dear gurus

i want to understand

what is the basic different of scp/rsync almost both working same way however rsync has great no of feature inside

1)next to clarify everytime when i transfer from 1 pc to other it ask me password, how can i bypass or keep save password

2) how can i crontab, as if this work overnight then passsword need someone to enter then this job run

3) where to see the log in centos/rhel if rsync work fail

4) how can i achive differential,incremental backup advice

regards
salim

No idea what you're trying to say here. "rsync has great no of feature"??? SCP and rsync are very different, and a quick Google search could tell you how. Basically, SCP is just a copy command, using an SSH tunnel. Just copies, that's all. Rsync can do that, and also can be set up just to copy what changed, not everything.
In what? Rsync or SSH? SSH, follow any one of the tutorials on how to generate a key, and swap it between machines.
2) how can i crontab, as if this work overnight then passsword need someone to enter then this job run
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Again, hard to follow here.
http://ubuntuforums.org/showthread.php?t=238672
If it's a cron job, look where the cron logs are kept. You can also write the log file anywhere you'd like when you set the job up.
You can read the man pages and tutorials on Rsync.

In addition, cron will normally email the job owner if it has a problem.
You might also want to check /var/log/messages.