Using launchd to run scheduled jobs

2011/08/29 14:25:22
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My new server’s predecessor ran Linux. I had a few scheduled jobs, like the ones that back up this blog, its database, and so on. On Linux the tool used for scheduling jobs is cron. Lo and behold, Apple has deprecated cron on their systems in lieu of an Apple-grown tool named launched. Launchd’s job is to replace cron, init, the rc scripts, the file alteration monitor, and a whole host of other normal Unix utilities in favor of one huge behemoth process that starts at boot. I’ve got mixed feelings about this concept, as it goes against the Unix philosophy of “doing one thing and doing it well”, but I chose OSX and have tried to learn their way of doing things.

I found several articles describing how one creates the plist files used by launchd. These XML files contain the information on the process you wish to run, including when to run it, how to run it, and who is allowed to run it. Here I will detail how I used it to merely run a scheduled job every night at midnight.

To start with, I have a simple script that backs up the database on the server:

# if we don't have a backup directory, make it
if [ ! -e $BACKUPDIR ]; then
  mkdir -p $BACKUPDIR
# run the remote database dump command
ssh $REMHOST "mysqldump --verbose --user=root --password='XXXXXXXXXXXX' --all-databases" > $OUTPUTFILE
# if the zipped output file already exists, move it before zipping the new one
if [ -f $OUTPUTFILE.bz2 ]; then
  mv $OUTPUTFILE.bz2 old-$OUTPUTFILE.bz2
chown smj:staff $OUTPUTFILE.bz2

This script is creatively entitled backup-database. I want it to run every night so I have an exact copy of this website’s database in case the web server goes down and I have to reinstall everything.

Under cron, I would run crontab -e and then put the following line into the editor that is brought up:

0 * * * *    $HOME/bin/backup-database

Apple made this simple step a lot more complicated than, in my opinion, it needed to be; but they gave me a tool that is a lot more powerful that mere cron.

To use launchd, I needed to make a file in the directory /Library/LaunchDaemons named org.littleprojects.backup.lawrence.db.plist that contains the following XML code:

<!--?xml version="1.0" encoding="UTF-8"?-->

I’m a little annoyed at how Apple handled the whole key-value syntax, but I’ll let that be. The file reads as follows: create a job named org.littleprojects.backup.lawrence.db that will run the program /Users/smj/bin/backup-database at the 0th hour and 0th minute of ever day (midnight). I’m not sure if this is any less difficult to read than the cron syntax, but it is more difficult to write to be sure.

Once I created the file, I then needed to load it into launchd, which could be done by running the following commands:

sudo launchctl load org.littleprojects.backup.lawrence.db.plist

If you do this you, do not need to reboot as others claim. This is Unix and reboots should only be necessary for major operating system changes. Rebooting in order to create a scheduled job is not only ridiculous, but time-wasting and leads to error-prone behavior like making many untested scheduled jobs at once in order to save on reboots. I always try to find a way around reboots to avoid error-prone behaviors!

So, I did the same thing for other scripts I wanted to run at certain times. I understand why apple decided to use launchd, I just wish they would have made it easier to configure. Lingon exists in the app store for the express purpose of helping you create these plist XML files, but it will not load them for you and instead recommends you reboot after each edit, which I noted my distaste for above.

In the future I will be exploring how to best use launchd for other purposes, like restarting services, and scheduling scripts to run when other system events occur.


Randomly crashing Mac Mini Server running Lion [Solution]

2011/08/18 19:08:24
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I’d been looking for a home server solution that offered me more disk space, but that ran cool and quieter than my server-closet behemoth. I wanted the server to also be able to execute scripts and scheduled jobs. I researched several options, including the Dell Zino, but none seemed to capture my attention like the Mac Mini Server.

The day it arrived, I set it up and spent hours pouring over Apple’s documentation. I got file sharing, SSH, WebDAV, and other services set up easily, but realized that I had to download Server Admin Tools separately in order to get access to DHCP and DNS. Some admins speculate that this is a separate download to prevent would be admins from accidentally turning on DHCP in a network that already had it. Screen sharing worked flawlessly. I had it using my 46″ television as a monitor, so watching online video and iTunes was awesome!

The next day I went back to my new toy, and… it had locked up. I was not pleased. Murmuring curses and threats of sending the server back didn’t bring the server back from its limbo. This was UNIX! UNIX was not supposed to do this! I felt betrayed by a company I’ve been growing in support for. Why did they do this to me?

I think I’ve solved the problem. Nothing useful was in the logs, but I came across a forum post where someone had suggested that there were bugs [1][2] in Lion’s display drivers that caused some Macs to lock up when returning from sleep.

I went into System Preferences, then chose Energy Saver. I slid the slider next to Display Sleep all the way to the right, choosing Never. I have not had a lockup since.

I can’t recommend this for everyone, because they don’t have a display that is either off, or using another input, like my TV, but it might work in a pinch if you don’t mind manually turning your display off. Also, I’m a little surprised that it’s affecting my brand new Mac Mini Server, because the articles I’ve found refer to older Macs.

Update: This may have been fixed by the 10.7.1 patch. I haven’t tested it yet.

Review: Safari 2 Go (for iPad)

2011/05/15 07:24:06
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A friend excitedly told me about Safari 2 Go (S2G) for iPad. Having an interest in the technical books Safari offers, I chose to try and download a book right then and there, on 3G. S2G refuses to let you log in at all, unless you have wifi.

Later, on my wifi network, I started the download of a book. It took 10 minutes. I’ve noticed that books in the Amazon Kindle app and iBooks generally take about 5 minutes to download, over 3G. I was quite disappointed with the Safari download speed.

Also, the first attempt was unsuccessful, for the iPad’s desire to sleep during inactivity had forced me to “babysit” the iPad, occasionally touching it to ensure that S2G didn’t terminate the download. Other iPad apps are able to override this inactivity setting so they can finish their downloads without issue, even finishing the download while the user is running other apps (you know, that whole multitasking thing that’s sweeping the nation).

Once I did get the book to download, the resulting product wasn’t fully available.

Some of the pages in the resulting book vanish when you try to read them. They’re quite the tease. It might be related to a watermark that vanishes after a picture is loaded. If you do a quick print screen you might be able to capture the missing page. :)

All in all, it’s not a pleasant experience.

The Safari web site works fine, but I was looking forward to the offline storage for when I didn’t have or want to use an internet connection.

At the moment S2G sucks. What the hell were they thinking releasing it like this?

Update (2011/06/08): The updates haven’t helped, crashing often and appearing to fix none of these issues. In fact, the latest version doesn’t even load, presenting the user with a black screen and no functionality.

Rebirth of a web site

2011/02/21 23:21:48
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I just spent time giving birth to an Old Dominion University Computer Science web site at  It’s just one page now, with some course information, and some links back to this site.  I know, not very interesting.  Under the covers, though, is a testbed for the Semantic Web.  So far, I’ve got Dublin Core and Friend of a Friend (FOAF) implemented for that one page.  In the future, I intend to bring the lessons learned there over to this site.  Eventually, the two will begin to look more alike in both style and function.

Perhaps Web Science and Digital Libraries is really my thing. I spend an awful lot of time blogging about my own web sites.

Validation of the Creative Commons Markup

I also discovered that the Creative Commons license markup wouldn’t validate with the W3C validator. This was because my page was initially specified as XHTML, but the Creative Commons license link contains additional RDF metadata used by search engines and other software. A blog post suggested just removing the metadata, but I didn’t want to do that because I knew that metadata had been placed there by Creative Commons for a reason. That metadata is part of the RDFa standard, which is a way of embedding metadata into normal HTML and XHMTL.

It turns out that once you put that Creative Commons markup into your XHTML, it becomes XHTML + RDFa, which is a completely different document type. I felt good that I’d figured this out, and changed the page to use this instead.

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML+RDFa 1.0//EN" "">

Now it validates fine as XHTML + RDFa.

Nifty floating image trick

You may wonder how I got the image to stay on the left and in the same spot, even though the visitor can scroll the page.  It is based on the following CSS, shamelessly stolen from the W3C themselves.

body {
    background-image: url(;
    background-repeat: no-repeat;
    background-position: top left;
    background-attachment: fixed;
   /* more CSS here */

That’s it. It’s quite elegant, and I spent more time making the image with Inkscape than I did actually writing the CSS to tell it to stay put.

Useful Links

I discovered the following useful sources for bringing that site to life:

Testing RAM on a Macbook Pro

2011/02/03 23:46:02
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My Macbook Pro came with 4GB of RAM in 2 2GB modules.  In December, I ordered 8 GB (2 4GB modules) of RAM from Crucial so I could max out the RAM in the machine, as I intend to keep it for a while.

Early in January I started to see applications crash.  VMWare Fusion became almost unusable.  I had never seen this behavior from my Mac before, so figured the new RAM had something to do with it, but wanted to verify my assumption before blindly blaming Crucial for bad RAM.  At that point I had to learn how to best test RAM on a Mac.

On a PC, I would have booted a CD running memtest86, but none of my Internet research indicated that was possible for the Mac.

Instead, I had to install memtest and boot the machine into single user mode in order to run memtest with the least amount of interference from running programs. I wanted to document what I had done in a blog post so I could remember next time.

Note: This article involves configuring your Mac in a way that could make it difficult for you to boot it normally if you don’t know what you’re doing. If you don’t feel comfortable following these instructions, take your Mac to a technician who not only feels comfortable fixing your computer, but is also competent to do so.

To tell the Mac to boot into verbose 64-bit single user mode, go to Applications/Utilities/Terminal, and type:

sudo nvram boot-args="-s arch=x86_64 -v"

You may be asked for the password you type to make changes to the system, type the password and hit enter.

Reboot your Mac.

Your Mac will boot to a prompt.  You may be surprised at all of the text that goes by.   These are kernel messages that are normally hidden by the nifty Apple logo.

At the prompt type:

memtest all

This will attempt to test as much RAM as it can.  Remember that some of your RAM is being taken up by minimal amount of software that booted your machine into single user mode (the kernel and bash), so you won’t be able to test everything, but if the flaw was in the RAM being used by the software running at this point, the machine would have failed to boot.

If your RAM is good, the test will take hours and you will see something like:

Memtest version 4.22 (64-bit)
Copyright (C) 2004 Charles Cazabon
Copyright (C) 2004-2008 Tony Scaminaci (Macintosh port)
Licensed under the GNU General Public License version 2 only
Mac OS X 10.6.6 (10J567) running in single user mode
Memory Page Size:  4096
System has 2 Intel core(s) with SSE
Requested memory: 7555MB (7922552832 bytes)
Available memory: 7555MB (7922552832 bytes)
NOTE:  Memory request is too large, reducing to acceptable value...
Allocated memory:  7343MB (7700721344 bytes) at local address 0x00000000101000000
Attempting memory lock... locked successfully
Partitioning memory into 2 comparison buffers...
Buffer A: 3671MB (3850360672 bytes) starts at local address 0x0000000101000000
Buffer B: 3671 MB (3850360672 bytes) starts at local address 0x00000001e67fd760
Running 1 test sequence... (CTRL-C to quit)
Test sequence 1 of 1:
Running tests on full 7343MB region...
  Stuck Address       : ok
  Linear PRN            : ok
Running comparison tests using 3671MB buffer...
  Random Value        : ok
  Compare XOR         : ok
  Compare SUB         : ok
  Compare MUL         : ok
  Compare DIV          : ok
  Compare OR           : ok
  Compare AND         : ok
  Sequential Increment:ok
  Solid Bits               :ok
  Block Sequential      :ok
  Checkerboard         :ok
  Bit Spread             :ok
  Bit Flip                  :ok
  Walking Ones         :ok
  Walking Zeroes       :ok
All tests passed!  Execution time:  7068 seconds.

If your RAM is bad, you will likely get a response really fast, and you will see something like:

*** Address Test Failed *** One or more DIMM address lines are non-functional.


FAILURE: possible bad address line at offset 0x06b3a4c8.


FAILURE: 0xbea1ce76 != 0xbea1ce7e at offset 0x06b3a4c8.


*** Memory Test Failed *** Please check transcript for details.

If you get a message like:

Attempting memory lock... ERROR: Memory lock failed - reason unknown.

WARNING: Testing with unlocked memory may be slower and less reliable

hit CTRL-C and make sure you booted into 64-bit mode by typing the following:

uname -m

If the command doesn’t return x86_64 then you are not running in 64-bit mode and your Mac can’t reach the RAM above 4GB. Please reference the above command for booting your Mac into single user mode, as it includes the flags to set it to 64-bit mode, then reboot.

If the command above does return x86_64, then you are running in 64-bit mode and this message is probably a preview of test failures to come.

If you have more than one module of RAM, then take all of them out, and place one in the machine, boot, and run the test. After that one finishes test, note the results, turn the Mac off, take the RAM module out, and put the next one in. This way you can figure out which module is bad.

When done, make sure you have good RAM in the machine, boot it again, and type:

sudo nvram boot-args=""

This will give you your “normal” Apple logo screen on boot.

I hope you will find this article as useful as I will the next time we need to test RAM on a Mac.

Today, Crucial was nice enough to send me some new RAM after I sent them the bad module.  So far, it appears to be passing memtest.  Cross your fingers for me!  :)