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Old 07-27-2015, 12:38 PM   #1
vincix
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understanding rsync --link-dest


I was checking out this link http://www.howtogeek.com/175008/the-...ta-with-rsync/ and I wanted to understand exactly how --link-dest works in that case. This is the command that I found on the site:

Quote:
rsync -avzhPR --chmod=Du=rwx,Dgo=rx,Fu=rw,Fgo=r --delete --stats --log-file=~/backup/rsync-`date +%F-%I%p`.log --exclude-from '~/exclude.txt' --link-dest=/home/geek2/files/`cat ~/backup/time2.txt` -e 'ssh -p 12345' /home/geek/files/ geek2@10.1.1.1:/home/geek2/files/`date +%F-%I%p`/
What I'm actually interested in knowing is, where does /home/geek2/files/`cat ~/backup/time2.txt` exist? Locally or on the 10.1.1.1?

If it's local, then for me it doesn't make too much sense. The idea is to create backups ON the server and create them relative to the previous backups still on the server. So how does it work? (I understand that rsync uses hard links and that's the idea of link-dest, so that part is more or less clear)
 
Old 07-27-2015, 02:19 PM   #2
mralk3
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I only briefly glanced at the tutorial you linked.

It appears that the time.txt is located on the local machine and is copied to the remote machine later on.

All of these commands are in fact doing what you are expecting. time.txt and time2.txt were created to track the snapshot date and time.

The commands you are running require that you understand the documentation of rsync, first. Just running commands is going to be plain confusing if you do not read the docs.

Code:
man rsync
 
Old 07-27-2015, 02:36 PM   #3
vincix
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Quote:
Originally Posted by mralk3 View Post
I only briefly glanced at the tutorial you linked.

It appears that the time.txt is located on the local machine and is copied to the remote machine later on.

All of these commands are in fact doing what you are expecting. time.txt and time2.txt were created to track the snapshot date and time.

The commands you are running require that you understand the documentation of rsync, first. Just running commands is going to be plain confusing if you do not read the docs.

Code:
man rsync
Would it be possible to actually answer my question specifically? I am sure that wouldn't be too much to ask.
 
Old 07-27-2015, 03:07 PM   #4
mralk3
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Quote:
Originally Posted by mralk3 View Post
It appears that the time.txt is located on the local machine and is copied to the remote machine later on.
I did answer your question to the best of my knowledge. I am not going to read the documentation for you and spell it out though.
 
Old 07-27-2015, 04:00 PM   #5
vincix
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Ok, I hadn't read your answer correctly.


First of all, I understand everything that is being used in that tutorial. That's not a problem. The rsync manual pages are of no help in this case.

So I have a client whose files I want to backup on that server. As I understand, --link-dest refers to the previous backup that was already made and creates hardlinks relative to that previous backup and adds any new/changed files. That much is clear. What I don't understand is why doesn't --link-dest refer to a path ON the server, instead of on the local machine? Indeed, I may not understand exactly how link-dest option works. Why does it compare it to something on the local machine, instead of with a previous backup made on the server? I hope my question is clear enough.

Thanks

Last edited by vincix; 07-27-2015 at 04:10 PM.
 
Old 07-27-2015, 06:30 PM   #6
rknichols
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It does refer to a path on the server, but the way you have it coded could be a problem. Your local shell is going to expand the `cat ~/backup/time2.txt` substitution and pass the result to the rsync process on the server. Whether the result of that expansion makes sense on the server is another matter. If you really wanted to get the content of a time2.txt file from the server, you would need to do it another way, either by copying that file to the local machine or by executing the cat command on the server with something like:
Code:
--link-dest=/home/geek2/files/`ssh geek2@10.1.1.1 cat backup/time2.txt`
 
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Old 07-27-2015, 06:48 PM   #7
vincix
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Quote:
Originally Posted by rknichols View Post
It does refer to a path on the server, but the way you have it coded could be a problem. Your local shell is going to expand the `cat ~/backup/time2.txt` substitution and pass the result to the rsync process on the server. Whether the result of that expansion makes sense on the server is another matter. If you really wanted to get the content of a time2.txt file from the server, you would need to do it another way, either by copying that file to the local machine or by executing the cat command on the server with something like:
Code:
--link-dest=/home/geek2/files/`ssh geek2@10.1.1.1 cat backup/time2.txt`
Thanks for the explanation.
So if I wanted something easier - if I had a backup and then I wanted only another single differential backup, after some files changed, what would the correct command be, ignoring in this example now the command expansions?

For instance, if I'd like to backup my home directory and have another differential backup on my server under the folder /backup/home2.

Code:
rsync -av --link-dest=/backup/home /home/user server:/backup/home2
Would that suffice as a basic one-time differential backup (after the initial one)? As I understand from what you've said, --link-dest=/backup/home refers to the path on the server (or more generally, on the destionation side).

And a second question: I'm trying to back up files from a Mac OS and the ownership of the files is different from the ones on the server (meaning that it changes the owner and the group when the files are synchronized on the server).

That does automatically mean that rsync will not create hardlinks, but will copy the files entirely? And is there a way to avoid this without having to create an analogous user/group on my linux server? Maybe break up the -a option and omit the -p (perms)? There must be an easier way, right? I'd prefer using rsync and not some other tool that works on top of rsync, as I'd really like to understand exactly how rsync works.

[later edit]
I did get it to work on my mac without having to bother with the ownership. So it did work like that directly.

Thank you for your answer.

Last edited by vincix; 07-27-2015 at 07:29 PM.
 
  


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