Resume transfer of a single file by rsync

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).

Be aware that --append implies --inplace, which itself implies --partial.

• By just using --partial you should cause rsync to leave partial transfers and resume them in subsequent attempts.

• By using --append you should cause rsync to both leave partial files and resume them next time. After transfer rsync should verify the checksum of transmitted data only.

• --append-verify includes the whole file in the checksum verification, including any portion transferred in a previous transfer.

• With either --append or --append-verify a failed checksum verification should cause the file to be re-transmitted completely (using --inplace)

You should be able to resume a mv or cp operation with rsync but you may want to use the --append-verify option for peace of mind.

Note that using --append causes rsync to copy only those files which have its size on the receiver shorter than the size on the sender (regardless of time stamps), or are absent on receiver. By documentation on this option:

If a file needs to be transferred and its size on the receiver is the same or longer than the size on the sender, the file is skipped.

More info in the man page

David Schwartz is correct, --partial (or better, -P) does do what you want. I verified this on a 37G file that was stopped ~8g into it, over a network. rsync quickly scanned the first parts of the partial (showing progress as it was going thanks to -P), and then resumed the transfer to the end of the partial file.