File-test operators that check information returned by stat.
Operator Description
-b Is filename a mountable disk (block device)?
-c Is filename an I/O device (character device)?
-s Is filename a non-empty file?
-t Does filename represent a terminal?
-A How long since filename accessed?
-C How long since filename's inode accessed?
-M How long since filename modified?
-S Is filename a socket?