====== diff ======
  * compare files in two directories, don't show common lines, output in 2 columns and ignore white space differences<code>
diff -y -b --suppress-common path1 path2
</code>
  * compare all files recursively, excluding any with a .pdf extension, and only print file names that differ<code>
diff -r -q --exclude="*.pdf" path1 path2
</code>
  * 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<code>
diff -r -q --exclude="*.pdf" path1 path2 | grep -v -E '^Common subdirectories|^Only in'
</code>

===== alternative output formats =====
  * unified<code>
diff --unified file1.txt file2.txt
</code>
  * context<code>
diff --context file1.txt file2.txt
</code>

===== create patch file =====
  * diff -Naur oldfile newfile > patchfile
  * then, to patch an original, copy the patchfile beside the file to be updated, and patch it like this<code>
patch -p0 < patchfile
</code>

===== count lines that are different =====
This outputs 2 columns, hiding lines that are the same, then pipes the result through the wc program with the flag to count newlines.
<code>
diff -y --suppress-common-lines file1.txt file2.txt | wc -l
</code>

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