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Many tools out there for syncing files aren't quite right for syncing you're home directory. Because of this, and to learn Ruby, I've been working on my own sync tool.

An issue I've encountered, which I think is in my sync tools, is what happens when the user renames a file or deletes a file. Just thinking about the first case (if the user renames it) is there a way that I can tell that this is not a new file but a file that has been renames? Maybe a unique ID of the file that I don't know about. File descriptors sounds like something to look into but I'm pretty sure that's a different concept.

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    You could keep track of the inode numbers. That's what you see when you use ls -i. – Kusalananda Jun 20 '16 at 21:46
  • Compare hashes? – phk Jun 20 '16 at 22:19
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There's no way to tell for sure whether a file has been renamed.

When a file is renamed, its inode number doesn't change. (This may not be true for “exotic” filesystems, such as network filesystems, but it's true for all “native” Unix filesystems.) However the converse is not true: if a file is deleted, a new file may be created with the same inode number. So even if you see that a file has the same inode number that another file had before, that doesn't necessarily imply that the file has been renamed.

The inode number uniquely identifies the file while it exists. That makes it suitable for detecting hard links, but not for detecting renamings.

File descriptors are associated with a specific process and are useless here.

You may consider a heuristic that if a file has the same size, modification time and inode number as before but a different name, then it means that it's been renamed. This heuristic is similar to what many synchronization tools do to detect unchanged files: same name, size and modification time.

All the tools I can think of that track file renaming do it based on the contents of the file. Unison detects renamed files if at least one of the endpoints is remote. Some modern version control systems such as git also detect renamed files.

I strongly suspect that whatever you want to do with file synchronization, between rsync, Unison and version control, it already exists.

  • If I selected it to sync my home directory are there options in Unison to not sync files in that directory, say large files like movies? And also does it allow you to say I don't want to sync the directories Dropbox & Downloads? – rangeme Jun 21 '16 at 19:06
  • Is rsync more of a backup program? Because I wanted what I said above plus automatic backup while doing this process. – rangeme Jun 21 '16 at 19:06
  • @wuttadowner rsync, unison, and really any file synchronization program allows excluding files by name patterns or whole directories. Rsync can certainly be used to make backups, but it's generally used by a higher-level program such as rsnapshot. – Gilles Jun 21 '16 at 19:36
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File descriptors are created for a program when it opens a file, and is only valid until the program closes it again. So just as you suspected: A different concept.

If the file was just renamed the inode(s) won't change, you can use that, but unless renames happen often I doubt it's worth it.

  • Why do you doubt it'd be worth it? – rangeme Jun 20 '16 at 22:37
  • If the file was just renamed, the inode won't change, but the converse is not true, which makes this observation not useful in practice. – Gilles Jun 20 '16 at 23:32

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