What is archive mode in rsync?

It's all of these:

-r, --recursive recurse into directories

-l, --links copy symlinks as symlinks

-p, --perms preserve permissions

-t, --times preserve modification times

-g, --group preserve group

-o, --owner preserve owner (super-user only)

-D same as --devices --specials

--devices preserve device files (super-user only)

--specials preserve special files

It excludes:

-H, --hard-links preserve hard links

-A, --acls preserve ACLs (implies -p)

-X, --xattrs preserve extended attributes

It's perfect for backups. My "default" set of switches is -avzP - archive mode, be verbose, use compression, preserve partial files, display progress.

Note: Invariably when the descriptions say "preserve", it means make the destination be like the source.

It make sure the permission, attributes, etc. is same at the both side. rsync only works in that way... if you want to synchronising files, you have to use -a parameter.

This is because rsync compare these permission, attribute, etc. at both side to determining if they are the SAME FILE and decide if it should be updated. if you use rsync -r xxx yyy instead rsync -a xxx yyy, some files will be copy again and again, coz, permission, attribute not be synchronised, and miss matched at both side...

The archive switch is just a shortcut to skip having to enter a bunch of switches that you'll normally use if you're using rsync to make backups (a common task).

It's the same as typing:

rsync -r -l -p -t -g -o -D

And it's much easier to remember. You can find definitions for all of those switches in the man page.

(The hyphens are optional when you're combining multiple single-letter switches.)