Wednesday, July 23, 2008

Backups With Rsync Using SSH and Cron

Sincronize Folders With Rsync Using SSH and Cron

Looking for how to set up RSync over SSH so that you can run it in a cron job, or without entering a password?

It's actually very simple. Just follow these few steps:

Step 1
As the user you are going to be running rsync as, and on the machine you will be running rsync on, type:

  # ssh-keygen -t rsa

Follow the prompts and use the defaults for the filenames it gives you. Don't enter in a passphrase, otherwise you will still be prompted for a password when trying to connect.

You should then have two new files in path_to_user_home/.ssh, id_rsa and

Step 2
Open path_to_user_home/.ssh/ and copy the text to the end of path_to_user_home/.ssh/authorized_keys file on the host you will be connecting to as the user you will be logging in as.

ex:in bash you can do it like this:
cat >> .ssh/authorized_keys

Note: if /.ssh/ or authorized_keys dont exist create them, this hapened to me in cygwin,
where i had to create .ssh/ and inside it i created the authorized_keys

Step 3
Now try it out. Try ssh'ing from the host you created the id_rsa* files on to the one you added a text to the end of the authorized_keys file. You won't be prompted for a password any more.

Step 4
Now you can use, cron (see Configuring a Cron Task below) to schedule rsync tasks using ssh, because you don't need to prompt the password

Configuring a Cron Task

The main configuration file for cron, /etc/crontab, contains the following lines:


# run-parts
01 * * * * root run-parts /etc/cron.hourly
02 4 * * * root run-parts /etc/cron.daily
22 4 * * 0 root run-parts /etc/cron.weekly
42 4 1 * * root run-parts /etc/cron.monthly

The first four lines are variables used to configure the environment in which the cron tasks are run. The value of the SHELL variable tells the system which shell environment to use (in this example the bash shell), and the PATH variable defines the path used to execute commands. The output of the cron tasks are emailed to the username defined with the MAILTO variable. If the MAILTO variable is defined as an empty string (MAILTO=""), email will not be sent. The HOME variable can be used to set the home directory to use when executing commands or scripts.

Each line in the /etc/crontab file has the format:

    *minute   *hour    *day    *month    *dayofweek   *command

minute — any integer from 0 to 59
hour — any integer from 0 to 23
day — any integer from 1 to 31 (must be a valid day if a month is specified)
month — any integer from 1 to 12 (or the short name of the month such as jan, feb, and so on)
dayofweek — any integer from 0 to 7 where 0 or 7 represents Sunday (or the short name of the week such as sun, mon, and so on)
command — the command to execute. The command can either be a command such as ls /proc >> /tmp/proc or the commandcustom script that you wrote.

For any of the above values, an asterisk (*) can be used to specify all valid values. For example, an asterisk for the month value means execute the command every month within the constraints of the other values.

A hyphen (-) between integers specifies a range of integers. For example, 1-4 means the integers 1, 2, 3, and 4.

A list of values separated by commas (,) specifies a list. For example, 3, 4, 6, 8 indicates those four specific integers.

The forward slash (/) can be used to specify step values. The value of an integer can be skipped within a range by following the range with /. For example, 0-59/2 can be used to define every other minute in the minute field. Step values can also be used with an asterisk. For instance, the value */3 can be used in the month field to run the task every third month.

Any lines that begin with a hash mark (#) are comments and are not processed.



# run custom script the first day of every month at 4:10AM
10 4 1 * * /



# run-parts
01 * * * * root run-parts /etc/cron.hourly
31 * * * * root run-parts /etc/cron.hourly
02 4 * * * root run-parts /etc/cron.daily
22 4 * * 0 root run-parts /etc/cron.weekly
42 4 1 * * root run-parts /etc/cron.monthly


echo "+================================+"  >> /root/.rsync.log
echo "Syncrhonization Attempt Started At:" >> /root/.rsync.log
date >> /your_log_dir/.rsync.log

rsync --delete -avz "/source_dir/"  -e ssh username@remote_machine_ip_or_dns_name:/destination_dir

echo "---------------------------------" >> /root/.rsync.log
echo "
Syncrhonization Attempt Ended At" >> /root/.rsync.log

date >> /root/.rsync.log

As you can see from the /etc/crontab file, it uses the run-parts script to execute the scripts in the /etc/cron.hourly, /etc/cron.daily, /etc/cron.weekly, and /etc/cron.monthly files on an hourly, daily, weekly, or monthly basis respectively. The files in these directory should be shell scripts.

If a cron tasks needs to be executed on a schedule other than hourly, daily, weekly, or monthly, it can be added to the /etc/cron.d directory. All files in this directory use the same syntax as /etc/crontab.

The cron daemon checks the etc/crontab file, the etc/cron.d/ directory, and the /var/spool/cron directory every minute for any changes. If any changes are found, they are loaded into memory. Thus, the daemon does not need to be restarted if a crontab file is changed.

Users other than root can configure cron tasks by using the crontab utility. All user-defined crontabs are stored in the /var/spool/cron directory and are executed using the usernames of the users that created them. To create a crontab as a user, login as that user and type the command crontab -e to edit the user's crontab using the editor specified by the VISUAL or EDITOR environment variable. The file uses the same format as /etc/crontab. When the changes to the crontab are saved, the crontab is stored according to username and written to the file /var/spool/cron/username.
Starting and Stopping the Service

To start the cron service, use the command /sbin/service crond start. To stop the service, use the command /sbin/service

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