[Bug-tar] gnu tar incremental backup saves unchanged directories

[Todd Brunhoff]

While writing yet another backup script using tar --listed-incremental, 
I found that there was no option to prevent archiving directories that 
had no changes. I see this as a problem because when doing an 
incremental backup of a system with 10's of thousands of directories, 
yet where only a few files have changed, all of the directories are 
archived. This makes for a huge incremental backup which is excessive 
and makes it difficult to identify what actually changed.

I did try some related options such as --newer-mtime=date and 
--atime-preserve={system,replace,}, but without success: all directories 
were always archived.

Eventually I settled on an external solution which really ought to be in 
tar. And that is to use --files-from=- and feed it with something like 
find <path> -newermt <date>.

I should note a related irritation. If the find found no files, tar 
would archive everything in the current directory. I understand that tar 
takes targets from the command line and, optionally from the 
--files-from argument, but there is no way to say to take files only 
from the --files-from argument.

Thanks,

Todd