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I have some files on a system volume that are already in a Time Machine backup, but would like to force Time Machine to back them up again. Is this possible?

I know I can do this by modifying the file(s) slightly, but I'd rather avoid doing that.

The system volume in question is an HFS+ volume, so my understanding is that Time Machine uses fsevents to track changes; is there a way to force an fsevent for a file that hasn't actually changed, such that Time Machine will back it up again?

I realise this is a strange question, but my reason for wanting to do this is that I've used HFS compression (via afsctool) to compress a large number of files, and I'd like the compressed version to be copied into Time Machine to eventually save space there as well.

So, is it possible to force Time Machine to fully backup a file that is already in the backup target?

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    Compression changes the file, so doesn't TM pick it up anyway?
    – nohillside ♦
    Commented Jun 24, 2021 at 10:06
  • You're not going to save space, as TM will hold the older uncompressed file AND the newer compressed version.
    – benwiggy
    Commented Jun 24, 2021 at 14:14
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    @benwiggy That's only true in the short term; eventually the uncompressed versions will be pruned away.
    – Haravikk
    Commented Jun 25, 2021 at 9:04
  • @nohillside strangely it doesn't; presumably compressing triggers some kind of fsevent but Time Machine seems to either ignore it, or maybe uses a checksum to decide that the file didn't actually change?
    – Haravikk
    Commented Jun 26, 2021 at 10:13
  • Isn't HFS compression happening on filesystem level, and invisible/transparent to any application accessing such files? This would explain why a new backup wasn't triggered, it also would mean that the touched file backuped by TM will not be compressed.
    – nohillside ♦
    Commented Jun 26, 2021 at 11:08

1 Answer 1

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If you go into Terminal you can use the command Touch to change the modification date of the file. That would them make Time Machine back it up as a "new revision of a file".

$> touch file.txt

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  • Weird, I could have sworn I'd already tried this but it appears to be the correct answer! It's strange that Time Machine picks up a modified time change, but not a fundamental change to the file (as compression moves all of the file's data into the resource fork, iirc). Oh well, this should let me force the backup, thanks!
    – Haravikk
    Commented Jun 26, 2021 at 10:15

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