Building a 7-zip command string in powershell - powershell

Question

My first excursion into a real app in Powershell; I could write the code in a traditional .Net language, but I think Powershell is ideal for console apps like this. I've searched StackOverflow and found the (nearly) exact script I need; my issue is that I want to take a folder of files (on server fsA) and create 7-Zip archives on fsB named by the file date. For instance, all the files that were last written on 7/21/12 would be found in archive 20120721.7z. I'm basing this on the solution found here: Create a 7z Archive for files created by date using Powershell
Here is what I have so far:
$prog_dir = "C:\Progra~1\7-Zip\7z.exe"
$archive_dir = "\\fsa\Backup"
$input_dir = "\\fsb\Xml\Archive\"
$7zOpts = "-m0=PPMd:o32:mem192M"
$groups = dir $input_dir | group-object -property {$_.LastWriteTime.Date}
$groups | foreach{$cmd = $prog_dir + " a " + $archive_dir + "\$((Get-Date $_.Name).ToString(`"yyyyMMdd`")).7z $([string]::join(`" `", $_.Group)) " + $7zOpts} #; invoke-expression $cmd}
#
#Where to put this: "+ $input_dir +"???
#
$cmd
I can't seem to find a way to specify the input directory in the foreach line; I can put it at the beginning of the join statement, but it only adds the input directory to the first file in the list. For instance:
$groups | foreach{$cmd = $prog_dir + " a " + $archive_dir + "\$((Get-Date $_.Name).ToString(`"yyyyMMdd`")).7z " + $input_dir + "$([string]::join(`" `", $_.Group)) " + $7zOpts} #; invoke-expression $cmd}
produces the output
C:\Progra~1\7-Zip\7z.exe a \\fsa\Backup\20120722.7z \\fsb\Xml\Archive\255IQR.xml 2573WV.xml 257RMC.xml
where the last two files do not have the full path prepended? What am I doing wrong?

Answer 1

PowerShell is a shell, so running commands is pretty much the thing it should do well. So generally, if you resort to Invoke-Expression when running commands you're doing something very wrong. Let's try a different way:
$prog_dir = 'C:\Progra~1\7-Zip\7z.exe' # you don't need 8.3 names here, by the way
$archive_dir = '\\fsa\Backup'
$input_dir = '\\fsb\Xml\Archive\'
$7zOpts = '-m0=PPMd:o32:mem192M'
$groups = dir $input_dir | group-object -property {$_.LastWriteTime.Date}
$groups | foreach {
$files = $_.Group | select -expand FullName
$date = (Get-Date $_.Name).ToString('yyyyMMdd')
& $prog_dir a $archive_dir\$date.7z $files $7zOpts
}