diff

• compare files in two directories, don't show common lines, output in 2 columns and ignore white space differences

  diff -y -b --suppress-common path1 path2

• compare all files recursively, excluding any with a .pdf extension, and only print file names that differ

  diff -r -q --exclude="*.pdf" path1 path2

• compare all files recursively, excluding any with a .pdf extension, and only print file names that differ, but suppress common subdirectory and Only in lines

  diff -r -q --exclude="*.pdf" path1 path2 | grep -v -E '^Common subdirectories|^Only in'

• unified

  diff --unified file1.txt file2.txt

• context

  diff --context file1.txt file2.txt

• diff -Naur oldfile newfile > patchfile
• then, to patch an original, copy the patchfile beside the file to be updated, and patch it like this

  patch -p0 < patchfile

This outputs 2 columns, hiding lines that are the same, then pipes the result through the wc program with the flag to count newlines.

diff -y --suppress-common-lines file1.txt file2.txt | wc -l

The diff utility has a man page, but the more comprehensive documentation is available in info format (info diff).