$ 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