Rsync Temporary File Extension – How to Set

To resume an interrupted copy, you should use rsync --append. From the man page's explanation of --append:

This causes rsync to update a file by appending data onto the end of the file, which presumes that the data that already exists on the receiving side is identical with the start of the file on the sending side. [...] Implies --inplace, [...]

Option --inplace makes rsync (over)write the destination file contents directly; without --inplace, rsync would:

1. create a new file with a temporary name,
2. copy updated content into it,
3. swap it with the destination file, and finally
4. delete the old copy of the destination file.

The normal mode of operation mainly prevents conflicts with applications that might have the destination file open, and a few other mishaps which are duly listed in the rsync manpage.

Note that, if a copy/update operation fails in steps 1.-3. above, rsync will delete the temporary destination file; the --partial option disables this behavior and rsync will leave partially-transferred temporary files on the destination filesystem. Thus, resuming a single file copy operation will not gain much unless you called the first rsync with --partial or --partial-dir (same effect as --partial, in addition instructs rsync to create all temporary files in a specific directory).

If you need to copy into a directory without write permission, add --inplace to command options. This is sometimes cheaper that it doesn't create a temporary copy, but during updating the file is in inconsistent state.

On the question why it doesn't check own rights beforehand: it could miss some additional specifics as extended access ACLs, so rsync doesn't believe into standard 3*3+3 scheme. OTOH this intentional policy doesn't make any real difference in result - anyway file isn't updated.