Shell And Wait

This pages describes the VBA Shell function and provides a procedure that will wait for a Shell'd function to terminate.

The Shell Function

The VBA Shell function can be used to start an external program or perform any operation for which you would normally use the Run item on the Windows start menu. The Shell function starts the command text and then immediately returns control back to the calling VBA code -- it does not wait for the command used in Shell to terminate. This page describes a function named ShellAndWait that will call the Shell function and then wait for the Shell'd process to terminate or for a prescribed timeout interval to expire.

The ShellAndWait Function

The ShellAndWait function calls Shell and then waits for the Shell'd process to terminate. You can specify an interval after which the function times out and returns to the caller. The declaration of ShellAndWait is as follows:

    Public Function ShellAndWait(ShellCommand As String, _
                        TimeOutMs As Long, _
                        ShellWindowState As VbAppWinStyle, _
                        BreakKey As ActionOnBreak) As ShellAndWaitResult

In the declaration, ShellCommand is the string that is passed to the VBA Shell function. TimeOutMs is the number of milliseconds to wait before returning a time-out result. ShellWindowsState is an item in the VbAppWinStyle enum that specifies the window style in which to open the Shell'd process. This value is simply passed through to the Shell function. BreakKey indicates how to handle the Application's Cancel key (CTRL BREAK). These parameters are described below.

ShellCommand

This parameter is the command text that is passed to the Shell function. This must be a valid command string; that is, a string that would work in the Run dialog on the Windows Start menu. If the text is invalid (such as attempting to execute a non-existant exe file), the function returns a result indicating failure.

TimeOutMs

This parameter specifies the number of milliseconds to wait for the Shell'd process to terminate. If the specified interval expires before the process ends, the wait is abandoned and the function returns a value indicating a time out occurred. If TimeOutMs is 0, the wait never expires and ShellAndWait will wait indefinitely for the process to terminate.

ShellWindowState

This parameter should be one of the values in the VbAppWinStyle enum that indicates the type of window (e.g., maximized or minimized) in which the Shell'd process should open. This value is simply passed through to the Shell command. If this value is not valid, the function returns a result indicating failure.

BreakKey

This parameter should be one of the values in the ActionOnBreak enum to indicate how the code should respond if the user presses CTRL Break. If BreakKey is IgnoreBreak, the cancel key is ignored. If BreakKey is AbandonWait, the wait is terminated and the function returns a valid indicating that the user abandoned the wait. If BreakKey is PromptUser, the function displays a vbYesNo message box asking the user whether to abandon or continue waiting. The ActionOnBreak enum is shown below.

    Public Enum ActionOnBreak
        IgnoreBreak = 0
        AbandonWait = 1
        PromptUser = 2
    End Enum

Return Values

The function returns as its result one of the values in the ShellAndWaitResult enum, shown below:

    Public Enum ShellAndWaitResult
        Success = 0
        Failure = 1
        TimeOut = 2
        InvalidParameter = 3
        SysWaitAbandoned = 4
        UserWaitAbandoned = 5
        UserBreak = 6
    End Enum

Success indicates that the Shell'd process terminated successfully before the TimeOutMs interval expired. M

Failure indicates that the Shell function failed, most likely due to an invalid command string in the ShellCommand parameter.

TimeOut indicates that the interval specified by TimeOutMs expired before the process terminated. This result is returned only if TimeOutMs is greater than zero.

InvalidParameter indicates that one of the parameters passed to ShellAndWait is invalid.

SysWaitAbandoned indicates that Windows abandoned the wait operation for internal reasons.

UserWaitAbandoned indicates that the user abandoned the wait operation by pressing CTRL Break. This is returne only if BreakKey is set to AbandonWait.

UserBreak indicates that the user press CTRL Break to terminate the wait and responded "no" to the "continue to wait" prompt. This value is returned only when BreakKey is set to PromptUser.

You can download a module file containing the code.

The complete code module is shown below.

Option Explicit
Option Compare Text

'''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''
'''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''
' modShellAndWait
' By Chip Pearson, chip@cpearson.com, www.cpearson.com
' This page on the web site: www.cpearson.com/Excel/ShellAndWait.aspx
' 9-September-2008
'
' This module contains code for the ShellAndWait function that will Shell to a process
' and wait for that process to end before returning to the caller.
'''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''
'''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''
Private Declare Function WaitForSingleObject Lib "kernel32" ( _
    ByVal hHandle As Long, _
    ByVal dwMilliseconds As Long) As Long

Private Declare Function OpenProcess Lib "kernel32.dll" ( _
    ByVal dwDesiredAccess As Long, _
    ByVal bInheritHandle As Long, _
    ByVal dwProcessId As Long) As Long

Private Declare Function CloseHandle Lib "kernel32" ( _
    ByVal hObject As Long) As Long

Private Const SYNCHRONIZE = &H100000

Public Enum ShellAndWaitResult
    Success = 0
    Failure = 1
    TimeOut = 2
    InvalidParameter = 3
    SysWaitAbandoned = 4
    UserWaitAbandoned = 5
    UserBreak = 6
End Enum

Public Enum ActionOnBreak
    IgnoreBreak = 0
    AbandonWait = 1
    PromptUser = 2
End Enum

Private Const STATUS_ABANDONED_WAIT_0 As Long = &H80
Private Const STATUS_WAIT_0 As Long = &H0
Private Const WAIT_ABANDONED As Long = (STATUS_ABANDONED_WAIT_0 + 0)
Private Const WAIT_OBJECT_0 As Long = (STATUS_WAIT_0 + 0)
Private Const WAIT_TIMEOUT As Long = 258&
Private Const WAIT_FAILED As Long = &HFFFFFFFF
Private Const WAIT_INFINITE = -1&


Public Function ShellAndWait(ShellCommand As String, _
                    TimeOutMs As Long, _
                    ShellWindowState As VbAppWinStyle, _
                    BreakKey As ActionOnBreak) As ShellAndWaitResult
'''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''
' ShellAndWait
'
' This function calls Shell and passes to it the command text in ShellCommand. The function
' then waits for TimeOutMs (in milliseconds) to expire.
'
'   Parameters:
'       ShellCommand
'           is the command text to pass to the Shell function.
'
'       TimeOutMs
'           is the number of milliseconds to wait for the shell'd program to wait. If the
'           shell'd program terminates before TimeOutMs has expired, the function returns
'           ShellAndWaitResult.Success = 0. If TimeOutMs expires before the shell'd program
'           terminates, the return value is ShellAndWaitResult.TimeOut = 2.
'
'       ShellWindowState
'           is an item in VbAppWinStyle specifying the window state for the shell'd program.
'
'       BreakKey
'           is an item in ActionOnBreak indicating how to handle the application's cancel key
'           (Ctrl Break). If BreakKey is ActionOnBreak.AbandonWait and the user cancels, the
'           wait is abandoned and the result is ShellAndWaitResult.UserWaitAbandoned = 5.
'           If BreakKey is ActionOnBreak.IgnoreBreak, the cancel key is ignored. If
'           BreakKey is ActionOnBreak.PromptUser, the user is given a ?Continue? message. If the
'           user selects "do not continue", the function returns ShellAndWaitResult.UserBreak = 6.
'           If the user selects "continue", the wait is continued.
'
'   Return values:
'            ShellAndWaitResult.Success = 0
'               indicates the the process completed successfully.
'            ShellAndWaitResult.Failure = 1
'               indicates that the Wait operation failed due to a Windows error.
'            ShellAndWaitResult.TimeOut = 2
'               indicates that the TimeOutMs interval timed out the Wait.
'            ShellAndWaitResult.InvalidParameter = 3
'               indicates that an invalid value was passed to the procedure.
'            ShellAndWaitResult.SysWaitAbandoned = 4
'               indicates that the system abandoned the wait.
'            ShellAndWaitResult.UserWaitAbandoned = 5
'               indicates that the user abandoned the wait via the cancel key (Ctrl+Break).
'               This happens only if BreakKey is set to ActionOnBreak.AbandonWait.
'            ShellAndWaitResult.UserBreak = 6
'               indicates that the user broke out of the wait after being prompted with
'               a ?Continue message. This happens only if BreakKey is set to
'               ActionOnBreak.PromptUser.
'''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''

Dim TaskID As Long
Dim ProcHandle As Long
Dim WaitRes As Long
Dim Ms As Long
Dim MsgRes As VbMsgBoxResult
Dim SaveCancelKey As XlEnableCancelKey
Dim ElapsedTime As Long
Dim Quit As Boolean
Const ERR_BREAK_KEY = 18
Const DEFAULT_POLL_INTERVAL = 500

If Trim(ShellCommand) = vbNullString Then
    ShellAndWait = ShellAndWaitResult.InvalidParameter
    Exit Function
End If

If TimeOutMs < 0 Then
    ShellAndWait = ShellAndWaitResult.InvalidParameter
    Exit Function
ElseIf TimeOutMs = 0 Then
    Ms = WAIT_INFINITE
Else
    Ms = TimeOutMs
End If

Select Case BreakKey
    Case AbandonWait, IgnoreBreak, PromptUser
        ' valid
    Case Else
        ShellAndWait = ShellAndWaitResult.InvalidParameter
        Exit Function
End Select

Select Case ShellWindowState
    Case vbHide, vbMaximizedFocus, vbMinimizedFocus, vbMinimizedNoFocus, vbNormalFocus, vbNormalNoFocus
        ' valid
    Case Else
        ShellAndWait = ShellAndWaitResult.InvalidParameter
        Exit Function
End Select

On Error Resume Next
Err.Clear
TaskID = Shell(ShellCommand, ShellWindowState)
If (Err.Number <> 0) Or (TaskID = 0) Then
    ShellAndWait = ShellAndWaitResult.Failure
    Exit Function
End If

ProcHandle = OpenProcess(SYNCHRONIZE, False, TaskID)
If ProcHandle = 0 Then
    ShellAndWait = ShellAndWaitResult.Failure
    Exit Function
End If

On Error GoTo ErrH:
SaveCancelKey = Application.EnableCancelKey
Application.EnableCancelKey = xlErrorHandler
WaitRes = WaitForSingleObject(ProcHandle, DEFAULT_POLL_INTERVAL)
Do Until WaitRes = WAIT_OBJECT_0
    DoEvents
    Select Case WaitRes
        Case WAIT_ABANDONED
            ' Windows abandoned the wait
            ShellAndWait = ShellAndWaitResult.SysWaitAbandoned
            Exit Do
        Case WAIT_OBJECT_0
            ' Successful completion
            ShellAndWait = ShellAndWaitResult.Success
            Exit Do
        Case WAIT_FAILED
            ' attach failed
            ShellAndWait = ShellAndWaitResult.Failure
            Exit Do
        Case WAIT_TIMEOUT
            ' Wait timed out. Here, this time out is on DEFAULT_POLL_INTERVAL.
            ' See if ElapsedTime is greater than the user specified wait
            ' time out. If we have exceed that, get out with a TimeOut status.
            ' Otherwise, reissue as wait and continue.
            ElapsedTime = ElapsedTime + DEFAULT_POLL_INTERVAL
            If Ms > 0 Then
                ' user specified timeout
                If ElapsedTime > Ms Then
                    ShellAndWait = ShellAndWaitResult.TimeOut
                    Exit Do
                Else
                    ' user defined timeout has not expired.
                End If
            Else
                ' infinite wait -- do nothing
            End If
            ' reissue the Wait on ProcHandle
            WaitRes = WaitForSingleObject(ProcHandle, DEFAULT_POLL_INTERVAL)
            
        Case Else
            ' unknown result, assume failure
            ShellAndWait = ShellAndWaitResult.Failure
            Exit Do
            Quit = True
    End Select
Loop

CloseHandle ProcHandle
Application.EnableCancelKey = SaveCancelKey
Exit Function

ErrH:
Debug.Print "ErrH: Cancel: " & Application.EnableCancelKey
If Err.Number = ERR_BREAK_KEY Then
    If BreakKey = ActionOnBreak.AbandonWait Then
        CloseHandle ProcHandle
        ShellAndWait = ShellAndWaitResult.UserWaitAbandoned
        Application.EnableCancelKey = SaveCancelKey
        Exit Function
    ElseIf BreakKey = ActionOnBreak.IgnoreBreak Then
        Err.Clear
        Resume
    ElseIf BreakKey = ActionOnBreak.PromptUser Then
        MsgRes = MsgBox("User Process Break." & vbCrLf & _
            "Continue to wait?", vbYesNo)
        If MsgRes = vbNo Then
            CloseHandle ProcHandle
            ShellAndWait = ShellAndWaitResult.UserBreak
            Application.EnableCancelKey = SaveCancelKey
        Else
            Err.Clear
            Resume Next
        End If
    Else
        CloseHandle ProcHandle
        Application.EnableCancelKey = SaveCancelKey
        ShellAndWait = ShellAndWaitResult.Failure
    End If
Else
    ' some other error. assume failure
    CloseHandle ProcHandle
    ShellAndWait = ShellAndWaitResult.Failure
End If

Application.EnableCancelKey = SaveCancelKey

End Function

This page last updated: 9-September-2008.