[bug-diffutils] bug#31577: Using -S / --starting-file causes diff to miss some files

[Maxim Baz]

Hello,

The manual describes -S option as "start with FILE when comparing directories". 
My intuition is that this should behave like -O option in git-diff [1]: show 
differences in FILE first, then show differences in all other files. However, 
it appears that diff behaves in a different (and quite weird) way: it first 
composes an ordered list of files to compare, then cuts off the beginning of 
this list until FILE, and only then starts comparing the remaining files. This 
is extremely unintuitive and I consider this a bug (and if happens to be "by 
design", then I propose a new feature request for -O option 🙂)


Specific example:

Create file structure as below, all files in the directory "one" should have 
word "one" as the contents, all files in the directory "two" should word "two" 
as the contents (so all files are different).

❯ tree -a
.
├── one
│   ├── dir
│   │   └── file
│   ├── .hidden-dir
│   │   └── file
│   ├── .hidden-file
│   └── zfile
└── two
    ├── dir
    │   └── file
    ├── .hidden-dir
    │   └── file
    ├── .hidden-file
    └── zfile

6 directories, 8 files

❯ diff -ur one two | grep '+++' | wc -l
4

❯ diff -ur -S zfile one two | grep '+++' | wc -l
1

❯ diff -ur -S zfile one two                     
diff --color --unified -ur -S zfile one/zfile two/zfile
--- one/zfile   2018-05-24 01:23:49.237899164 +0200
+++ two/zfile   2018-05-24 01:24:58.260920662 +0200
@@ -1 +1 @@
-one
+two


I expect both diff calls to output differences in 4 files, only the second call 
to print differences in the file "zfile" first.


[1]: https://git-scm.com/docs/git-diff#git-diff--Oltorderfilegt



Regards,
Maxim Baz