[OS X TeX] file permissions

[Daniel Becker]

>> Do you suggest I need to reinstall OS X. Or simply to run Disk Utility?
> 
> 
> Disk Utility. It adjusts the permissions to what is recorded in the receipts of the installed packages. These receipts need to be on the disk, which they usually do, except some user removes them... (The final @ in the UNIX permissions of /bin signifies that "some ACLs [Access Control List] are applied.")

Disk Utility fixed *a lot* permissions and TeX Live Utility works again. I think I should reinstall ghostscript and imagemagick that I installed via macports.

What is a good source to learn (just enough of) unix file permissions to avoid such stupidness on my side the next time? That also explains OS X specials, why the permissions in /usr are not 755 for all files and directories below etc.? What are world writable directory is and whether it is a security risk?

Thanks for your help!

Daniel