Can rsync verify contents before syncing

Question

Can Rsync be configured to verify the contents of the file before they are being synced. I have heard about checksum, but I came to know that checksum only does a sampling. I want to transfer a file only if it is contents are changed and not timestamp, is there a way to do it with any of the rsync modes. In my scenario, say file sample.text will be created every week and I want to sync it with a remote server only if the contents of sample.text are changed, since it is created every week, the time stamp would obviously change. But I want the transfer only on a content change.

Solution 1

Yes:

$ man rsync | grep "\--checksum"
    -c, --checksum              skip based on checksum, not mod-time & size

Rsync is pretty complicated. I recommend cuddle time with the man page and experimentation with test data before using it for anything remotely important.

Most of the time, people use rsync -ac source dest.

$ man rsync | grep "\--archive"
 -a, --archive               archive mode; same as -rlptgoD (no -H)

And that -rlptgoD garbage means: recursive (r), copy symlinks as symlinks (l), preserve permissions (p), preserve times (t), preserve group (g), preserve owner (o), preserve device files (D, super-user only), preserve special files (also part of D).

The -c or --checksum is really what you are looking for (skip based on checksum, not mod-time & size). Your supposition that rsync only samples mtime and size is wrong.

Solution 2

See the --checksum option on the rsync man page.

Also, the --size-only option will be a faster choice if you know for sure that a change of contents also means a change of size.

Comments

[Luke Cousins]

+ for cuddle time with the man page and experimentation with test data - Extremely good advice

[ishmaelMakitla]

You should really add some explanation as to why this code should work - you can also add comments in the code itself - in its current form, it does not provide any explanation which can help the rest of the community to understand what you did to solve/answer the question.

[oldbowman]

If there was no change the second row of diff.txt in verbose mode of rsync is empty, sed examine this second line.