Fetching errors after running grep command from java

Question

I am running grep command via my Java program. Running grep on the command line sometimes writes an error on stderr of the kind: No such file or directory. I want to detect in my Java program whenever this error happens as a result of executing the grep command via the program. How can I achieve this goal of mine? This is what I've written so far:

    Runtime rt = Runtime.getRuntime();
    String[] cmd = {"/bin/sh", "-c", "grep -c 'Search_String' /path/to/file(s)/being/searched"};
    Process proc = rt.exec(cmd);
    BufferedReader is = new BufferedReader(new 
    InputStreamReader(proc.getInputStream()));
    String line;
    while ((line = is.readLine()) != null) {
        System.out.println(line);
    }

Any reason you can't check that the file exists before passing it to grep? — Joe C
– Joe C, Commented Apr 24, 2017 at 20:59
Check the exitValue. Grep returns 0 for success. If it returns a non-zero value, you're going to have to parse the output. — Dakoda
– Dakoda, Commented Apr 24, 2017 at 21:04
@JoeC I can check but this would make it more comfortable for me when I am trying to run the command against lots of files — Harshit Sharma
– Harshit Sharma, Commented Apr 24, 2017 at 21:11
For your code: int retVal = proc.waitFor(); // wait for return value — Dakoda
– Dakoda, Commented Apr 24, 2017 at 21:34
Don't check Process.exitValue(). It won't be there until it returns. It creates another Thread to execute the command. waitFor() will wait until it returns. — Dakoda
– Dakoda, Commented Apr 24, 2017 at 21:40

John Weldon · Accepted Answer · 2017-04-24 22:07:18Z

1

You can detect if the process returned with an error; as @Dakoda mentions, exitValue() won't have the exit value until the process ends, but using waitFor() will block until the process ends and returns the exit value:

int rv = rt.waitFor();
if (rv != 0) { ... }

Error output is usually on stderr rather than stdout, so to read errors you'd use:

BufferedReader is = new BufferedReader(
  new InputStreamReader(proc.getErrorStream()));

edited Apr 24, 2017 at 22:07

answered Apr 24, 2017 at 21:34

John Weldon

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