Resume uploading with built-in Windows FTP client

No, the Windows command-line ftp.exe does not support transfer resuming.

But you can just automatically download any small 3rd party portable command-line FTP client that supports automatic resume and use that.

For example the following PowerShell code downloads WinSCP .NET assembly package, extracts it and starts a resumable upload:

$winscp_assembly = "WinSCPnet.dll"
if (Test-Path $winscp_assembly)
{
    Write-Host "WinSCP already downloaded"
}
else
{
    $webclient = New-Object System.Net.WebClient

    $padUrl = "https://winscp.net/pad/winscp.xml"
    $pad = [xml]$webclient.DownloadString($padUrl)
    $winscpVersion = $pad.XML_DIZ_INFO.Program_Info.Program_Version
    if (-not $winscpVersion)
    {
        throw "Cannot find latest version of WinSCP"
    }

    Write-Host "Latest version of WinSCP is $winscpVersion"
    
    $winscpArchive = "WinSCP-$winscpVersion-Automation.zip"
    Write-Host "Downloading $winscpArchive ..."
    $url =
        "https://sourceforge.net/projects/winscp/files/WinSCP/" +
        $winscpVersion + "/" + $winscpArchive + "/download"
    $webclient.DownloadFile($url, $winscpArchive)
    Write-Host "Done"

    Write-Host "Extracting $winscpArchive ..."
    Expand-Archive $winscpArchive .

    Write-Host "Done"
}

Add-Type -Path $winscp_assembly

$ftp_host = "ftp.example.com"
$ftp_path = "/target/path/"
$upload_path = "C:\big\file.dat"
Write-Host "Starting resumable upload of $upload_path to $ftp_host ..."
# Setup session options
$sessionOptions = New-Object WinSCP.SessionOptions
$sessionOptions = New-Object WinSCP.SessionOptions -Property @{
    Protocol = [WinSCP.Protocol]::Ftp
    HostName = $ftp_host
    UserName = "username"
    Password = "password"
}

$session = New-Object WinSCP.Session
$session.Open($sessionOptions)

$session.PutFiles($upload_path, $ftp_path).Check()

To run the PowerShell script (upload.ps1) use:

powershell.exe -File upload.ps1 -ExecutionPolicy Bypass

The script uses PowerShell 5 Expand-Archive cmdlet. If you need to use older version of PowerShell, check an old revision of this answer for a solution with use of Shell.Application object.

(I'm the author of WinSCP)

Another option is to implement the resume manually using FtpWebRequest.

See How to continue or resume FTP upload after interruption of internet.

Again you can use the FtpWebRequest from a PowerShell script. See Upload files with FTP using PowerShell.

Resume uploading with built-in Windows FTP client

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