Can the PowerShell commandlet move-item move a currently open file? - powershell

Question

I want to avoid moving files that are currently open by another process. Is there any way the move-item PowerShell command can move, or even worse copy, a currently open file?
We currently have a situation where we have two processes that need data files transferred from process A's output folder to process B's input folder. The idea is that process A writes a file, and then a PowerShell script moves the files to the folder that process B reads.
We are having an issue sometimes that the same file is transferred twice, and it is not a partial file either time.
The below code is executed at 00, 10, 20, 30, 40, 50 minutes past the hour. Process B on the Samba server runs at 05, 15, 25, 35, 45, 55 minutes past the hour and moves the files out of the folder the PowerShell script puts them in, once process B has finished processing the files. There are only ever up to about a dozen 1 KB files being moved at a time.
Process A is not controlled by us and can write files to that location at any time. It seems there is some race condition happening with Process A creating a file just before the PowerShell script moves the file, where the script copies the file and then moves it 10 minutes later when the script runs again.
With the below code is the only possibility that Process A is making the file twice if two entries are logged for the same file with "Moved File" in the log file?
$source = "C:\folder\*.txt"
$target_dir = "\\samba-server\share\"
$bad_dir = "C:\folder\bad_files\"
$log = "C:\SystemFiles\Logs\transfer.log"
$files = Get-ChildItem $source
foreach ($file in $files){
if ($file.name -eq $null) {
# Nothing to do, Added this in since for some reason it executes the conditions below
}
elseif (test-path ($target_dir + $file.name)) {
# If there is a duplicate file, write to the log file, then copy it to the bad dir with
# the datetime stamp in front of the file name
$log_string = ((Get-Date -format G) + ",Duplicate File," + "'" + $file.name + "', " + $file.LastWriteTime)
write-output ($log_string) >> $log
$new_file = ($bad_dir + (get-date -format yyyy.MM.dd.HHmmss) + "_" + $file.name)
move-item $file.fullname $new_file
}
else {
# The file doesnt exist on the remote source, so we are good to move it.
move-item $file.fullname $target_dir
if ($?) { # If the last command completed successfully
$log_string = ((Get-Date -format G) + ",Moved File," + "'" + $file.name + "', " + $file.LastWriteTime)
} else {
$log_string = ((Get-Date -format G) + ",Failed to Move File," + "'" + $file.name + "', " + $file.LastWriteTime)
}
write-output ($log_string) >> $log
}
}

Answer 1

This is the classic producer-consumer problem, which is a well researched topic.
Some solutions you might try are checking the file's last write time. If it is well enough in the past, it can be moved without issues. Another one would be trying to open the file with exclusive access. If it fails, the file is still being used by the producer process. Othervise, close the file and move it.
Some examples are like so,
# List files that were modified at least five minutes ago
gci | ? { $_.lastwritetime -le (get-date).addminutes(-5) }
# Try to open a file with exclusive mode
try {
$f1 = [IO.File]::Open("c:\temp\foo.txt", [IO.Filemode]::Open, [IO.FileAccess]::Read, [IO.FileShare]::None)
# If that didn't fail, close and move the file to new location
$f1.Close()
$f1.Dispose()
Move-Item "c:\temp\foo.txt" $newLocation
} catch [System.IO.IOException]{
"File is already open" # Catch the file is locked exception, try again later
}