I am using powermock to mock some native command invocation using process builder. the strange thing is these test pass sometimes and fail sometimes giving a NPE. Is this a powermock issue or some gotcha in the program. the snippet of the class under test is.. public void method1(String jsonString, String filename) { try { JSONObject jObj = new JSONObject(jsonString); JSONArray jArr = jObj.getJSONArray("something"); String cmd = "/home/y/bin/perl <perlscript>.pl<someConstant>" + " -k " + <someConstant> + " -t " + <someConstant>; cmd += vmArr.getJSONObject(i).getString("jsonKey"); ProcessBuilder pb = new ProcessBuilder("bash", "-c", cmd); pb.redirectErrorStream(false); Process shell = pb.start(); shell.waitFor(); if (shell.exitValue() != 0) { throw new RuntimeException("Error in Collecting the logs. cmd="+cmd); } StringBuilder error = new StringBuilder(); InputStream iError = shell.getErrorStream(); BufferedReader bfr = new BufferedReader( new InputStreamReader(iError)); String line = null; while ((line = bfr.readLine()) != null) { error.append(line + "\n"); } if (!error.toString().isEmpty()) { LOGGER.error(error`enter code here`); } iError.close(); bfr.close(); } catch (Exception e) { throw new RuntimeException(e); } and the unit test case is .. @PrepareForTest( {.class, ProcessBuilder.class,Process.class, InputStream.class,InputStreamReader.class, BufferedReader.class} ) @Test(sequential=true) public class TestClass { @Test(groups = {"unit"}) public void testMethod() { try { ProcessBuilder prBuilderMock = createMock(ProcessBuilder.class); Process processMock = createMock(Process.class); InputStream iStreamMock = createMock(InputStream.class); InputStreamReader iStrRdrMock = createMock(InputStreamReader.class); BufferedReader bRdrMock = createMock(BufferedReader.class); String errorStr =" Error occured"; String json = <jsonStringInput>; String cmd = "/home/y/bin/perl <perlscript>.pl -k "+<someConstant>+" -t "+<someConstant>+" "+<jsonValue>; expectNew(ProcessBuilder.class, "bash", "-c", cmd).andReturn(prBuilderMock); expect(prBuilderMock.redirectErrorStream(false)).andReturn(prBuilderMock); expect(prBuilderMock.start()).andReturn(processMock); expect(processMock.waitFor()).andReturn(0); expect(processMock.exitValue()).andReturn(0); expect(processMock.getErrorStream()).andReturn(iStreamMock); expectNew(InputStreamReader.class, iStreamMock) .andReturn(iStrRdrMock); expectNew(BufferedReader.class, iStrRdrMock) .andReturn(bRdrMock); expect(bRdrMock.readLine()).andReturn(errorStr); expect(bRdrMock.readLine()).andReturn(null); iStreamMock.close(); bRdrMock.close(); expectLastCall().once(); replayAll(); <ClassToBeTested> instance = new <ClassToBeTested>(); instance.method1(json, fileName); verifyAll(); } catch (Exception e) { Assert.fail("failed while collecting log.", e); } } I get an error on execution an the test case fails.. Caused by: java.lang.NullPointerException at java.lang.ProcessBuilder.start(ProcessBuilder.java:438) Note : I do not get this error on all executions. Sometimes it passes and sometimes it fails. I am not able to understand this behavior.