$ gedit ~/.bashrc

add the following lines at the end of the file and save:

# LD_LIBRARY_PATH
OTCL_LIB=/home/shojaru/ns-allinone-XXX/otcl-XXX
NS2_LIB=/home/shojaru/ns-allinone-XXX/lib
X11_LIB=/usr/X11R6/lib
USR_LOCAL_LIB=/usr/local/lib
export LD_LIBRARY_PATH=$LD_LIBRARY_PATH:$OTCL_LIB:$NS2_LIB:$X11_LIB:$USR_LOCAL_LIB

# TCL_LIBRARY
TCL_LIB=/home/shojaru/ns-allinone-XXX/tclXXX/library
USR_LIB=/usr/lib
export TCL_LIBRARY=$TCL_LIB:$USR_LIB

# PATH
XGRAPH=/home/shojaru/ns-allinone-XXX/bin:/home/shojaru/ns-allinone-XXX/tclXXX/unix:/home/shojaru/ns-allinone-XXX/tkXXX/unix
NS=/home/shojaru/ns-allinone-XXX/ns-XXX/
NAM=/home/shojaru/ns-allinone-XXX/nam-XXX/
PATH=$PATH:$XGRAPH:$NS:$NAM


replace the XXX with your own versions of ns2, otcl, tcl,tk,nam and also edit the path with your own path-name

Now to take effect, type :

$ source ~/.bashrc

Now,validate and save the output comments:
$ cd ns-2.28
$ ./validate > output_validate 2>&1