With my scripted backup, I squash directories into tarballs. It includes the date as part of the filename.tar.bz2. For April, cleanup will involve removing March; "rm *-201003*.tar.bz2"
Now the part I haven't automated is the actual file removal. My backup script does not yet remove the outdated copies.
Alternatively, I know some of the rsync based backup options use links. This is to save on space; if a file hasn't changed, simply include a link from the current backup to that older file
backup 1 = file1.ext
backup 2 = link-file1.ext, file2.ext
backup 3 = link-file1.ext, file3,ext
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