how can i check if the process got output?

Question

is there a way to indicate if a process like standard calculator got an output or not,
i need it because i have this line :

sr = p1.StandardOutput;

and i need to do this :

s = sr.ReadLine();

only if there's an output from p1 in calculator for example there's no output so the program stuck after the ReadLine.
thanks all.

the code :

while (i < asProcesses.Length - 1)
            {
                if ((i + 1) == asProcesses.Length - 1 && sOutRedirect != "")
                    break;
                p1.StartInfo.RedirectStandardOutput = true;
                p1.StartInfo.FileName = asProcesses[i];
                p1.StartInfo.UseShellExecute = false;
                if(i==0)
                    p1.Start();
                sr = p1.StandardOutput;
                Process p2 = new Process();
                p2.StartInfo.RedirectStandardInput = true;
                p2.StartInfo.FileName = asProcesses[i + 1];
                p2.StartInfo.UseShellExecute = false;
                p2.Start();
                sw = p2.StandardInput;
                while (!sr.EndOfStream && s != null)
                {
                    s = sr.ReadLine();
                    if (s != null)
                    {
                        sw.WriteLine(s);
                    }
                }
                if (sw != null)
                    sw.Close();
                if (sr != null)
                    sr.Close();
                i++;
            }

I posted a few comments on a similar question that may be relevant: [Execute Process Chain](http://stackoverflow.com/questions/5170273/execute-process-chain) — Tamschi, Mar 03 '11 at 01:46

Jack B Nimble · Answer 1 · 2011-03-02T19:38:00.997

1

void foo()
{
    System.Diagnostics.Process p = new System.Diagnostics.Process();
    p.StartInfo.FileName = "c:\\windows\\system32\\ping.exe";
    p.StartInfo.RedirectStandardOutput = true;
    p.StartInfo.UseShellExecute = false;
    p.EnableRaisingEvents = true;
    p.OutputDataReceived += new System.Diagnostics.DataReceivedEventHandler(p_OutputDataReceived);
    p.Start();
    p.BeginOutputReadLine();
}           

void p_OutputDataReceived(object sender, System.Diagnostics.DataReceivedEventArgs e)
{
    string s = e.Data;        
    // process s
}

edited Mar 02 '11 at 19:38

answered Mar 02 '11 at 19:04

Jack B Nimble

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@Jack i did what you said and my program stops after the while loop – Roy Gavrielov Mar 02 '11 at 19:08
You should wait for calc.exe to exit, otherwise the contents of `s` depend on a race condition. – Tamschi Mar 02 '11 at 19:08
@Tamaschi and if i don't want to wait for exit,i just want to continue with the program? – Roy Gavrielov Mar 02 '11 at 19:11
@Roy stops as in hangs? or just jumps over it? Of course calc.exe does not generate console output. – Jack B Nimble Mar 02 '11 at 19:15
@Roy You could make it time out by adding a counter and Thread.Sleep to the stream redirection loop. I'll post it as answer in a second... – Tamschi Mar 02 '11 at 19:28
added p.BeginOutputReadLine(); which you apparently need to REALLY receive output. – Jack B Nimble Mar 02 '11 at 19:37

Tamschi · Answer 2 · 2011-03-03T00:32:57.003

If all processes exit after executing, use this instead of the inner while:

p1.WaitForExit();
sw.Write(sr.ReadToEnd());

If you want the processes to time out:

int i = 0;
while (!p1.HasExited && i < maxWaits)
{
    Thread.Sleep(delay);
    i++;
}
sw.Write(sr.ReadToEnd());

//Kill process if running:
if (!p1.HasExited)
{
    try { p1.Kill(); }
    catch { }
}

Edit:

It seems you're trying to chain the output from each process to the next one. If that's the case, you're missing a p1 = p2 at the end of the loop.
Also, consider to move the first starting process out of the loop: It will make your code much more readable. Setting p1's StartInfo should be moved into the if (i == 0) block if you leave it this way. Moving the output read last process out wouldn't be a bad idea either, in my opinion...

Edit:

This is my solution (with timing out):

        int maxWaits = 10; // Wait 1 second at most.
        int delay = 100;

        var p = new Process();
        p.StartInfo.RedirectStandardOutput = true;
        p.StartInfo.FileName = asProcesses[0];
        p.StartInfo.UseShellExecute = false;
        p.Start();

        foreach (var path in asProcesses.Skip(1))
        {
            var p2 = new Process();
            p2.StartInfo.FileName = path;
            p2.StartInfo.RedirectStandardInput = true;
            p2.StartInfo.RedirectStandardOutput = true;
            p2.StartInfo.UseShellExecute = false;

            {
                int i = 0;
                while (!p.HasExited && i < maxWaits)
                {
                    p2.StandardInput.Write(p.StandardOutput.ReadToEnd()); //Redirect IO. This line means that the second process can start calculations if the first is long-running and writes its output progressively.
                    Thread.Sleep(delay);
                    i++;
                }
            }

            p2.StandardInput.Write(p.StandardOutput.ReadToEnd()); //Redirect last output from p.

            {
                //Kill process if still running:
                if (!p.HasExited)
                {
                    try { p.Kill(); }
                    catch { }
                }
            }
        }

        {
            int i = 0;
            while (!p.HasExited && i < maxWaits)
            {
                Thread.Sleep(delay);
                i++;
            }
        }

        string result = p.StandardOutput.ReadToEnd();
        {
            if (!p.HasExited)
            {
                try { p.Kill(); }
                catch { }
            }
        }

Edit:

Algorithm that waits for each process to exit:

        var p = new Process();
        p.StartInfo.RedirectStandardOutput = true;
        p.StartInfo.FileName = asProcesses[0];
        p.StartInfo.UseShellExecute = false;
        p.Start();

        foreach (var path in asProcesses.Skip(1))
        {
            var p2 = new Process();
            p2.StartInfo.FileName = path;
            p2.StartInfo.RedirectStandardInput = true;
            p2.StartInfo.RedirectStandardOutput = true;
            p2.StartInfo.UseShellExecute = false;

            p.WaitForExit();
            p2.StandardInput.Write(p.StandardOutput.ReadToEnd());
        }

        p.WaitForExit();
        string result = p.StandardOutput.ReadToEnd();

I moved the first process out of the loop to get rid of the conditional. The control flow is simpler this way and it's easier to add code affection the first process specifically.

thanks for the answer, you're right i'm trying to implement piping, and i don't want to kill any process. for example, if the command is calc|calc|calc - the calculator should be opened 3 times — Roy Gavrielov, Mar 02 '11 at 20:50

how can i check if the process got output?

2 Answers2

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