Script to enforce backup retention policy - powershell

Question

I have a need for a script that can look at our backup folders (we take full backups of machines) and ensure that we never store more than the two most recent backups for each computer. I am currently working on a powershell loop that should compare each file name to every other file in the backup folder. Each machine has multiple backup files that can be identified by a matching prefix and a unique ID number separated by a delimiting character.
prefix = machine name and unique ID denotes each backup.
I am working on setting up the outer loop and running into a wall. I am storing the list of files to be checked in $array.
I have a nested for loop that I want to have check each file against every other file in the array. I have a query that should rebuild the array and exclude the current reference file each iteration.
for ($i=0; $i -lt 5; $i++) {
$array = Get-ChildItem -Path C:\Users\Aaron.Trujillo\Desktop\test
$array = Get-ChildItem -Path C:\Users\Aaron.Trujillo\Desktop\test -Exclude $array[$i]
for ($j=0; $j -lt 5; $j++){
Write-Host "i: $i j: $j"
Write-Host "Array i: $($array[$i].name) Array j: $($array[$j].name)"
if($array[$i].name -ne $array[$j].name)
{
Write-Host "truthval=false"
$truthval="false"
}
else
{
Write-Host "truthval=true"
$truthval = "true"
}
}
}
I was expecting this loop to compare the first five files in the array against each other (excluding the current reference file) and change the variable for each file, but for some reason the reference file isn't getting excluded.
Something must be wrong with the logic, but I'm struggling to spot it.

Answer 1

I see several flaws with your code.
As $array is presumably populated by a Get-ChildItem (which on ntfs formatted drives returns a sorted output) the nested loops aren't neccessary at all.
When iterating with one for simply compare with the predessor/follower index +/- 1.
As Names include per your description an ID, they are always different. So you've to split the name at the nebulous delimiter and check if the machine name is the same.
your if command doesn't emit anything, it just overwrites the same variable on every iteration.
With #Lee_Dailey's good idea of grouping you can split the names on the fly and check the count per group, sort files per group by LastWriteTime and only keep the newest 2.
So given a hypothetical tree of files
> tree /F
A:.
└───Backup
Alpha_8.bak
Bravo_2.bak
Bravo_5.bak
Bravo_6.bak
Bravo_7.bak
Bravo_9.bak
Charlie_1.bak
Charlie_10.bak
Charlie_3.bak
Charlie_4.bak
this small script:
$BackupDir = 'A:\Backup'
$Delim = '_'
Get-ChildItem -Path $BackupDir -File |
Group-Object {($_.BaseName -split $Delim)[0]}
yields:
Count Name Group
----- ---- -----
1 Alpha {Alpha_8.bak}
5 Bravo {Bravo_2.bak, Bravo_5.bak, Bravo_6.bak, Bravo_7.bak...}
4 Charlie {Charlie_1.bak, Charlie_10.bak, Charlie_3.bak, Charlie_4.bak}
You can then iterate the groups and delete files per group by whatever means,
for example delete all but the 2 newest (defined per LastWriteTime) files per machine:
Get-ChildItem -Path $BackupDir -File |
Group-Object {($_.BaseName -split $Delim)[0]} | ForEach-Object {
$_.Group | Sort LastWriteTime -Desc | Select -skip 2 | Remove-Item -WhatIf
}
If the output looks OK, remove the trailing -WhatIf

Answer 2

I think it's the brackets around the $array[$j.name] i.e. it should be:
if($array[$i].name -ne $array[$j].name)
Edit:
For troubleshooting try manually outputting your information:
for ($i=0; $i -lt 5; $i++) {
for ($j=$i+1; $j -lt 5; $j++){
Write-Host "i: $i j: $j"
Write-Host "Array i: $($array[$i].name) Array j: $($array[$j].name)"
if($array[$i].name -ne $array[$j].name)
{
Write-Host "truthval=false"
$truthval="false"
}
else
{
Write-Host "truthval=true"
$truthval = "true"
}
}
}