pipe - Getting output from a shell/dos app into a Delphi app

Question

I have a commandline application coded in delphi that I need to call from a normal desktop application (also coded in delphi). In short, I want to call the commandline app and display the text it outputs "live" in a listbox.

It's been ages since I have played around with the shell, but I distinctly remember that in order to grab the text from a commandline app - I have to use the pipe symbol ">". Like this:

C:/mycmdapp.exe >c:/result.txt

This will take any text printed to the shell (using writeLn) and dump it to a textfile called "result.txt".

But.. (and here comes the pickle), I want a live result rather than a backlog file. A typical example is the Delphi compiler itself - which manages to report back to the IDE what is going on. If my memory serves me correctly, I seem to recall that I must create a "pipe" channel (?), and then assign the pipe-name to the shell call.

I have tried to google this but I honestly was unsure of how to formulate it. Hopefully someone from the community can point me in the right direction.

Updated: This question might be identical to How do I run a command-line program in Delphi?. Some of the answers fit what I'm looking for, although the title and question itself is not identical.

Answer 1

As ever so often Zarco Gajic has a solution: Capture the output from a DOS (command/console) Window. This is a copy from his article for future reference:

The example runs 'chkdsk.exe c:' and displays the output to Memo1. Put a TMemo (Memo1) and a TButton (Button1) on your form. Put this code in the OnCLick event procedure for Button1:

procedure RunDosInMemo(DosApp: string; AMemo:TMemo);
const
    READ_BUFFER_SIZE = 2400;
var
    Security: TSecurityAttributes;
    readableEndOfPipe, writeableEndOfPipe: THandle;
    start: TStartUpInfo;
    ProcessInfo: TProcessInformation;
    Buffer: PAnsiChar;
    BytesRead: DWORD;
    AppRunning: DWORD;
begin
    Security.nLength := SizeOf(TSecurityAttributes);
    Security.bInheritHandle := True;
    Security.lpSecurityDescriptor := nil;

    if CreatePipe({var}readableEndOfPipe, {var}writeableEndOfPipe, @Security, 0) then
    begin
        Buffer := AllocMem(READ_BUFFER_SIZE+1);
        FillChar(Start, Sizeof(Start), #0);
        start.cb := SizeOf(start);

        // Set up members of the STARTUPINFO structure.
        // This structure specifies the STDIN and STDOUT handles for redirection.
        // - Redirect the output and error to the writeable end of our pipe.
        // - We must still supply a valid StdInput handle (because we used STARTF_USESTDHANDLES to swear that all three handles will be valid)
        start.dwFlags := start.dwFlags or STARTF_USESTDHANDLES;
        start.hStdInput := GetStdHandle(STD_INPUT_HANDLE); //we're not redirecting stdInput; but we still have to give it a valid handle
        start.hStdOutput := writeableEndOfPipe; //we give the writeable end of the pipe to the child process; we read from the readable end
        start.hStdError := writeableEndOfPipe;

        //We can also choose to say that the wShowWindow member contains a value.
        //In our case we want to force the console window to be hidden.
        start.dwFlags := start.dwFlags + STARTF_USESHOWWINDOW;
        start.wShowWindow := SW_HIDE;

        // Don't forget to set up members of the PROCESS_INFORMATION structure.
        ProcessInfo := Default(TProcessInformation);

        //WARNING: The unicode version of CreateProcess (CreateProcessW) can modify the command-line "DosApp" string. 
        //Therefore "DosApp" cannot be a pointer to read-only memory, or an ACCESS_VIOLATION will occur.
        //We can ensure it's not read-only with the RTL function: UniqueString
        UniqueString({var}DosApp);

        if CreateProcess(nil, PChar(DosApp), nil, nil, True, NORMAL_PRIORITY_CLASS, nil, nil, start, {var}ProcessInfo) then
        begin
            //Wait for the application to terminate, as it writes it's output to the pipe.
            //WARNING: If the console app outputs more than 2400 bytes (ReadBuffer),
            //it will block on writing to the pipe and *never* close.
            repeat
                Apprunning := WaitForSingleObject(ProcessInfo.hProcess, 100);
                Application.ProcessMessages;
            until (Apprunning <> WAIT_TIMEOUT);

            //Read the contents of the pipe out of the readable end
            //WARNING: if the console app never writes anything to the StdOutput, then ReadFile will block and never return
            repeat
                BytesRead := 0;
                ReadFile(readableEndOfPipe, Buffer[0], READ_BUFFER_SIZE, {var}BytesRead, nil);
                Buffer[BytesRead]:= #0;
                OemToAnsi(Buffer,Buffer);
                AMemo.Text := AMemo.text + String(Buffer);
            until (BytesRead < READ_BUFFER_SIZE);
        end;
        FreeMem(Buffer);
        CloseHandle(ProcessInfo.hProcess);
        CloseHandle(ProcessInfo.hThread);
        CloseHandle(readableEndOfPipe);
        CloseHandle(writeableEndOfPipe);
    end;
end;

procedure TForm1.Button1Click(Sender: TObject);
begin {button 1 code}
   RunDosInMemo('chkdsk.exe c:',Memo1);
end;

Update: The above example reads the output in one step. Here is another example from DelphiDabbler showing how the output can be read while the process is still running:

function GetDosOutput(CommandLine: string; Work: string = 'C:'): string;
var
  SA: TSecurityAttributes;
  SI: TStartupInfo;
  PI: TProcessInformation;
  StdOutPipeRead, StdOutPipeWrite: THandle;
  WasOK: Boolean;
  Buffer: array[0..255] of AnsiChar;
  BytesRead: Cardinal;
  WorkDir: string;
  Handle: Boolean;
begin
  Result := '';
  with SA do begin
    nLength := SizeOf(SA);
    bInheritHandle := True;
    lpSecurityDescriptor := nil;
  end;
  CreatePipe(StdOutPipeRead, StdOutPipeWrite, @SA, 0);
  try
    with SI do
    begin
      FillChar(SI, SizeOf(SI), 0);
      cb := SizeOf(SI);
      dwFlags := STARTF_USESHOWWINDOW or STARTF_USESTDHANDLES;
      wShowWindow := SW_HIDE;
      hStdInput := GetStdHandle(STD_INPUT_HANDLE); // don't redirect stdin
      hStdOutput := StdOutPipeWrite;
      hStdError := StdOutPipeWrite;
    end;
    WorkDir := Work;
    Handle := CreateProcess(nil, PChar('cmd.exe /C ' + CommandLine),
                            nil, nil, True, 0, nil,
                            PChar(WorkDir), SI, PI);
    CloseHandle(StdOutPipeWrite);
    if Handle then
      try
        repeat
          WasOK := ReadFile(StdOutPipeRead, Buffer, 255, BytesRead, nil);
          if BytesRead > 0 then
          begin
            Buffer[BytesRead] := #0;
            Result := Result + Buffer;
          end;
        until not WasOK or (BytesRead = 0);
        WaitForSingleObject(PI.hProcess, INFINITE);
      finally
        CloseHandle(PI.hThread);
        CloseHandle(PI.hProcess);
      end;
  finally
    CloseHandle(StdOutPipeRead);
  end;
end;