When working with process chains in Windows CMD, developers often face the challenge that every start or direct command invocation creates a new process. This breaks pipe continuity and changes the process ID (PID), making it difficult to maintain:

• Consistent I/O pipe connections
• The same process identifier throughout the chain
• Proper process tree management

In Unix environments, the exec command replaces the current process image with a new one while maintaining:

# Unix example
#!/bin/sh
echo "This process will be replaced"
exec python3 script.py  # Takes over current PID and file descriptors

1. Using cmd /c with Special Syntax

The closest native Windows equivalent combines cmd /c with the & operator:

REM This maintains STDIO handles but changes PID
cmd /c "first_command.exe & second_command.exe"

2. PowerShell's Start-Process -NoNewWindow

PowerShell provides better control with the -NoNewWindow parameter:

# PowerShell example
Start-Process -FilePath "program.exe" -ArgumentList "params" -NoNewWindow -Wait

For true exec-like behavior, we need to use Windows API calls. Here's a C++ implementation:

#include <windows.h>
#include <tchar.h>

void ExecuteWithReplace(LPCTSTR application, LPTSTR commandLine) {
    STARTUPINFO si;
    PROCESS_INFORMATION pi;
    
    ZeroMemory(&si, sizeof(si));
    si.cb = sizeof(si);
    ZeroMemory(&pi, sizeof(pi));
    
    CreateProcess(
        application,
        commandLine,
        NULL, NULL,
        TRUE,
        0,
        NULL, NULL,
        &si,
        &pi
    );
    
    WaitForSingleObject(pi.hProcess, INFINITE);
    CloseHandle(pi.hProcess);
    CloseHandle(pi.hThread);
}

For those needing a script-based solution, this batch pattern comes closest:

@echo off
:: This maintains STDIO but not PID
setlocal
if "%1"=="internal" (
    shift
    %*
    exit /b
)

cmd /c "%~f0 internal %*"

When you must maintain the original PID for process management:

1. Create a helper process that monitors the target
2. Use job objects for process grouping
3. Implement IPC between the original and new processes

For projects targeting both Windows and Unix, consider these alternatives:

Scenario	Windows Solution	Unix Solution
Simple command replacement	cmd /c chaining	exec
Full process control	Windows API	execvp
Script portability	PowerShell wrapper	exec wrapper

Here's how a Python script might handle this cross-platform:

import os
import sys
import subprocess

def execute_replacement(command):
    if os.name == 'nt':
        # Windows implementation
        CREATE_NO_WINDOW = 0x08000000
        subprocess.call(command, creationflags=CREATE_NO_WINDOW)
    else:
        # Unix implementation
        os.execvp(command[0], command)

---

When transitioning from Unix to Windows environments, one particularly thorny issue arises: how to replicate the behavior of Unix's exec command in Windows CMD. The fundamental requirement is to replace the current process with a new one without creating a child process, thereby preserving:

• Process ID (PID) continuity
• Existing I/O pipe connections
• Process tree relationships

Windows doesn't have a direct 1:1 equivalent to Unix's exec, but we can approximate the behavior through several methods:

@echo off
:: Method 1: Using cmd /c with exit
cmd /c "processB.exe & exit /b %ERRORLEVEL%"

:: Method 2: Using call with errorlevel propagation
call processB.exe
exit /b %ERRORLEVEL%

For more robust solutions, PowerShell provides better alternatives:

# PowerShell equivalent using Start-Process -NoNewWindow
Start-Process -NoNewWindow -Wait -FilePath "processB.exe" -PassThru

# Advanced version with error handling
try {
    $process = Start-Process -NoNewWindow -Wait -FilePath "processB.exe" -PassThru
    exit $process.ExitCode
}
catch {
    Write-Error $_.Exception.Message
    exit 1
}

For programmers willing to dive deeper, the Windows API provides CreateProcess with specific flags:

// C++ example using CreateProcess
STARTUPINFO si = { sizeof(si) };
PROCESS_INFORMATION pi;

if (CreateProcess(
    NULL,               // Application name
    "processB.exe",     // Command line
    NULL,               // Process attributes
    NULL,               // Thread attributes
    TRUE,               // Inherit handles
    0,                  // Creation flags
    NULL,               // Environment
    NULL,               // Current directory
    &si,                // Startup info
    &pi))               // Process info
{
    WaitForSingleObject(pi.hProcess, INFINITE);
    CloseHandle(pi.hThread);
    CloseHandle(pi.hProcess);
}

For batch scripting scenarios, consider these patterns:

:: Redirect all output to parent process
processB.exe >&2 | findstr /R /C:"^"

:: Alternative using temporary files
processB.exe > output.log 2>&1
type output.log

To maintain the process hierarchy as expected in Unix systems:

1. Use WMIC to track process relationships
2. Implement job objects for process grouping
3. Consider using START /B for background execution

:: Example using WMIC for process tracking
wmic process where (name='processB.exe') get processid,parentprocessid