This is safe using rsync for combining two dirs to another one?

[Azq2]

For e.g. I want to merge all files from overlay/ and main/ to one dir destination/
And files in overlay/ must to have priority on files from main/

I use this command:

rsync -av --delete overlay/ main/ destination/

This works... but I'm not sure if that safe.

Is it guaranteed that files from the first source will always be in priority?
I see in the code this algorithm:

• get all files from all source dirs
• sort this files by fsort() (by name)
• remove all duplicates, use only first file with concrete name

But will the file from the first source always come first after fsort?
Or this is undefined behavior?

P.S. I want to use single rsync command.

[Azq2]

Explanation of my case

# create test data
$ mkdir overlay/ main/ destination/
$ echo "file from overlay" > overlay/file
$ echo "file from main" > main/file

# rsync files
$ rsync -av --delete overlay/ main/ destination/
sending incremental file list
./
file

sent 168 bytes  received 38 bytes  412.00 bytes/sec
total size is 12  speedup is 0.06

# check result
$ cat destination/file
file from overlay

[Azq2]

Also I read this in wiki:

In computer science, merge sort (also commonly spelled as mergesort) is an efficient, general-purpose, and comparison-based sorting algorithm. Most implementations produce a stable sort, which means that the order of equal elements is the same in the input and output.

Merge sort (fsort), used in rsync, is an stable sort?
I'm not specialist in algorithm and fsort code don't tell me anything about of stability :(

[WayneD]

The flist.c file has comments about this, but I didn't find it mentioned in the manpage. Some of the comments:

/* This file-struct sorting routine makes sure that any identical names in
** the file list stay in the same order as they were in the original list.
** This is particularly vital in inc_recurse mode where we expect a sort
** on the flist to match the exact order of a sort on the dir_flist. */

/* If one is a dir and the other is not, we want to
** keep the dir because it might have contents in the
** list. Otherwise keep the first one. */

So, with identically named files, rsync always chooses the first one encountered. If the second folder has a dir named the same as a file in the first folder, the dir is sent instead.

[Azq2]

Thanks for explanation. Rsync is awesome :)