Conex_unix_persistencySourceFile system operations using Unix
exists is Sys.exists.
mkdir ~mode name creates a directory name, or errors.
file_type path is the file type of path or error.
read_file path is the contents of path or error.
write_file ~mode path data writes data under path or error.
write_replace ~mode path data writes data under path.tmp and renames path.tmp to path or error.