touch(1posix) - change file access and modification times
-a     Change the access time of file. Do not change the modification time unless -m is also specified.
-c     Do not create a specified file if it  does  not  exist.  Do  not  write  any  diagnostic  messages
       concerning this condition.
-m     Change the modification time of file. Do not change the access time unless -a is also specified.
-r  ref_file
       Use the corresponding time of the file named by the pathname ref_file instead of the current time.
-t  time
       Use  the specified time instead of the current time. The option-argument shall be a decimal number
       of the form: