Hmmm.. I'm really not able to wrap my head around the current diff. (Your idea is sound. But the implementation is sketchy.)

• I am not able to instantly grok the purpose of using PID-specific tmp dir. Is it avoid deleting a pre-existing non-empty tmp dir?
• I do not see the point of "$@" hook to jekyll build incantation. --blank is only valid for jekyll new. jekyll build doesn't recognise --blank.

The best outcome would probably be a Cucumber test..

I agree with this thought.
In fact, there's already a scenario testing jekyll build --blank (features/create_sites.feature:6). However, It just checks the presence of specific source files instead of running jekyll build and validating those source files.
IMHO, it'd be best if you can scrap the current proposal and enhance the above Cucumber test with additional 3steps (to cd into scaffolded blank site, run jekyll build and assert a zero exit status).

I am not able to instantly grok the purpose of using PID-specific tmp dir. Is it avoid deleting a pre-existing non-empty tmp dir?

Yeah, I was thinking that we'd invoke this twice and it'd be nice to not blow away the first one, but I can revert if you prefer! Let me know.

I was thinking that we'd invoke this twice and it'd be nice to not blow away the first one..

If we are indeed continuing down the "Bash script" route (as against the "Cucumber test" route), I feel we can address your primary concern (about duplicated code) using just "Bash functions" to respect DRY and the secondary concern (about keeping directories separate and untouched) by exposing $@ to jekyll new.

setup_tmp_dir() { ... }

...

setup_tmp_dir()
[[ $@ ]] && args="$@" || args="default-site"
bundle exec jekyll new "$args"
...

script/default-site

script/default-site --blank

Update: I wasn't thinking straight. The above script isn't gonna work. Besides, dirname as PID hampers readability of both build log and file system tree itself.
I will employ DRY in my pull request itself.

Updated to remove the PID-specific dir.

In general I’m trying to simplify, so adding another file with bash functions doesn’t feel like it simplifies.

Thanks @parkr
@jekyllbot: merge +dev