Process.Start() differences between Windows and Linux

Question

I'm experiencing some differences in Process.Start() in a .NET Core 2.2 project. You can find the full source in this gist: https://gist.github.com/jchristn/5a2a301baedeed787a2e57cd528e46d6

I have a method for starting processes:

private static void ExecuteShell(
    string filename, 
    string args, 
    bool useShellExecute,
    bool redirectStdOut,
    bool redirectStdErr,
    out int returnCode, 
    out string consoleOutput)
{
    returnCode = 0;
    consoleOutput = null;

    if (String.IsNullOrEmpty(filename)) throw new ArgumentNullException(nameof(filename));

    // fileName, i.e. "cmd.exe"
    // args, i.e.     "/c dir /w"

    Process process = new Process();
    process.StartInfo.FileName = filename;
    process.StartInfo.Arguments = args;
    process.StartInfo.UseShellExecute = useShellExecute;
    process.StartInfo.RedirectStandardOutput = redirectStdOut;
    process.StartInfo.RedirectStandardError = redirectStdErr;
    process.Start();
    if (process.StartInfo.RedirectStandardOutput) consoleOutput = process.StandardOutput.ReadToEnd();
    process.WaitForExit();
    returnCode = process.ExitCode;
}

And the caller looks like this:

while (true)
{
    try
    {
        Console.WriteLine("");
        Console.WriteLine("Example: cmd.exe /c dir /w");
        Console.Write("Command: ");
        string userInput = Console.ReadLine();
        if (String.IsNullOrEmpty(userInput)) break;

        string[] parts = userInput.Split(new char[] { ' ' }, 2);

        string filename = parts[0];
        string arg = null;
        if (parts.Length > 1) arg = parts[1];

        bool useShellExecute = InputBoolean("  Use shell execute  : ", false);
        bool redirectStdOut =  InputBoolean("  Redirect stdout    : ", false);
        bool redirectStdErr =  InputBoolean("  Redirect stderr    : ", false);

        int returnCode;
        string consoleOutput;

        ExecuteShell(
            filename,
            arg,
            useShellExecute,
            redirectStdOut,
            redirectStdErr,
            out returnCode, out consoleOutput);

        Console.WriteLine("Return code    : " + returnCode);
        Console.WriteLine("Console output : " + consoleOutput);
    }
    catch (Exception e)
    {
        Console.WriteLine("Exception: " + Environment.NewLine + SerializeJson(e, true));
    }
}

The simplest way to reproduce the larger issue I'm having is this. Assume there is a file on the file system and I want to type testfile > testfile2, i.e. to pipe to another file.

On Windows, if I use cmd.exe /c type testfile > testfile2 it works great (with the three Boolean values set to false).

i.e.

C:\Code\ExecuteShell\ExecuteShell\bin\Debug
etcoreapp2.2>dotnet ExecuteShell.dll

Example: cmd.exe /c dir /w
Command: cmd.exe /c type testfile > testfile2
  Use shell execute  :  [y/N]?
  Redirect stdout    :  [y/N]?
  Redirect stderr    :  [y/N]?
Return code    : 0
Console output :

Example: cmd.exe /c dir /w
Command:
C:\Code\ExecuteShell\ExecuteShell\bin\Debug
etcoreapp2.2>dir
 Volume in drive C is OS
 Volume Serial Number is 72E2-466A

 Directory of C:\Code\ExecuteShell\ExecuteShell\bin\Debug
etcoreapp2.2

... portions removed ...
04/09/2020  05:41 PM                15 testfile
04/09/2020  05:41 PM                15 testfile2

When I try this on Ubuntu 14.04 it fails.

~/code/ExecuteShell/ExecuteShell/bin/Debug/netcoreapp2.2/publish$ dotnet ExecuteShell.dll

Example: cmd.exe /c dir /w
Command: cat testfile > testfile2
  Use shell execute  :  [y/N]?
  Redirect stdout    :  [y/N]?
  Redirect stderr    :  [y/N]?
Hello, world!

cat: >: No such file or directory
cat: testfile2: No such file or directory
Return code    : 1
Console output :

If I try it with useShellExecute set to true, I get this strange xdg-open issue:

~/code/ExecuteShell/ExecuteShell/bin/Debug/netcoreapp2.2/publish$ dotnet ExecuteShell.dll

Example: cmd.exe /c dir /w
Command: cat testfile > testfile2
  Use shell execute  :  [y/N]? y
  Redirect stdout    :  [y/N]?
  Redirect stderr    :  [y/N]?
xdg-open: unexpected argument 'testfile'
Try 'xdg-open --help' for more information.
Return code    : 1
Console output :

Any idea how to make this work?

bk2204 · Accepted Answer

In this case, you're confusing the use of the word “shell” in its Unix context (a command line interpreter) with its Windows and C# use:

The word "shell" in this context (UseShellExecute) refers to a graphical shell (similar to the Windows shell) rather than command shells (for example, bash or sh) and lets users launch graphical applications or open documents.

So useShellExecute actually means that you can give the program a document of some type and expect it to be opened by a suitable program. That's what xdg-open does, so that's probably why C# invokes it.

In your case, the command you want to run is sh -c 'cat testfile > testfile2'. That's the equivalent of your cmd invocation. However, your code won't work if you do that now because you split on whitespace. So you'll end up with the arguments sh, -c, 'cat, testfile, >, and testfile2'. Unlike cmd, which is responsible for its own argument processing, sh does not concatenate its commands with whitespace, and this won't work.

If you were passing this as an array of arguments, you'd want your arguments to be sh, -c, and cat testfile > testfile2; that is, the entire string you want to pass to the shell should be one complete argument.

Process.Start() differences between Windows and Linux

Answers (2)

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