transfers to default to  comparing  nanosec‐
              onds, you can create a ~/.popt file and put these lines in it:

                  rsync alias ‐a ‐a@‐1
                  rsync alias ‐t ‐t@‐1

              With that as the default, you’d need to specify --modify‐window=0
              (aka  -@0)  to override it and ignore nanoseconds, e.g. if you’re
              copying between ext3 and ext4, or if the receiving rsync is older
              than 3.1.3.

       --checksum, -c
              This changes the way rsync checks if the files have been  changed
              and are in need of a transfer.  Without this option, rsync uses a
              "quick  check"  that  (by default) checks if each file’s size and
              time of last modification match between the sender and  receiver.
              This  option  changes this to compare a 128‐bit checksum for each
              file that has a matching size.  Generating  the  checksums  means
              that  both  sides  will  expend a lot of disk I/O reading all the
              data in the files in the transfer, so this can slow  things  down
              significantly (and this is prior to any reading that will be done
              to transfer changed files)

              The  sending  side  generates its checksums while it is doing the
              file‐system scan that builds the list  of  the  available  files.
              The  receiver  generates  its  checksums  when it is scanning for
              changed files, and will checksum any file that has the same  size
              as  the  corresponding sender’s file: files with either a changed
              size or a changed checksum are selected for transfer.

              Note that rsync always verifies that each  transferred  file  was
              correctly  reconstructed  on  the  receiving  side  by checking a
              whole‐file checksum that is generated as the file is transferred,
              but that automatic after‐the‐transfer verification has nothing to
              do with this option’s before‐the‐transfer "Does this file need to
              be updated?" check.

              The checksum used is auto‐negotiated between the client  and  the
              server,  but can be overridden using either the --checksum‐choice
              (--cc) option or an environment variable  that  is  discussed  in
              that option’s section.

       --archive, -a
              This  is equivalent to -rlptgoD.  It is a quick way of saying you
              want recursion and want to preserve almost everything.  Be  aware
              that  it  does  not  include  preserving  ACLs (-A), xattrs (-X),
              atimes (-U), crtimes (-N), nor  the  finding  and  preserving  of
              hardlinks (-H).

              The  only exception to the above equivalence is when --files‐from
              is specified, in which case -r is not implied.

       --no‐OPTION
              You may turn off one or more implied options by prefixing the op‐
              tion name with "no‐".  Not all positive options  have  a  negated
              opposite,  but a lot do, including those that can be used to dis‐
              able an implied option (e.g.  --no‐D, --no‐perms) or have differ‐
              ent defaults  in  various  circumstances  (e.g.  --no‐whole‐file,
              --no‐blocking‐io, --no‐dirs).  Every valid negated option accepts
              both  the  short  and the long option name after the "no‐" prefix
              (e.g. --no‐R is the same as --no‐relative).

              As an example, if you want to use --archive (-a) but  don’t  want
              --owner  (-o),  instead  of  converting  -a into -rlptgD, you can
              specify -a --no‐o (aka --archive --no‐owner).

              The order of the options is important: if you specify  --no‐r -a,
              the  -r  option  would  end  up  being turned on, the opposite of
              -a --no‐r.  Note also