Now that we can take advantage of rsync to minimize the data to transfer when backup. But it's still a little uncomfortable if we need to do this manually everyday, right? Well, cron is here to solve the pain.

Cron is kind of a system service that automatically do some job as you specified. Backup, for example, is a perfect kind of job that we can count on cron.

First, we need to specify a job that we want cron to do. In my case, I want cron to automatically sync my source tree folder on remote data center and my local backup folder. A simple rsync command seems meet my need. But actually, there are more to consider:

  • I don't want to copy the obj files, since they are normally large in size and change frequently, but can be easily re-generated. But I also don't want to skip the entire build folder when do rsync since there are some configure files in there.

  • The backup process should be totally automated. More specifically, no password is needed when do rysnc.

Towards the first need, I can use ssh to send remote command to do necessary clean up work before rysnc. And the second need can be meted according to my previous post about ssh/scp without password.

So my final backup script looks like this:

# ~/


date >> $LOG_FILE 
echo "Synchronization start..." >> $LOG_FILE 
ssh b@B 'cd ~/src/build; make clean; rm -rf obj/" >> $LOG_FILE 
rsync -avz --exclude "tags" $SOURCE_DIR $TARGET_DIR >> $LOG_FILE 
echo "Synchronization done" >> $LOG_FILE

Once we figure out what to do, we need to tell cron about our job. The configure file of cron is /etc/crontab. A job description is like follows:

# Example of job definition: 
# .----------------minute (0 - 59) 
# | .------------- hour (0 - 23) 
# | | .---------- day of month (1 - 31) 
# | | | .------- month (1 - 12) OR jan,feb,mar,apr ... 
# | | | | .---- day of week (0 - 6) (Sunday=0 or 7) OR sun,mon,tue,wed,thu,fri,sat 
# | | | | | 
# * * * * * user-name command to be executed 0 0 * * * jack ~/

I want to do backup every day on midnight so I set the minute and hour both to 0. The asterisk (*) symbol in day/month means any valid values.

Now we are done. The back up process is completely automated and scheduled.