I create a static library project in my xCode and build a .a named mylib.a. There some code (.m files) and another static lib file named common.a to inform the static library nylib.a. But I doubt that if the contend of common.a are pack in the mylib.a really. So how can I observe the content of the mylib.a in mac OS? I knew there are some command such as ar, nm to complete this task in Linux. but It does't run in mac OS.
"nm" most certainly does exist on the Mac (in the "/usr/bin/" folder).
If you do not see it there, then you probably need to install the "Command Line Tools for Xcode" that you'll find at https://developer.apple.com/downloads/index.action (you'll need an Apple Developer login to get access to this).
"nm" will dump all the symbols in a static library file for you. Here is the manual page for it.