linux - Creating a directory with a concatenated name Ubuntu -

i need create directory name concatenation between static string "test_" , today's date. instance: test_2015-04-28

i have tried command not work

mkdir test_'date +%f' 

any appreciated.

mkdir test_$(date +%y%m%d) 

this create directory test_20150428 . can make this:

mkdir test_$(date +%y-%m-%d) 

which create directory test_2015-04-28.

hope helps !!!