Quiero ejecutar comandos NS2 (que es un programa externo en Linux) a través de Java, usando ProcessBuilder cuando obtengo el error de comando ns no encontrado

 /home/maria/Documents/test.sh: line 4: ns: command not found Execution failed. org.apache.commons.exec.ExecuteException: Process exited with an error: 127 (Exit value: 127) at org.apache.commons.exec.DefaultExecutor.executeInternal(DefaultExecutor.java:404) at org.apache.commons.exec.DefaultExecutor.execute(DefaultExecutor.java:166) at org.apache.commons.exec.DefaultExecutor.execute(DefaultExecutor.java:153) at test.opencmd.runScript(opencmd.java:18) at test.opencmd.main(opencmd.java:30)

mi codigo java es

 package test; import java.io.*; public class test2 { public static void main(String args[]) { String s = null; try { // run the Unix "}; //System.out.print(System.getProperty("user.home")); Process p = Runtime.getRuntime().exec( "ns /home/maria/ns-allinone-2.35/ns-2.35/indep-utils/cmu-scen-gen && cbrgen.tcl -type cbr -nn 10 -seed 1 -mc 5 -rate 5.0"); BufferedReader stdInput = new BufferedReader(new InputStreamReader(p.getInputStream())); BufferedReader stdError = new BufferedReader(new InputStreamReader(p.getErrorStream())); // read the output from the command System.out.println("Here is the standard output of the command:\n"); while ((s = stdInput.readLine()) != null) { System.out.println(s); } // read any errors from the attempted command System.out.println("Here is the standard error of the command (if any):\n"); while ((s = stdError.readLine()) != null) { System.out.println(s); } System.exit(0); } catch (IOException e) { System.out.println("exception happened - here's what I know: "); e.printStackTrace(); System.exit(-1); } } }

Supongo que no estoy haciendo una llamada de programa correcta.