magpiebrain

Now we’ve worked out what files you want backed up, and we’ve sorted automatic authentication, the only thing left to do is schedule our backups. Unfortunately we can’t make use of the very handy iSync or Backup tools to do this (Apple don’t seem keen on opening up these tools up, as they no-doubt help sell expensive .Mac accounts).

h3. cron

cron is available on all *NIX systems, and is a well established solution to running all kinds of house keeping tasks. On OSX 10.4 however cron is being phased out in favour of launchd which we’ll look at next.

launchd

As of OSX 10.4 a new daemon called launchd has been added. Eventually this will replace tools such as cron, rc and xinetd – however it’s low on the userfriendly scale right now. Unlike cron’s simple crontab file, launchd requires XML files which can be a pain to edit without something like the shareware program launchd editor. Hopefully later OSX releases might bundle a decent UI for managing it.

Anacron + launchd – for those who like to switch their computers off

Both cron and the newer launchd suffer from the same problem when it comes to running routine housekeeping tasks – if the computer isn’t on when a task is scheduled, the task won’t get run, and even when you switch the computer on again, neither tool can work out that it should try and catch up by running the task on startup.

So using either cron or launchd means that if we use either task to manage our backups, the computer has to be on when the backup is scheduled to run.

Anacron attempts to solve these deficiencies. It stores a record of when each job was last run, so on startup it can work out if it needs to catch up. This is especially handy for laptops which are probably only on for a few hours a day.

Depending on your operating system, you can either get anacron setup to work with cron, or launchd. Assuming you take the anacron + launcd approach, grab Ronald Florence’s highly useful Anacron for OSX 10.4 installer. Once installed, you should have a file /etc/anacrontab which will look like this:

	
#period delay   job-identifier  command
1       5       cron.daily      periodic daily
7       10      cron.weekly     periodic weekly
30      15      cron.monthly    periodic monthly

The first field is the frequency of the job – 1 means daily, 7 means weekly and 30 means monthly. The second value is the delay in minutes between anacron realising a job needs to be run, and it actually getting scheduled. The third field is a unique identifier for the job, and finally the command itself.

To simplify things we should create a simple script which contains our rsync command(s) – for now you can place it into your bin directory, and call it something sensible like backup. Next, assuming you want to run the backup job daily, add the following line to /etc/anacrontab:

	
1       10      backup.user      /Users/johnsmith/bin/backup

To make sure anacron is working (and is picking up your job) simply start Console.app, and select Open System Log from the File menu – you should see its output. If you need to tweak your backup job, you can remove the record of the previous run /var/spool/anacron/ – it should contain one file per record in anacrontab. Then, you can force anacron to rerun the job by executing anacron -n.

And there you have it! Trouble free, secure backups that will get run even if you switch your computer on for only a few hours each day.