How do I force Robocopy to overwrite files?

Question

In general, Robocopy ignores files for which lastwrittendate and filesize are the same. How can we escape this design? I'd like to force overwriting with Robocopy.

I expected that dst\sample.txt should be written test001. But these file are recognized as the same files by Robocopy and not overwritten. The "/IS" option is not effective in this case.

New-Item src -itemType Directory
New-Item dst -itemType Directory
New-Item src\sample.txt -itemType File -Value "test001"
New-Item dst\sample.txt -itemType File -Value "test002"
Set-ItemProperty src\sample.txt -Name LastWriteTime -Value "2016/1/1 15:00:00"
Set-ItemProperty dst\sample.txt -Name LastWriteTime -Value "2016/1/1 15:00:00"

ROBOCOPY.exe src dst /COPYALL /MIR
Get-Content src\sample.txt, dst\sample.txt
> test001
> test002

ROBOCOPY.exe src dst /COPYALL /MIR /IS
Get-Content src\sample.txt, dst\sample.txt
> test001
> test002

Answer 1

From the documentation:

/is Includes the same files. /it Includes "tweaked" files.

"Same files" means files that are identical (name, size, times, attributes). "Tweaked files" means files that have the same name, size, and times, but different attributes.

robocopy src dst sample.txt /is      # copy if attributes are equal
robocopy src dst sample.txt /it      # copy if attributes differ
robocopy src dst sample.txt /is /it  # copy irrespective of attributes

This answer on Super User has a good explanation of what kind of files the selection parameters match.

With that said, I could reproduce the behavior you describe, but from my understanding of the documentation and the output robocopy generated in my tests I would consider this a bug.

PS C:\temp> New-Item src -Type Directory >$null
PS C:\temp> New-Item dst -Type Directory >$null
PS C:\temp> New-Item src\sample.txt -Type File -Value "test001" >$null
PS C:\temp> New-Item dst\sample.txt -Type File -Value "test002" >$null
PS C:\temp> Set-ItemProperty src\sample.txt -Name LastWriteTime -Value "2016/1/1 15:00:00"
PS C:\temp> Set-ItemProperty dst\sample.txt -Name LastWriteTime -Value "2016/1/1 15:00:00"
PS C:\temp> robocopy src dst sample.txt /is /it /copyall /mir
...
  Options : /S /E /COPYALL /PURGE /MIR /IS /IT /R:1000000 /W:30

------------------------------------------------------------------------------

                           1    C:\temp\src\
            Modified                   7        sample.txt

------------------------------------------------------------------------------

               Total    Copied   Skipped  Mismatch    FAILED    Extras
    Dirs :         1         0         0         0         0         0
   Files :         1         1         0         0         0         0
   Bytes :         7         7         0         0         0         0
...
PS C:\temp> robocopy src dst sample.txt /is /it /copyall /mir
...
  Options : /S /E /COPYALL /PURGE /MIR /IS /IT /R:1000000 /W:30

------------------------------------------------------------------------------

                           1    C:\temp\src\
            Same                       7        sample.txt

------------------------------------------------------------------------------

               Total    Copied   Skipped  Mismatch    FAILED    Extras
    Dirs :         1         0         0         0         0         0
   Files :         1         1         0         0         0         0
   Bytes :         7         7         0         0         0         0
...
PS C:\temp> Get-Content .\src\sample.txt
test001
PS C:\temp> Get-Content .\dst\sample.txt
test002

The file is listed as copied, and since it becomes a same file after the first robocopy run at least the times are synced. However, even though seven bytes have been copied according to the output no data was actually written to the destination file in both cases despite the data flag being set (via /copyall). The behavior also doesn't change if the data flag is set explicitly (/copy:d).

I had to modify the last write time to get robocopy to actually synchronize the data.

PS C:\temp> Set-ItemProperty src\sample.txt -Name LastWriteTime -Value (Get-Date)
PS C:\temp> robocopy src dst sample.txt /is /it /copyall /mir
...
  Options : /S /E /COPYALL /PURGE /MIR /IS /IT /R:1000000 /W:30

------------------------------------------------------------------------------

                           1    C:\temp\src\
100%        Newer                      7        sample.txt

------------------------------------------------------------------------------

               Total    Copied   Skipped  Mismatch    FAILED    Extras
    Dirs :         1         0         0         0         0         0
   Files :         1         1         0         0         0         0
   Bytes :         7         7         0         0         0         0
...
PS C:\temp> Get-Content .\dst\sample.txt
test001

An admittedly ugly workaround would be to change the last write time of same/tweaked files to force robocopy to copy the data:

& robocopy src dst /is /it /l /ndl /njh /njs /ns /nc |
  Where-Object { $_.Trim() } |
  ForEach-Object {
    $f = Get-Item $_
    $f.LastWriteTime = $f.LastWriteTime.AddSeconds(1)
  }
& robocopy src dst /copyall /mir

Switching to xcopy is probably your best option:

& xcopy src dst /k/r/e/i/s/c/h/f/o/x/y