Mistakenly ran chmod 644 /usr/* Any way to recover my installation?

Question

I ran chmod 644 /usr/* on my installation of Mac OSX 10.6.

Now, everything is not working, from chmod to every other command. Obviously those commands are in /usr/bin, and I do not have execute permissions for chmod to give myself execute permissions.

Answer 1

The Typical Method

OSX has the built-in application Disk Utility to fix issues like this. Follow Apple's guide for troubleshooting permissions issues in Mac OS X, and learn about Disk Utility's Repair Disk Permissions feature.

For Mac OS X 10.2 or later, open Disk Utility (/Applications/Utilities/). Select your Mac OS X startup volume in the column on the left of the Disk Utility window, then click the First Aid tab. Click the Repair Disk Permissions button.

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The Extreme Cases Methods

Method 1

1. Download AppleJack
2. Mount the AppleJack disk image, and run the AppleJack.pkg installer
3. Now we will run AppleJack:
   1. Restart your computer
   2. When the "bong" sounds, hold down the ⌘ and s keys until you get lots of text appearing on a black screen. You are now in single user mode.
   3. Type applejack auto restart at the prompt and hit return

Method 2 (with help from macstrategy.com)

1. Find someone with an 10.6.* OSX install disc (or burn/write your own)
2. Boot your Mac from the disc (do one of the following):
   1. Insert your install disc, restart your computer and hold down the c key (recommended).
   2. Insert your install disc, go to Apple menu > System Preferences > Startup Disk and select the install disc, then restart your computer.
   3. Insert your install disc, restart your computer and hold down the alt (option) key, select the install disc, click the up or right arrow.
3. Select your required language.
4. Don't install Mac OS X but instead go to the Utilities menu and select Disk Utility.
5. Select the disk icon in the top left of the drive you want to repair permissions on e.g. Macintosh HD.
6. Click on the First Aid tab at the top.
7. Click on the Repair Disk Permissions button.
8. When this has finished quit the Disk Utility application and restart your computer normally.

Answer 2

Well, most of the directories and files have 755 as the default so I would start there to allow you to execute common utilities. You can then later figure out if you want to tighten security.

sudo chmod -R 755 /usr

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This page has a variety of ideas, depending on what you have access to. Example, if you have execute permissions on cp, mv and cat you could use the following sequence of commands to "copy" permissions from another file (in this example /bin/ls).

/bin/cp /bin/ls /tmp/chmod
/bin/cat /bin/chmod > /tmp/chmod
/bin/mv /tmp/chmod /bin/chmod

Notes: The first commands copies a file (including permissions). The second command copies the contents of chmod to the new file (does not overwrite permissions). The last command overwrites the original chmod file.

Answer 3

Another option is to run in Superuser mode, or run the CD

Superuser Mode

1. Turn off the Mac
2. Hold Command+s as you press Power
3. When it starts, you'll be presented with a ROOT prompt. Mount the file system (as shows in the instructions when you start), then add your Execute permission

Run from the CD (recommended)

1. Insert the Mac OSX DVD and shutdown the Mac
2. Press and hold the Alt key as you turn on the Mac
3. Select the Mac OSX Install DVD option
4. Select your language and click Next
5. Select Tools->Disk Utility
6. Select your hard drive partition from the left side
7. On the First Aid tab, click Repair Disk Permissions and wait

Once either of these are done, reboot your Mac