Running Get Current Date action displays date as 20181019 to take backup of crons while on server it displays crontab_0181019_bkp.
I'm not sure, but $ mark is not needed in the command line.
Did you get unexpected result when filename is "crontab_<releaseDate>_bkp"?
I tried your command without RA, and then the result was same as yours.
When $ mark is removed, it seems date is shown correctly.
I'd like to ask you to try your command locally before implement on RA.
Yes i tried without $ and it worked.