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.