Capturing output from WshShell.Exec using Windows Script Host

Question

I wrote the following two functions, and call the second (\"callAndWait\") from JavaScript running inside Windows Script Host. My overall intent is to call one command line

Answer 1

Yes, the Exec function seems to be broken when it comes to terminal output.

I have been using a similar function function ConsumeStd(e) {WScript.StdOut.Write(e.StdOut.ReadAll());WScript.StdErr.Write(e.StdErr.ReadAll());} that I call in a loop similar to yours. Not sure if checking for EOF and reading line by line is better or worse.

Answer 2

First, your loop is broken in that it always tries to read from oExec.StdOut first. If there is no actual output then it will hang until there is. You wont see any StdErr output until StdOut.atEndOfStream becomes true (probably when the child terminates). Unfortunately, there is no concept of non-blocking I/O in the script engine. That means calling read and having it return immediately if there is no data in the buffer. Thus there is probably no way to get this loop to work as you want. Second, WShell.Run does not provide any properties or methods to access the standard I/O of the child process. It creates the child in a separate window, totally isolated from the parent except for the return code. However, if all you want is to be able to SEE the output from the child then this might be acceptable. You will also be able to interact with the child (input) but only through the new window (see SendKeys).

As for using ReadAll(), this would be even worse since it collects all the input from the stream before returning so you wouldn't see anything at all until the stream was closed. I have no idea why the example places the ReadAll in a loop which builds a string, a single if (!WScript.StdIn.AtEndOfStream) should be sufficient to avoid exceptions.

Another alternative might be to use the process creation methods in WMI. How standard I/O is handled is not clear and there doesn't appear to be any way to allocate specific streams as StdIn/Out/Err. The only hope would be that the child would inherit these from the parent but that's what you want, isn't it? (This comment based upon an idea and a little bit of research but no actual testing.)

Basically, the scripting system is not designed for complicated interprocess communication/synchronisation.

Note: Tests confirming the above were performed on Windows XP Sp2 using Script version 5.6. Reference to current (5.8) manuals suggests no change.

Answer 3

You cannot read from StdErr and StdOut in the script engine in this way, as there is no non-blocking IO as Code Master Bob says. If the called process fills up the buffer (about 4KB) on StdErr while you are attempting to read from StdOut, or vice-versa, then you will deadlock/hang. You will starve while waiting for StdOut and it will block waiting for you to read from StdErr.

The practical solution is to redirect StdErr to StdOut like this:

sCommandLine = """c:\Path\To\prog.exe"" Argument1 argument2"
Dim oExec
Set oExec = WshShell.Exec("CMD /S /C "" " & sCommandLine & " 2>&1 """)

In other words, what gets passed to CreateProcess is this:

CMD /S /C " "c:\Path\To\prog.exe" Argument1 argument2 2>&1 "

This invokes CMD.EXE, which interprets the command line. /S /C invokes a special parsing rule so that the first and last quote are stripped off, and the remainder used as-is and executed by CMD.EXE. So CMD.EXE executes this:

"c:\Path\To\prog.exe" Argument1 argument2 2>&1

The incantation 2>&1 redirects prog.exe's StdErr to StdOut. CMD.EXE will propagate the exit code.

You can now succeed by reading from StdOut and ignoring StdErr.

The downside is that the StdErr and StdOut output get mixed together. As long as they are recognisable you can probably work with this.

Answer 4

Another technique which might help in this situation is to redirect the standard error stream of the command to accompany the standard output. Do this by adding "%comspec% /c" to the front and "2>&1" to the end of the execStr string. That is, change the command you run from:

zzz

to:

%comspec% /c zzz 2>&1 

The "2>&1" is a redirect instruction which causes the StdErr output (file descriptor 2) to be written to the StdOut stream (file descriptor 1). You need to include the "%comspec% /c" part because it is the command interpreter which understands about the command line redirect. See http://technet.microsoft.com/en-us/library/ee156605.aspx
Using "%comspec%" instead of "cmd" gives portability to a wider range of Windows versions. If your command contains quoted string arguments, it may be tricky to get them right: the specification for how cmd handles quotes after "/c" seems to be incomplete.

With this, your script needs only to read the StdOut stream, and will receive both standard output and standard error. I used this with "net stop wuauserv", which writes to StdOut on success (if the service is running) and StdErr on failure (if the service is already stopped).

Answer 5

You might have hit the deadlock issue described on this Microsoft Support site.

One suggestion is to always read both from stdout and stderr. You could change readAllFromAny to:

function readAllFromAny(oExec)
{
  var output = "";

  if (!oExec.StdOut.AtEndOfStream)
    output = output + oExec.StdOut.ReadLine();

  if (!oExec.StdErr.AtEndOfStream)
    output = output + "STDERR: " + oExec.StdErr.ReadLine();

  return output ? output : -1;
}