Thanks for the review, it's a big patch. 😅 I'm a bit busy at the moment, I will respond to the other comments later this week or sometime next week.

Rebase after changes in D82808.

Rebase after changes in D82808.

Rebase after changes in D82808.

Addressed review comment: replace shows -> show.

Please review D82791: "[lit] Improve lit's output with default settings and --verbose." before reviewing this revision.

Please review D82791: "[lit] Improve lit's output with default settings and --verbose." and D82811 "[lit] Highlight failing line with colored output." before reviewing this revision.

I've split this patch up into four different patches. Here is the suggested review order:

• [docs] [lit] Add a more helpful description for lit.py's -s flag.
• [NFC] [lit] Separate verbose and showOutput.
• [lit] Improve lit's output with default settings and --verbose.

Include missing commit separating verbose and showOutput.

On second thought, I'm going to update the documentation alongside the behavior change, to make sure the two are in sync.
Patch 1: Change behavior of -v. (and default and -vv behavior). Update https://llvm.org/docs/CommandGuide/lit.html as necessary.
Patch 2: Add highlighting/colored output. Update https://llvm.org/docs/CommandGuide/lit.html as necessary.
Patch 3: Rename echo_all_commands to runWithCommandOutput.

Quick question: We show the command output up to and including the failing line. Should we do the same for the script part, i.e., skip the un-executed part of the script? Is it ever useful to know which script line/command would have been executed next?

@JDevlieghere, that is part of the code that is writing out the command line.

Hmm, Phabricator attached my inline comment to my other larger comment, that's confusing.
I'm guessing I was supposed to make an out-of-line comment paired with the inline one,
that's what the "not submitted" was about.

Fixed some variable names, added some doc comments, cleaned up some logic, added some more CHECK lines to tests.

Ah, I just realized there are some inconsistent variable names, some are snake_case, some are camelCase. Should I be consistently using camelCase? I did see a bunch of snake_case variables, that's probably what tripped me up...

Consider the test file:

Closing. I have a PR up against apple/llvm-project. https://github.com/apple/llvm-project/pull/979

Addressed John's suggestion to replace OS X -> macOS.