Those codes give you a line number and a type of difference which has been found:
a stands for added
8c8,9 means that line number 8 in the first file (
--from-file) has changed to something and this something is stored in the lines 8-9 in the second file. Similarly
2d1 means that line 2 from the first file has been deleted, expected after line 1 in the second file, etc.
I don't think you really want to get rid of those codes, they are quite useful, but if you insist on removing them (or changing to something else), then just play with the following six output formats (here I assume you are using GNU diff):
I will not cover all details of the (rather complex) syntax here, you will find their description in the
info diff under If-then-else chapter. The bellow will give you exactly what you are asking, i.e. no codes at all, just pure
diff --old-line-format=$'< %l\n' --new-line-format=$'> %l\n' \
--old-group-format=$'%<' --new-group-format=$'%>' \
--changed-group-format=$'%>' --unchanged-group-format='' file1 file2
You can make alias of that if you want.