Free disk space by removing TimeMachine local snapshots

Mac OSX TimeMachine's "local snapshots" are basically backup files stored on your local hard-disk (as opposed to regular ones stored on the external TimeMachine server). This is a useful feature for people who travel a lot, so they can revert files to recent versions even when they are not connected to their home TimeMachine.

Unfortunately, this feature tends to take a lot of disk space whenever you deal with big files (like movies etc). In some cases, you might want to temporarily disable it in order to claim back a few GigaBytes. Note that your regular TimeMachine backups will NOT be affected; you'll just lose the ability to revert to recent (i.e. from last week) versions without being connected to the external TimeMachine.

TimeMachine is managed from the command line with the tmutil command. You can type man tmutil to see all options. To disable the snapshots and delete those big files (which are found under /.MobileBackups, by the way), use "sudo tmutil disablelocal". Once snapshots are purged, you can restart the feature (if you need it) with "sudo tmutil enablelocal" (yeah, not exactly rocket science).

In my case, this procedure freed some 139 GB on a 512 GB disk. Not bad!

VmWare tip

This might be obvious to most, but it's easy for novice VmWare users to forget.

When you take a snapshot, VmWare has to write down the full current status of your image, including RAM. If you take a snapshot while the image is running, VmWare will have to save the full RAM content, which might run up to several GBs. If you take the snapshot after your image has properly shut down, RAM content will simply be discarded, and VmWare will only have to deal with the actual disk.

TL;DR: Always shut down your image before snapshotting, and you'll save a lot of disk space.

Encrypting and Decrypting SQLDeveloper 3 Passwords

Some Oracle products are fairly sweet, let's be honest. One of them is the revamped SQLDeveloper, which has finally caught up (mostly) with MSSQL Management Studio.

One of its best features is the ability to import and export list of connections via XML (right-click on Connections to find the relevant menu). The resulting file is very readable, and hence easily manipulable. The only opaque item is the encrypted password, but it turns out that they are not particularly hardened. This is what you have to do to be able to manipulate them for fun and profit.

1. Get Jython, or your favourite choice of JVM dialect that can work with jars. I picked Jython because 1) it's Python! and 2) Oracle ships it with most products, under oracle_common\util\jython.
2. Load ojmisc.jar and db-ca.jar. These can be found in different places depending on your SQLDeveloper version.
3. Import oracle.jdevimpl.db.adapter.DatabaseProviderHelper. That class has the two methods you need: goingOut (i.e. encrypt) and comingIn (i.e. decrypt).

So here's a complete Jython script for the Windows version of SQLDeveloper:

# set this to the path where you extracted SQLDeveloper
SQLDEV_ROOT = r'C:\sqldeveloper' 
# here's the real stuff
import sys
from os.path import join
sys.path.append(join(SQLDEV_ROOT,r'sqldeveloper\extensions\oracle.datamodeler\lib\ojmisc.jar'))
sys.path.append(join(SQLDEV_ROOT,r'sqldeveloper\modules\oracle.adf.model_11.1.1\db-ca.jar'))
from oracle.jdevimpl.db.adapter.DatabaseProviderHelper import goingOut as encrypt, comingIn as decrypt

if __name__=='__main__':
    print "Encrypted 'password': " + encrypt('password')
    print "Decrypted 'password': " + decrypt(encrypt('password'))

Oracle XE 11.2.0.2 + Oracle Client 11.2.0.1 = cannot "connect / as sysdba"

While having fun with some complicated (and likely illegal!) scenarios I won't go into now, I ended up this morning with an Oracle XE database to which I wanted to connect with the usual DBA routine:

sqlplus /nolog
connect / as sysdba

I kept getting the dreadful "ORA-12560: TNS:protocol adapter error" and couldn't understand why. Surely I didn't break it that much? The service was still up and I could connect regularly in every way except that.

It turns out this is what happens when you try to do too many things on the same machine. I had installed an Oracle Client on top, because I needed a few things from there (OleDB provider etc) and because other products are guaranteed to work with that client rather than XE.

The problem is that the sqlplus version that comes with the Client is older (!) than the one from XE, but because of how the Windows PATH system variable gets manipulated by the installer, the old version takes over and gives you this problem.

Solution? In your PATH, make sure the folder from the oraclexe directory comes before the one from any client you might have there. However, understand that this is likely to break your Client, so either make sure the change is temporary, or set it locally only when you need it (batch files!).

How to compile PyObjC for Python 3 on OSX 10.8 Mountain Lion

Another one for teh Google...

It so happens that I am curious about PyObjC, the Python bindings for Objective-C, which is the "native" language of choice for OSX/iOS.

As usual, my timing is completely wrong: recent versions of Xcode dropped support for PyObjC, and the project has shrunk to basically one person (that Ronald Oussoren I previously mentioned). The version on PyPI seems to work with Python 2.x only. Even the official page on Sourceforge is basically abandoned, and packages available from there are obsolete. This is a problem because I'm really trying hard to do everything with Python 3 these days, and the PyObjC version shipped with OSX 10.8 "Mountain Lion" is for 2.7 (the only Python version Apple ships and supports).

Luckily, from my past tribolations I knew that Ronald had his own repository on BitBucket, so I tried that and it worked fine. However, the documentation on how to build PyObjC from source is quite scarce (in fine geek tradition), and I had to figure out the following principles in the hard way:

• Ronald's repository is split into many separate packages that have to be individually built. This is very fine-grained, but a bit cumbersome for the general case.
• Do not use the setup.py script you'll find under /pyobjc . These are just for people pulling from PyPI, i.e. post-release.
• /pyobjc-xcode is obsolete, and there's nothing to build there.
• /pyobjc-framework-XgridFoundation simply refuses to build under ML. Xgrid is a somewhat obscure, proprietary Apple technology for highly-parallel computation. If you don't know what it is, chances are that you won't need it. I personally don't care about it.
• /pyobjc-core is a requirement for all other packages, so it should be built first.
• In order of importance, /pyobjc-framework-Cocoa, -Quartz and -CoreData are dependencies of other packages, so they should be built in this order before any other pyobjc-framework-*.
• Python 3 support is occasionally shaky. In one occasion, one file had to be patched to remove unicode literals (the u'mystring' notation from Python 2 that was dropped in Python 3.0), but that's just a temporary snag: Python 3.3 will reintroduce that syntax as a compatibility hack for exactly this type of situation. I've submitted a patch anyway, but if you can't wait for Ronald to consider it, it is available in the below-mentioned repository.
• Looking at BitBucket, I noticed there's at least one significant fork that is arguably targeting Python 3 more consistently. You might want to try that if Ronald's version is not good enough for you.

Because I don't plan to do this sort of work every day, I've put together a script so that I won't have to remember all this stuff when starting a new virtualenv environment. It's now available from my utils repo on BitBucket. There is no documentation but OMG IT'S FULL OF COMMENTS so there. As usual, any feedback is more than welcome.