I‘ve compile and installed a library at /usr/local/lib/libapp2.so -> libapp2.so.1.4.3. How do I set the Library path under Linux operating systems?

You need to use ldconfig config file and ldconfig command which creates the necessary links and cache to the most recent shared libraries found in the directories specified on the command line, in the file /etc/ld.so.conf, and in the trusted directories such as /lib64 or /usr/lib64 (/lib or /usr/lib on 32 bit systems). The /etc/ld.so.conf contains lib settings which can be used to add or delete paths. However, you need to simply drop your config file in /etc/ld.so.conf.d/ directory and it will be used by /sbin/ldconfig to configure dynamic linker run time bindings.

Add Your Path

Create a file called /etc/ld.so.conf.d/myapp.conf:
# vi /etc/ld.so.conf.d/myapp.conf
Add the following path:

/usr/local/lib

Save and close the file.

Activate Your Library Path

You must run the following command to activate path:
# ldconfig

Verify Your New Library Path

# ldconfig -v | less
OR
# ldconfig -v | grep /usr/local/lib
Sample outputs:

/usr/local/lib:
        libGeoIP.so.1 -> libGeoIP.so.1.4.6
        libGeoIPUpdate.so.0 -> libGeoIPUpdate.so.0.0.0
/usr/lib64/mysql:
        libmysqlclient_r.so.15 -> libmysqlclient_r.so.15.0.0
        libmysqlclient.so.15 -> libmysqlclient.so.15.0.0
/lib:
        libutil.so.1 -> libutil-2.5.so

How Do I Delete The Library Path?

Simply, delete the file:
# rm /etc/ld.so.conf.d/myapp.conf
# ldconfig

How Do I Edit The Library Path?

Simply edit the file and reload the changes:
# vi /etc/ld.so.conf.d/myapp.conf
# ldconfig

How Do I Compile Program With Shared Libs And GNU GCC?

You can use the following gcc
$ gcc -Wl,-R/path/to/lib -I/path/to/include -L/path/to/lib -o myAppName mycode.c -llibapp2
See gcc man page for further details.

Further readings:

• Linux Commands For Shared Library Management & Debugging Problem
• See the following man pages for more information:

ldconfig(8)
ld(1)
gcc(1)