How to show merged differences between two files?

Question

How can I get only diff letters between two files?

For example,

file1:

 aaa;bbb;ccc 123;456;789 a1a;b1b;c1c 

file2:

 aAa;bbb;ccc 123;406;789 a1a;b1b;c5c 

After diff I should get only this string of difference from the second file: A05

Answer 1

diff -y --suppress-common-lines <(fold -w 1 file1) <(fold -w 1 file2) |
sed 's/.*\(.\)$/\1/' | paste -s -d '' -

This uses process substitution with fold to turn each file into a column of characters that's one character wide and then compares them with diff .

The -y option prints lines next to each other, and --suppress-common-lines skips lines that are the same between both files. Until here, the output looks like this:

$ diff -y --suppress-common-lines <(fold -w 1 file1) <(fold -w 1 file2)
a                                 | A
5                                 | 0
1                                 | 5

We're only interested in the the last character of each line. We use sed to discard the rest:

$ diff -y --suppress-common-lines <(fold -w 1 file1) <(fold -w 1 file2) |
> sed 's/.*\(.\)$/\1/'
A
0
5

To get these into a single line, we pipe to paste with the -s option (serial) and the empty string as the delimiter ( -d '' ). The dash tells paste to read from standard in.

$ diff -y --suppress-common-lines <(fold -w 1 file1) <(fold -w 1 file2) |
> sed 's/.*\(.\)$/\1/' | paste -s -d '' -
A05

---

An alternative, if you have the GNU diffutils at your disposal, is cmp :

$ cmp -lb file1 file2 | awk '{print $5}' | tr -d '\n'
A05

cmp compares files byte by byte. The -l option ("verbose") makes it print all the differences, not just the first one; the -b options make it add the ASCII interpretation of the differing bytes:

$ cmp -lb file1 file2
 2 141 a    101 A
18  65 5     60 0
34  61 1     65 5

The awk command reduces this output to the fifth column, and tr removes the newlines.

Answer 2

For the example given, you could compare the files character by character and if there is a difference, print the character of the second file. Here's one way to do that:

paste <(fold -w1 file1) <(fold -w1 file2) | \
while read c1 c2; do [[ $c1 = $c2 ]] || printf $c2; done

For the given example, this will print A05 .