How do I determine the target architecture of static library (.a) on Mac OS X?

I'm interested in verifying if a given iPhone static library has been built for ARM or Intel.

It's more curiosity than anything. Is there some kind of Mac OS X or BSD specific tool to do this? This post gives an example in Linux.

Another option is lipo; its output is brief and more readable than otool's.

An example:

% lipo -info /usr/lib/libiodbc.a 
Architectures in the fat file: /usr/lib/libiodbc.a are: x86_64 i386 ppc
% lipo -info libnonfatarchive.a
input file libnonfatarchive.a is not a fat file
Non-fat file: libnonfatarchive.a is architecture: i386
%

file will probably tell you. otool certainly should be able to. But I'd try file first, e.g.

logan:/Users/logan% file d2
d2: Mach-O executable ppc

Example with archive:

logan:/Users/logan% file /usr/lib/libMallocDebug.a
/usr/lib/libMallocDebug.a: Mach-O universal binary with 2 architectures
/usr/lib/libMallocDebug.a (for architecture i386):      current ar archive random library
/usr/lib/libMallocDebug.a (for architecture ppc):       current ar archive

As mentioned earlier, file does not always work. otool -hv -arch all is probably the closest thing that is guaranteed to work - it gives architecture information for every single object file in the library.

Example:

% otool -hv /sw/lib/libfftw3.a
Archive : /sw/lib/libfftw3.a
/sw/lib/libfftw3.a(align.o):
Mach header
magic cputype cpusubtype  caps    filetype ncmds sizeofcmds      flags
MH_MAGIC_64  X86_64        ALL  0x00      OBJECT     3        336 SUBSECTIONS_VIA_SYMBOLS
/sw/lib/libfftw3.a(alloc.o):
Mach header
magic cputype cpusubtype  caps    filetype ncmds sizeofcmds      flags
MH_MAGIC_64  X86_64        ALL  0x00      OBJECT     3        416 SUBSECTIONS_VIA_SYMBOLS
...

This bash script will help you programmatically get a list of architectures into a variable.

list_archs.sh:

#! /bin/bash
lipo -info $1 | sed -En -e 's/^(Non-|Architectures in the )fat file: .+( is architecture| are): (.*)$/\3/p'

Usage example:

./list_archs.sh /usr/lib/libc.dylib
x86_64 i386

If anyone comes here looking for answers about how to tell whether a library (or the object files in it) are intended for Mac Catalyst, use otool -l to dump the load commands. Find the LC_BUILD_VERSION section for any object. Mac Catalyst is identified by platform 6 rather than platform 1.

As an alternative, I've found objdump can work well. As an example, in my environment I build library archives with vxWorks and need to link those into other projects. To test whether the archive is the correct architecture, I could do something like the following (bash syntax):

if [ "$(objdumpsparc -a ${ARCHIVE_FILE} 2>&1 | ggrep -cvP 'elf32-sparc-vxworks')" -ne "0" ]; then
  echo "Cannot build with ${ARCHIVE_FILE}, it contains one or more non-sparc components"
fi;

This example isn't precisely correct, because some lines DO show up that don't say elf32-sparc-vxworks, but it's easy enough to adapt this.

One nice benefit of this is that objdump, or a similarly named variant, is installed on most *nix operating systems, whereas tools suggested in other responses aren't.

edit It just occurred to me the OP was asking on OSX. My apologies.