How can I get the current date in YYYY-MM-DD format in (OS X) bash?

Try

 cp myfile.ext myfile.`date "+%Y-%m-%d"`.ext

and for a proper solution, decompose the name first into basename and extension, assign the date and then reassemble for final targetname.

The key is that

 date +FORMAT

allows for very rich format strings. See date --help or the rather fine manual.

cp myfile.ext myfile.`date +%Y-%m-%d`.ext

or use shorthand format

date +%F

Related