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Repro: Steve's Nose

Step by step this time. Slowly.


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Some people seem to be having difficulties with this. Word has it several prominent bloggers (whatever that's supposed to mean and/or import) don't understand it and can't reproduce it either - which is hardly surprising.

And so if Steve's Nose wasn't explicit enough then perhaps a step by step hand-holding version will help.

The challenge begins now. This might be rocket science to you so pay close attention.

1. Get the Image

This challenge will use a universally accessible image currently available at the following URL.

http://www.apple.com/macosx/

That's Apple's microsite for Mac OS X 10.6 Snow Leopard. The image you want is here.

http://images.apple.com/macosx/images/overview_headline_20090824.jpg

This is the image depicted immediately below.



Save this image to disk (your hard drive on your 'Mac') in your (already cluttered) ~/Downloads folder.

Proceed to step 2 when you have completed this first step.

2. Terminal.app

Open Terminal.app and navigate to ~/Downloads. List the file 'overview_headline_20090824.jpg'. [This is the file you just saved to disk in case you already forgot.]

$ ls -l overview_headline_20090824.jpg
-rw-r--r--  1 macuser  staff  8311 Sep 29 15:15 overview_headline_20090824.jpg

Note the size of the file. It's 8311 bytes. Do your best to keep that in mind. Also note the modified time stamp. [Yes yours will differ. It won't be Sep 29 15:15 in all likelihood. Don't ask why.]

Finally note the permissions. They are '-rw-r--r--'. If you don't understand what that means then stop reading this article at once and go back to Quark.

You are now going to change permissions on the file but you are not going to use Finder because Finder is a toy. You are instead going to use Terminal.app.

$ chmod 0400 overview_headline_20090824.jpg
$ ls -l overview_headline_20090824.jpg
-r--------  1 macuser  staff  8311 Sep 29 15:15 overview_headline_20090824.jpg

You'll now notice (if you're still reading and your eyes haven't glazed over) that there's a single 'r' in the file's new permissions. You can also figure out (perhaps by using a pencil and counting from the right) that the only user account able to access the file is 'macuser'. No other account has any access permissions at all.

[You're going to have to do a mental substitution in your head each time you see the account name 'macuser'. For that's an account on another machine - it's not on your machine, silly! So each time you see 'macuser' you say your own account name to yourself. Then you won't get confused. Alright? Good!]

3. Preview.app

You're now going to open the file in Preview.app. And you're going to twirl it (⌘R) twice in rapid succession so you get the following. Note the red button is 'dirty', meaning you have made changes to the file that you have not yet saved. KEEP THIS IN MIND.

You're now going to save the file. Note the red button's no longer 'dirty': your changes have been saved.

4. Terminal.app Redux

You're now back in Terminal.app. You will now list 'overview_headline_20090824.jpg' again.

$ ls -l overview_headline_20090824.jpg
-r--------  1 macuser  staff  12272 Sep 29 15:17 overview_headline_20090824.jpg

You will now notice a number of things.

  • The file permissions have not changed. They're still '-r--------', meaning only you can access the file - and all you can do is read it. You still can't write to it.
  • The modified time stamp has changed. This new stamp is more recent than the original.
  • The size of the file has now changed as well. Previously it was 8311 bytes; now it's 12272 bytes.

Now this might still take a day or two to sink in but here's what you're going to arrive at: you personally modified the file 'overview_headline_20090824.jpg' without having 'write' permission to do so.

You may now return to your work with Quark.

People aren't as dumb as you think. They're dumber.
 - Stig Anderson

See Also
Learning Curve: Steve's Nose
Coldspots: Snow Leopard Ignores Unix File Permissions

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