We have a long standing Mercurial 'alias' (it's not an alias in the Git sense) called 'hg sdiff' that provides diffs in non-context form, because for system administrator usage we're often changing things where the context of standard (context) diffs isn't useful and we want the terseness of standard diffs. For a long time we've had a little irritation, where if you changed only one file in a Mercurial repository 'hg sdiff' wouldn't show you the file name, but if you changed several files, 'hg sdiff' would. Today I dug into what was going on and it is more peculiar than I expected.

Mercurial has no native 'hg diff' option to do non-context diffs, so we actually do this through the standard Extdiff extension, which allows you to use external programs to provide diffs. We configure our 'sdiff' custom extdiff command to run 'diff -Nr', which on our Ubuntu machines uses GNU diffutils. However, GNU Diff has no command line option to print filenames. Nor are the file names printed by the code of the Extdiff extension itself when it runs your external diff command.

The clue to what is going on is in the '-r' argument to diff, or alternately this sentence in the Extdiff documentation:

The external diff programs are called with a configurable set of options and two non-option arguments: paths to directories containing snapshots of files to compare, or paths to the files themselves if only one file is modified.

If we run 'hg sdiff' and only one file has been changed in the repository, Extdiff will invoke 'diff -Nr file1.old file1' (to simplify the arguments a bit), and diff itself won't print any filenames. However, if we run 'hg sdiff' and we've changed two or more files, Extdiff will create two directories and invoke 'diff -Nr directory1 directory2', and then since it's running in recursive mode, diff will print the filenames along with the changes in the files.

(I believe things may be slightly more complex if you run Extdiff to compare two revisions, instead of comparing a revision to the working tree, but even then our 'hg sdiff' doesn't print the file name for single-file changes.)

As far as I can tell there's no Extdiff option to always create directories and do recursive diffs, which would do what we want here. Extdiff does have the '--per-file' option to do the reverse and always pass your external program two files. One could write a cover script for Extdiff usage that detects the two-files case and prints filenames appropriately, but we're going to just continue to live with the situation.

(We're not interested in switching to zero-context unified or context diffs, both of which would print the filenames but be more verbose and potentially confusing.)