To create symlink /opt/foo –> /usr/bin/bar, (i.e., create symlink at /opt/foo which references to file /usr/bin/bar) do:
ln -s /usr/bin/bar /opt/foo
References :
http://askubuntu.com/questions/56339/how-to-create-a-soft-or-symbolic-link
https://en.wikipedia.org/wiki/Symbolic_link
http://manpages.ubuntu.com/manpages/wily/en/man1/ln.1posix.html