Difference between lib, lib32, lib64, libx32, and libexec

Question

My 64 bit Ubuntu 13.04 system has the following directories in /:

lib
lib32
lib64
libx32
libexec

In the /usr directory there is:

lib
lib32
libx32
libexec

This seemed like something that could be easily answered with a search, but I found nothing online, other than that these directories store shared libraries (except libexec). But which shared libraries go in which folders (other than 32bit in lib32 and 64 bit in lib64)? Could someone explain the difference between all of these directories?

Answer 1

Ah yes this is a very confusing part if you've dealt with Unixes for any length of time. There is a standard that most Unixes "try" to follow called FHS - Filesystem Hierarchy Standard.

Given I primarily use Red Hat based distros I'm most familiar with their take on FHS for Fedora, CentOS, and RHEL Linux distros. But I've used Debian & BSD based distros as well and they're not all that different in terms of where things are kept, filesystem wise.

Now to your questions. I would take a look at the FHS document, which loosely governs these directory structures. In general:

Directory - /lib

Contains essential shared libraries and kernel modules.

Purpose: The /lib directory contains those shared library images needed to boot the system and run the commands in the root filesystem, ie. by binaries in /bin and /sbin.

Note1: Shared libraries that are only necessary for binaries in /usr (such as any X Window binaries) must not be in /lib. Only the shared libraries required to run binaries in /bin and /sbin may be here.

Note2: Given the primary purpose of /lib is to contain libraries for tools deployed to the directories /bin & /sbin, the libraries in /lib can be either 32-bit or 64-bit.

For example (Fedora 14 64-bit system)

$ uname -a
Linux grinchy 2.6.35.14-106.fc14.x86_64 #1 SMP Wed Nov 23 13:07:52 UTC 2011 x86_64 x86_64 x86_64 GNU/Linux

Here's a sampling of the files from my /lib

./libpam.so.0.82.2:             ELF 32-bit LSB shared object, Intel 80386, version 1 (SYSV), dynamically linked, stripped
./libplc4.so:                   ELF 32-bit LSB shared object, Intel 80386, version 1 (SYSV), dynamically linked, stripped
./libidn.so.11.6.1:             ELF 32-bit LSB shared object, Intel 80386, version 1 (SYSV), dynamically linked, stripped
./upstart/telinit:              ELF 64-bit LSB executable, x86-64, version 1 (SYSV), dynamically linked (uses shared libs), for GNU/Linux 2.6.32, stripped
./upstart/runlevel:             ELF 64-bit LSB executable, x86-64, version 1 (SYSV), dynamically linked (uses shared libs), for GNU/Linux 2.6.32, stripped
./upstart/shutdown:             ELF 64-bit LSB executable, x86-64, version 1 (SYSV), dynamically linked (uses shared libs), for GNU/Linux 2.6.32, stripped
./upstart/reboot:               ELF 64-bit LSB executable, x86-64, version 1 (SYSV), dynamically linked (uses shared libs), for GNU/Linux 2.6.32, stripped
./libdb-4.8.so:                 ELF 32-bit LSB shared object, Intel 80386, version 1 (SYSV), dynamically linked, stripped
./firmware/mixart/miXart8.elf:  ELF 32-bit MSB executable, PowerPC or cisco 4500, version 1 (SYSV), statically linked, not stripped
./libtinfo.so.5.7:              ELF 32-bit LSB shared object, Intel 80386, version 1 (SYSV), dynamically linked, stripped

Directory - /lib<qual>

Alternate format essential shared libraries (optional). These would be the directories /lib32, /lib64, etc.

Purpose: There may be one or more variants of the /lib directory on systems which support more than one binary format requiring separate libraries. This is commonly used for 64-bit or 32-bit support on systems which support multiple binary formats, but require libraries of the same name.

Note: In this case, /lib32 and /lib64 might be the library directories, and /lib a symlink to one of them.

Directory - /usr/lib

Libraries for programming and packages.

Purpose: /usr/lib includes object files, libraries, and internal binaries that are not intended to be executed directly by users or shell scripts.

Note1: Miscellaneous architecture-independent application-specific static files and subdirectories must be placed in /usr/share.

Applications may use a single subdirectory under /usr/lib. If an application uses a subdirectory, all architecture-dependent data exclusively used by the application must be placed within that subdirectory.

Note2: For example, the perl5 subdirectory for Perl 5 modules and libraries.

Directory - /usr/lib<qual>

Alternate format libraries (optional).

Purpose: /usr/lib<qual> performs the same role as /usr/lib for an alternate binary format, except that the symbolic links /usr/lib<qual>/sendmail and /usr/lib<qual>/X11 are not required.

Note: The case where /usr/lib and /usr/lib<qual> are the same (one is a symbolic link to the other) these files and the per-application subdirectories will exist.

TLDR;

In general:

If there are libraries that are required by an executable in either the /bin or /sbin directories, those libraries should be going in the /lib* directories.

If there are libraries for use programs and packages they go in /usr/lib/*. If there are executables that are needed by a particular library, but these executables aren't suppose to be called by the users directly or by root, they go in /usr/libexec.