-a, --archive
This is equivalent to -rlptgoD. It is a quick way of saying you want recursion and want to
preserve almost everything (with -H being a notable omission). The only exception to the above
equivalence is when --files-from is specified, in which case -r is not implied.
Note that -a does not preserve hardlinks, because finding multiply-linked files is expensive. You
must separately specify -H.
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-u, --update
This forces rsync to skip any files which exist on the destination and have a modified time that
is newer than the source file. (If an existing destination file has a modification time equal to
the source file’s, it will be updated if the sizes are different.)
Note that this does not affect the copying of symlinks or other special files. Also, a difference
of file format between the sender and receiver is always considered to be important enough for an
update, no matter what date is on the objects. In other words, if the source has a directory
where the destination has a file, the transfer would occur regardless of the timestamps.
This option is a transfer rule, not an exclude, so it doesn’t affect the data that goes into the
file-lists, and thus it doesn’t affect deletions. It just limits the files that the receiver
requests to be transferred.
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-t, --times
This tells rsync to transfer modification times along with the files and update them on the remote
system. Note that if this option is not used, the optimization that excludes files that have not
been modified cannot be effective; in other words, a missing -t or -a will cause the next transfer
to behave as if it used -I, causing all files to be updated (though rsync’s delta-transfer
algorithm will make the update fairly efficient if the files haven’t actually changed, you’re much
better off using -t).
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