zsh on macOS date modify output of given date (without script)

Question

I'm aware of date +%u to get the day of the week for today.

I'd like to get that integer for any arbitrary date i input - if possible in the format I choose (eg %YYmmdd)

Answer 1

ok, found it finally:

date -j -f %Y%m%d +%u 20200910

this is, because date on macOS doesn't take a switch for putting in custom date (fyi for those folks, how try to make -v work, like me^^)

in addition, -f affects only input format (it's literally the second word in the man page, but I managed to overlook more than once)

-j is needed to use -f without setting the date.

hope this will spare someone time in the future ;)

edit: it seems to be important, to specify input format before output format (see comment from @chepner below)

(also be careful with quotes)

Answer 2

$ date +%u -d "2020-09-10"
4