I think it would be useful to display an error if the generation process fails.

In most cases, we could remove the >dev/null 2>/dev/null to get status and error messages. They're currently fairly verbose. One advantage is that this will translate properly and can more easily give users detailed diagnostic information (like "no permission to write file"). A disadvantage is that errors like "arc not in path" won't have much context.

Been poking around at the code. The following function looks promising: https://secure.phabricator.com/diffusion/P/browse/master/src/applications/differential/parser/DifferentialHunkParser.php;a366f85c118bf6b857155792446c397dac03e0a3$229

Haha, noobs calling everything "Phabricator".

Also, Python's built-in difflib also does the right thing (at least for this small example).

How do you decide how to apply light-green vs dark-green highlighting? Is that something diff gives you?

This came up with a custom Arcanist linter we have (I didn't write it, but I'm now trying to fix the exception). The lintPath() function looks like this: