Assuming I want to test if a library is installed and usable by a program. I can use ldconfig -p | grep mylib to find out if it's installed on the system. but what if the library is only known via setting LD_LIBRARY_PATH?

In that case, the program may be able to find the library, but ldconfig won't. How can I check if the library is in the combined linker path?

I'll add that I'm looking for a solution that will work even if I don't actually have the program at hand (e.g. the program isn't compiled yet), I just want to know that a certain library exists in ld's paths.

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    You could use ldd <binary> to check if all libraries linked are in the path. Maybe there is a more elegant way. – Thomas May 10 '16 at 9:33
  • @Thomas I think you should make an answer of your comment. ldd serves exactly this purpose. – lgeorget May 10 '16 at 9:41
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    @Thomas - but what if I the program isn't compiled yet, and that library is needed for compilation? – nbubis May 10 '16 at 9:41
  • @Igeorget - see my edit / comment – nbubis May 10 '16 at 9:44
  • @nbubis: when you need the library for compilation, you normally have to use LIBRARY_PATH environment variable which is looked up e.g. by the gcc compiler. The LIBRARY_PATH environment variable also has a colon seperated list of directories. – Thomas May 10 '16 at 10:09

ldconfig can list all the libraries it has access to. These libraries are also stored in its cache.

/sbin/ldconfig -v -N will crawl all the usual library paths, list all the available libraries, without reconstructing the cache (which is not possible if you're a non-root user). It does NOT take into account libraries in LD_LIBRARY_PATH (contrarily to what this post said before edit) but you can pass additional libraries to the command line by using the line below:

/sbin/ldconfig -N -v $(sed 's/:/ /g' <<< $LD_LIBRARY_PATH)
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  • ldconfig is a tool from the 1980s that was given up in the 1990s. For this reason, ldconfig is not portable as it only applies to implementations that are based on the SunOS-4.0 a.out based dynamic linker from 1987. – schily May 10 '16 at 10:00
  • How does that help listing libs which are only on LD_LIBRARY_PATH? I guess I could write a script to parse that and then read that through ldconfig, but it seems a bit like an over kill. – nbubis May 10 '16 at 10:10
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    @nbubis you can always pass the libraries in LD_LIBRARY_PATH to ldconfig. Like /sbin/ldconfig -N -v $(sed 's/:/ /' <<< $LD_LIBRARY_PATH) not too much of an overkill to my taste. – lgeorget May 10 '16 at 10:32
  • @schily Really? It works pretty well with ELF libraries as far as I can tell. And it's still used, ld.so relies on the cache built by ldconfig. – lgeorget May 10 '16 at 10:33
  • The Linux people hacked the old Sun linker source-code to support ELF. The new ELF implementation from Sun removed ldconfig in an agreement with AT&T as the linker caching was too sensitive to errors. Sun later added the tree stooges: moe, lari and crle as helper programs for ELF management. – schily May 10 '16 at 10:52

Globally substitute (space) for : with LD_LIBRARY_PATH

/sbin/ldconfig -N -v $(sed 's/:/ /g' <<< $LD_LIBRARY_PATH)
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You can compile a simple test program with gcc and link your library. Then you can check the used libraries with ldd. I use something like this:

echo "int main(){}" | gcc -x c++ -Wl,--no-as-needed -lmylib - && ldd a.out | grep mylib

-Wl,--no-as-needed prevents the linker from discarding the library, because no symbols from the library are used.

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