Where is the Ld_library_path set?

You can set it in your ~/. profile and/or specific init file of your shell (e.g. ~/. bashrc for bash, ~/. zshenv for zsh).

What is Ld_library_path in Linux?

LD_LIBRARY_PATH is the predefined environmental variable in Linux/Unix which sets the path which the linker should look in to while linking dynamic libraries/shared libraries. … The best way to use LD_LIBRARY_PATH is to set it on the command line or script immediately before executing the program.