a character string specifying the test to be performed.
Unary tests (only x is used) are "-f" (existence and
not being a directory), "-d" (existence and directory) and
"-x" (executable as a file or searchable as a directory).
Binary tests are "-nt" (strictly newer than, using the modification
dates) and "-ot" (strictly older than): in both cases the
test is false unless both files exist.
character vectors giving file paths.
‘Existence’ here means being on the file system and accessible
by the stat system call (or a 64-bit extension) -- on a
Unix-alike this requires execute permission on all of the directories in
the path that leads to the file, but no permissions on the file