing only differences inside line

Question

I want to compare lines in two files, but to minimize noise in the output, I want only the actual differences in the lines to be printed.

For instance, given the two files below:

a.txt

a b c d e f g h i j k l m n o p q r s t u v w x y z

b.txt

a B c d e f g h i j k l m n o p q r s t u v w x y z

(the difference between them is the case of letter b)

I want the output to be something like:

[-b-]{+B+}

Currently, the best approach I found was to use git diff --word-diff, but it outputs the whole line:

a [-b-]{+B+} c d e f g h i j k l m n o p q r s t u v w x y z

Is there a more direct way to do it, other than manually parsing the output? Also, ideally I would prefer to use something more commonly available than git diff, e.g. a POSIX shell tool that would not require the user to install extra packages.

• Barmar: It would be nice if you used an example where the differences were more visible. I had to squint to see that those two characters are not the same.
• anol: Sorry, I added a note describing the difference between the lines.
• Barmar: Why not just use b and B so it's obvious? I understand that this was probably the actual difference, but for purposes of the question you can make it easier.
• anol: I wanted to avoid solutions that would only work on ASCII characters, but since the proposed solution does not depend on it, I changed it. However, I cannot update the answer to reflect the new changes since the edit would be smaller than 6 characters long.
• Ciro Santilli OurBigBook.com: related: unix.stackexchange.com/questions/11128/diff-within-a-line

Answer 1

Using wdiff:

$ wdiff -3 a.txt b.txt
======================================================================
 [-b-] {+B+}
======================================================================

The -3 or ---no-common option will remove words that are common between the two files and only show the differences.

The ===... banner (and empty lines) may be removed with grep:

$ wdiff -3 a.txt b.txt | grep -vx '=*'
 [-b-] {+B+}

wdiff may also read unified diff data if you give it the -d or --diff-input option, for example from git:

git diff somefile | wdiff -d -3

Although wdiff is not a POSIX tool, it is commonly available.

• scohe001: It might be worth noting that if your terminal supports ANSI escapes, you can make wdiff print fancy colored output that's (imo) easier to read with this in your bashrc: alias wdiff="wdiff -n -w $'033円[30;41m' -x $'033円[0m' -y $'033円[30;42m' -z $'033円[0m'" (taken from here).