I have deleted the Lucida Grande font from my system, and system bars and menus are full of weird characters. I cannot use my MBA properly. How can I restore system fonts without losing my data?
Place LucidaGrande.ttc in /System/Library/Fonts/.
You can extract fonts from your OS X install media using Pacifist or grab a copy from your backup or another Mac with similar build/version.
For more information regarding recovering system files, see:
You can also copy the font from the recovery partition:
diskutil mount Recovery\ HD;hdiutil mount /Volumes/Recovery\ HD/com.apple.recovery.boot/BaseSystem.dmg;sudo cp /Volumes/OS\ X\ Base\ System/System/Library/Fonts/Base/LucidaGrande.ttc /System/Library/Fonts
You can also use
sudo fontrestore default.
From the man page:
NAME fontrestore -- Restore the available system fonts to a pristine state, if possible. SYNOPSIS fontrestore default [-n] DESCRIPTION fontrestore detects fonts in /Library/Fonts, /System/Library/Fonts, and ~/Library/Fonts which are not part of the system install and moves them to /Library/Fonts (Removed), /System/Library/Fonts (Removed), and ~/Library/Fonts/Fonts (Removed) directories. Critical fonts required for operation of the user interface are also restored. The [-n] option performs the same operation without touching the disk, describing what would be done without actually doing it.