Shell Function

Runs an executable program and returns a Variant (Double) representing the program's task ID if successful, or zero if unsuccessful.

Syntax

Shell(pathname, [windowstyle])

Parameters

• pathname - Required. String expression specifying the name of the program to execute, along with any required arguments or command-line switches. May include directory or folder path.
• windowstyle - Optional. Variant (Integer) corresponding to the style of the window in which the program is to be run. If omitted, the program is started minimized with focus.

Window Style Values

Return Value

Returns a Variant (Double) containing the task ID of the started program: - If successful: Returns the task ID (a unique identifier for the process) - If unsuccessful: Returns 0 - Task ID can be used with AppActivate statement to give focus to the window

Remarks

The Shell function runs an executable program asynchronously. This means that a program started with Shell might not finish executing before the statements following the Shell function are executed.

Key characteristics: - Executes programs asynchronously (doesn't wait for completion) - Returns immediately after starting the program - Can launch any executable file (.exe, .com, .bat, .cmd, etc.) - Can include command-line arguments in pathname - Task ID can be used with AppActivate to switch focus - If program cannot be started, returns 0 - On error, generates Error 5 (Invalid procedure call) or Error 53 (File not found)

The pathname can include: - Full path to executable: "C:\Windows\notepad.exe" - Relative path: "....\tools\mytool.exe" - Program in system PATH: "notepad.exe" - Command with arguments: "notepad.exe C:\readme.txt"

Important considerations: - Shell executes asynchronously - use AppActivate or API calls to synchronize - No direct way to know when shelled program terminates from VB6 - Can't capture standard output/error directly (use API or temp files) - Security: Be cautious with user-supplied paths to avoid injection - Long filenames with spaces should be enclosed in quotes

Typical Uses

1. Launch Applications: Open external programs from your VB6 app
2. Open Documents: Launch files with associated applications
3. Run Batch Files: Execute .bat or .cmd scripts
4. Execute Commands: Run command-line tools
5. System Tools: Open Windows utilities (calc, notepad, etc.)
6. Background Tasks: Start processes that run independently
7. Integration: Interact with other applications
8. File Operations: Use command-line tools for file manipulation

Basic Examples

' Example 1: Open Notepad
Dim taskId As Double
taskId = Shell("notepad.exe", vbNormalFocus)
If taskId = 0 Then
MsgBox "Failed to start Notepad"
End If

' Example 2: Open file with Notepad
Dim taskId As Double
taskId = Shell("notepad.exe C:\readme.txt", vbNormalFocus)

' Example 3: Run Calculator maximized
Dim taskId As Double
taskId = Shell("calc.exe", vbMaximizedFocus)

' Example 4: Execute batch file hidden
Dim taskId As Double
taskId = Shell("C:\Scripts\backup.bat", vbHide)

Common Patterns

Pattern 1: SafeShell

Execute program with error handling

Function SafeShell(programPath As String, Optional windowStyle As Integer = vbNormalFocus) As Double
On Error Resume Next
SafeShell = Shell(programPath, windowStyle)

If Err.Number <> 0 Then
MsgBox "Error starting program: " & Err.Description, vbExclamation
SafeShell = 0
End If
End Function

Pattern 2: ShellAndWait

Execute program and wait for completion (using AppActivate)

Function ShellAndWait(programPath As String, windowStyle As Integer) As Boolean
Dim taskId As Double
Dim startTime As Double

On Error GoTo ErrorHandler

taskId = Shell(programPath, windowStyle)
If taskId = 0 Then
ShellAndWait = False
Exit Function
End If

' Give the program time to start
DoEvents

' Wait for program window to exist
startTime = Timer
Do While Timer - startTime < 30  ' 30 second timeout
On Error Resume Next
AppActivate taskId
If Err.Number = 0 Then Exit Do
Err.Clear
DoEvents
Loop

ShellAndWait = True
Exit Function

ErrorHandler:
ShellAndWait = False
End Function

Pattern 3: QuotePath

Ensure path is properly quoted for spaces

Function QuotePath(path As String) As String
If InStr(path, " ") > 0 And Left(path, 1) <> """" Then
QuotePath = """" & path & """"
Else
QuotePath = path
End If
End Function

' Usage:
taskId = Shell(QuotePath("C:\Program Files\MyApp\app.exe"), vbNormalFocus)

Pattern 4: OpenFileWithApp

Open file with specific application

Function OpenFileWithApp(appPath As String, filePath As String, _
Optional windowStyle As Integer = vbNormalFocus) As Boolean
Dim commandLine As String
Dim taskId As Double

' Quote paths if they contain spaces
If InStr(appPath, " ") > 0 Then appPath = """" & appPath & """"
If InStr(filePath, " ") > 0 Then filePath = """" & filePath & """"

commandLine = appPath & " " & filePath

On Error Resume Next
taskId = Shell(commandLine, windowStyle)
OpenFileWithApp = (taskId <> 0 And Err.Number = 0)
End Function

Pattern 5: ExecuteCommand

Execute command-line command

Function ExecuteCommand(command As String, Optional waitSeconds As Integer = 0) As Double
Dim taskId As Double
Dim endTime As Double

On Error GoTo ErrorHandler

' Run command via cmd.exe
taskId = Shell("cmd.exe /c " & command, vbHide)

If waitSeconds > 0 Then
endTime = Timer + waitSeconds
Do While Timer < endTime
DoEvents
Loop
End If

ExecuteCommand = taskId
Exit Function

ErrorHandler:
ExecuteCommand = 0
End Function

Pattern 6: LaunchAndActivate

Launch program and bring to front

Function LaunchAndActivate(programPath As String) As Boolean
Dim taskId As Double
Dim attempts As Integer

On Error Resume Next
taskId = Shell(programPath, vbNormalFocus)

If taskId = 0 Then
LaunchAndActivate = False
Exit Function
End If

' Try to activate window
For attempts = 1 To 10
DoEvents
AppActivate taskId
If Err.Number = 0 Then
LaunchAndActivate = True
Exit Function
End If
Err.Clear
Next attempts

LaunchAndActivate = False
End Function

Pattern 7: CheckProgramExists

Verify program exists before shelling

Function CheckProgramExists(programPath As String) As Boolean
On Error Resume Next
CheckProgramExists = (Dir(programPath) <> "")
End Function

' Usage:
If CheckProgramExists("C:\Tools\mytool.exe") Then
taskId = Shell("C:\Tools\mytool.exe", vbNormalFocus)
Else
MsgBox "Program not found"
End If

Pattern 8: ShellWithTimeout

Execute with timeout detection

Function ShellWithTimeout(programPath As String, timeoutSeconds As Integer) As Boolean
Dim taskId As Double
Dim startTime As Double

On Error GoTo ErrorHandler

taskId = Shell(programPath, vbNormalFocus)
If taskId = 0 Then
ShellWithTimeout = False
Exit Function
End If

startTime = Timer
Do While Timer - startTime < timeoutSeconds
DoEvents
Loop

ShellWithTimeout = True
Exit Function

ErrorHandler:
ShellWithTimeout = False
End Function

Pattern 9: OpenDocument

Open document with default application

Function OpenDocument(filePath As String) As Boolean
Dim taskId As Double

On Error Resume Next

' Use "start" command to open with default app
taskId = Shell("cmd.exe /c start """" """ & filePath & """", vbHide)

OpenDocument = (taskId <> 0 And Err.Number = 0)
End Function

Pattern 10: RunBatchFile

Execute batch file with parameters

Function RunBatchFile(batchPath As String, parameters As String, _
Optional hideWindow As Boolean = True) As Double
Dim commandLine As String
Dim windowStyle As Integer

If InStr(batchPath, " ") > 0 Then
commandLine = """" & batchPath & """"
Else
commandLine = batchPath
End If

If Len(parameters) > 0 Then
commandLine = commandLine & " " & parameters
End If

windowStyle = IIf(hideWindow, vbHide, vbNormalFocus)

On Error Resume Next
RunBatchFile = Shell(commandLine, windowStyle)
End Function

Advanced Usage

Example 1: ProcessLauncher Class

Manage launching and tracking external processes

' Class: ProcessLauncher
Private m_processes As Collection

Private Type ProcessInfo
TaskId As Double
ProgramPath As String
LaunchTime As Date
Description As String
End Type

Private Sub Class_Initialize()
Set m_processes = New Collection
End Sub

Public Function LaunchProcess(programPath As String, _
Optional description As String = "", _
Optional windowStyle As Integer = vbNormalFocus) As Double
Dim taskId As Double
Dim procInfo As ProcessInfo

On Error GoTo ErrorHandler

taskId = Shell(programPath, windowStyle)

If taskId <> 0 Then
procInfo.TaskId = taskId
procInfo.ProgramPath = programPath
procInfo.LaunchTime = Now
procInfo.Description = description

m_processes.Add procInfo, CStr(taskId)
End If

LaunchProcess = taskId
Exit Function

ErrorHandler:
LaunchProcess = 0
End Function

Public Function ActivateProcess(taskId As Double) As Boolean
On Error Resume Next
AppActivate taskId
ActivateProcess = (Err.Number = 0)
End Function

Public Function GetProcessCount() As Long
GetProcessCount = m_processes.Count
End Function

Public Function GetProcessInfo(taskId As Double) As String
Dim procInfo As ProcessInfo

On Error Resume Next
procInfo = m_processes(CStr(taskId))

If Err.Number = 0 Then
GetProcessInfo = "Program: " & procInfo.ProgramPath & vbCrLf & _
"Description: " & procInfo.Description & vbCrLf & _
"Launched: " & procInfo.LaunchTime & vbCrLf & _
"Task ID: " & procInfo.TaskId
Else
GetProcessInfo = "Process not found"
End If
End Function

Public Sub ClearProcesses()
Set m_processes = New Collection
End Sub

Example 2: CommandExecutor Module

Execute command-line commands with output capture

' Module: CommandExecutor

Public Function ExecuteCommandWithOutput(command As String, _
ByRef output As String) As Boolean
Dim tempFile As String
Dim fileNum As Integer
Dim commandLine As String
Dim taskId As Double

On Error GoTo ErrorHandler

' Create temp file for output
tempFile = Environ("TEMP") & "\cmdout_" & Format(Now, "yyyymmddhhnnss") & ".txt"

' Redirect output to temp file
commandLine = "cmd.exe /c " & command & " > """ & tempFile & """ 2>&1"

taskId = Shell(commandLine, vbHide)
If taskId = 0 Then
ExecuteCommandWithOutput = False
Exit Function
End If

' Wait for command to complete (primitive wait)
Sleep 1000  ' Would need to declare Sleep API

' Read output file
fileNum = FreeFile
Open tempFile For Input As #fileNum
output = Input(LOF(fileNum), #fileNum)
Close #fileNum

' Clean up
Kill tempFile

ExecuteCommandWithOutput = True
Exit Function

ErrorHandler:
If fileNum > 0 Then Close #fileNum
On Error Resume Next
If Dir(tempFile) <> "" Then Kill tempFile
ExecuteCommandWithOutput = False
End Function

Public Function RunCommandHidden(command As String) As Boolean
Dim taskId As Double

On Error Resume Next
taskId = Shell("cmd.exe /c " & command, vbHide)
RunCommandHidden = (taskId <> 0 And Err.Number = 0)
End Function

Public Function RunCommandVisible(command As String) As Double
On Error Resume Next
RunCommandVisible = Shell("cmd.exe /k " & command, vbNormalFocus)
End Function

Example 3: ApplicationLauncher Class

Launch applications with comprehensive error handling

' Class: ApplicationLauncher
Private m_lastError As String
Private m_lastTaskId As Double

Public Function LaunchApplication(programPath As String, _
Optional arguments As String = "", _
Optional windowStyle As Integer = vbNormalFocus, _
Optional verifyExists As Boolean = True) As Boolean
Dim fullCommand As String

On Error GoTo ErrorHandler

m_lastError = ""
m_lastTaskId = 0

' Verify program exists
If verifyExists Then
If Dir(programPath) = "" Then
m_lastError = "Program not found: " & programPath
LaunchApplication = False
Exit Function
End If
End If

' Build command line
fullCommand = programPath
If InStr(fullCommand, " ") > 0 And Left(fullCommand, 1) <> """" Then
fullCommand = """" & fullCommand & """"
End If

If Len(arguments) > 0 Then
fullCommand = fullCommand & " " & arguments
End If

' Launch
m_lastTaskId = Shell(fullCommand, windowStyle)

If m_lastTaskId = 0 Then
m_lastError = "Shell function returned 0"
LaunchApplication = False
Else
LaunchApplication = True
End If

Exit Function

ErrorHandler:
m_lastError = "Error " & Err.Number & ": " & Err.Description
LaunchApplication = False
End Function

Public Function LaunchAndActivate(programPath As String, _
Optional arguments As String = "") As Boolean
Dim success As Boolean
Dim attempts As Integer

success = LaunchApplication(programPath, arguments, vbNormalFocus)

If Not success Then
LaunchAndActivate = False
Exit Function
End If

' Try to activate
For attempts = 1 To 20
DoEvents
On Error Resume Next
AppActivate m_lastTaskId
If Err.Number = 0 Then
LaunchAndActivate = True
Exit Function
End If
Err.Clear
Next attempts

LaunchAndActivate = False
End Function

Public Property Get LastError() As String
LastError = m_lastError
End Property

Public Property Get LastTaskId() As Double
LastTaskId = m_lastTaskId
End Property

Public Function OpenFileWithDefaultApp(filePath As String) As Boolean
Dim commandLine As String

' Use Windows "start" command
commandLine = "cmd.exe /c start """" """ & filePath & """"

On Error Resume Next
m_lastTaskId = Shell(commandLine, vbHide)

OpenFileWithDefaultApp = (m_lastTaskId <> 0 And Err.Number = 0)

If Not OpenFileWithDefaultApp Then
m_lastError = "Failed to open file: " & Err.Description
End If
End Function

Example 4: BatchFileRunner Module

Execute batch files with enhanced functionality

' Module: BatchFileRunner

Public Function ExecuteBatchFile(batchPath As String, _
Optional parameters As String = "", _
Optional visible As Boolean = False, _
Optional workingDir As String = "") As Double
Dim commandLine As String
Dim windowStyle As Integer
Dim originalDir As String

On Error GoTo ErrorHandler

' Verify batch file exists
If Dir(batchPath) = "" Then
MsgBox "Batch file not found: " & batchPath, vbExclamation
ExecuteBatchFile = 0
Exit Function
End If

' Quote path if needed
If InStr(batchPath, " ") > 0 Then
commandLine = """" & batchPath & """"
Else
commandLine = batchPath
End If

' Add parameters
If Len(parameters) > 0 Then
commandLine = commandLine & " " & parameters
End If

' Change working directory if specified
If Len(workingDir) > 0 Then
originalDir = CurDir
ChDir workingDir
End If

' Execute
windowStyle = IIf(visible, vbNormalFocus, vbHide)
ExecuteBatchFile = Shell(commandLine, windowStyle)

' Restore directory
If Len(workingDir) > 0 Then
ChDir originalDir
End If

Exit Function

ErrorHandler:
If Len(workingDir) > 0 Then
On Error Resume Next
ChDir originalDir
End If
ExecuteBatchFile = 0
End Function

Public Function RunBatchWithLog(batchPath As String, logPath As String) As Boolean
Dim commandLine As String
Dim taskId As Double

' Quote paths
If InStr(batchPath, " ") > 0 Then batchPath = """" & batchPath & """"
If InStr(logPath, " ") > 0 Then logPath = """" & logPath & """"

' Redirect output to log
commandLine = "cmd.exe /c " & batchPath & " > " & logPath & " 2>&1"

On Error Resume Next
taskId = Shell(commandLine, vbHide)

RunBatchWithLog = (taskId <> 0 And Err.Number = 0)
End Function

Public Function CreateAndRunBatch(commands() As String, _
Optional visible As Boolean = False) As Boolean
Dim batchPath As String
Dim fileNum As Integer
Dim i As Long
Dim taskId As Double

On Error GoTo ErrorHandler

' Create temp batch file
batchPath = Environ("TEMP") & "\temp_" & Format(Now, "yyyymmddhhnnss") & ".bat"

' Write commands
fileNum = FreeFile
Open batchPath For Output As #fileNum
Print #fileNum, "@echo off"
For i = LBound(commands) To UBound(commands)
Print #fileNum, commands(i)
Next i
Close #fileNum

' Execute
taskId = Shell(batchPath, IIf(visible, vbNormalFocus, vbHide))

CreateAndRunBatch = (taskId <> 0)
Exit Function

ErrorHandler:
If fileNum > 0 Then Close #fileNum
CreateAndRunBatch = False
End Function

Error Handling

The Shell function can generate the following errors:

• Error 5 (Invalid procedure call or argument): Invalid windowstyle parameter
• Error 53 (File not found): Program or path not found
• Error 76 (Path not found): Directory in path doesn't exist

Always use error handling when executing external programs:

On Error Resume Next
taskId = Shell(programPath, vbNormalFocus)
If Err.Number <> 0 Or taskId = 0 Then
MsgBox "Error: " & Err.Description
End If

Performance Considerations

• Shell executes asynchronously and returns immediately
• No performance impact on VB6 app after launch
• Multiple programs can be launched simultaneously
• Task ID allows tracking and activation
• Consider resource usage when launching many programs
• Use DoEvents to allow UI updates after Shell

Best Practices

1. Error Handling: Always wrap Shell in error handling
2. Path Quoting: Quote paths with spaces using double quotes
3. Verify Existence: Check file exists before shelling (Dir function)
4. Security: Validate user input to prevent command injection
5. Resource Management: Track launched processes
6. Window Style: Choose appropriate window style for user experience
7. Wait Strategy: Use AppActivate or API for synchronization if needed
8. Return Value: Check return value (0 = failure)
9. Documentation: Document external dependencies
10. Testing: Test with various paths and edge cases

Comparison with Related Functions

Platform Considerations

• Available in VB6, VBA (Windows only)
• Windows-specific function
• Task ID is Windows-specific process identifier
• Path separators are backslashes ()
• Case-insensitive on Windows
• Long filename support (use quotes)
• Windows versions may affect behavior

Limitations

• No built-in way to wait for completion
• Cannot capture standard output/error directly
• Limited to Windows operating system
• Task ID becomes invalid when process terminates
• No parent-child process relationship tracking
• Cannot pass structured data to launched program
• Security risks with user-supplied paths
• No control over process priority or environment

Related Functions

• AppActivate: Activates a running application window by task ID
• CreateObject: Creates automation objects for inter-app communication
• SendKeys: Sends keystrokes to active window
• Dir: Verifies file existence before shelling
• Environ: Gets environment variables for path construction