Anomaly in powershell script doesn't make sense

Question

We have a SQL Server on a Windows 2016 server that creates a SQL Server backup of the .MDF and Log files once a weekday basis scheduled for 23:30. I created a housekeeping script to start deleting these .bak files that are older than two working days.

However, the check for a Tuesday, functions with an anomaly. In the script below the part AddDays(4) retains files for Monday, Friday AND Thursday instead of Monday and Friday only. But if I change this to AddDays(3) it ONLY retains Monday's files.

Why is this behaving like this?

# Determine day of week
$weekDay = (get-date).DayOfWeek.ToString()
$firstAbbrevCheck = $weekDay.SubString(0,3)
# write-host $firstAbbrevCheck
# Write date to log file to ensure sripts ran
$todaysDate = (get-date).ToString("ddMMyyyy")
Write-output $todaysDate | Out-file <path>\HouseKeepLog.txt -append
# If not a sunday or Monday then process
if($firstAbbrevCheck -ne "Sun") {
  if($firstAbbrevCheck -ne "Mon") {
    $secondAbbrevCheck = $weekDay.SubString(0,2)
    # If retaining two work days of backups then for Tuesday delete 
      files greater than 4 days old
    if($secondAbbrevCheck -eq "Tu") {
     Get-ChildItem –Path <Path to backup folder> -recurse -Filter 
       *.bak -exclude master*, msdb* | Where-Object 
      {($_.LastWriteTime -lt (Get-Date).AddDays(-4))} | Remove-Item
    }
    else {
    # Else Wed - Sat and delete files greater than 2 days old 
    Get-ChildItem –Path <Path to backup folder> -recurse -Filter 
      *.bak -exclude master*, msdb* | Where-Object 
      {($_.LastWriteTime -lt (Get-Date).AddDays(-2))} | Remove-Item
    }
  }
 }

That will be hard to tell without the exact inputs. Code looks fine - it might be possible that since you are using (get-date).adddays(), you are including the TIME the script runs at and thus leading to unexpected output. Try and add debugging information to your script to find out where the logical error occurs. — hcm
– hcm, Commented Dec 28, 2019 at 14:50

avarice · Accepted Answer · 2019-12-28 16:24:17Z

1

You can also use UFormat. Here's how:

$today = Get-Date 
$wd = Get-Date -UFormat "%A"
Write-output ('{0:ddMMyyyy}' -f $today >><path>\HouseKeepLog.txt
if (($wd -notmach "Su.*") -and ($wd -notmatch "Mo.*")) {
   $rd = switch -Regex ($wd) {
     "Tu.*" { $today.AddDays(-4).Date }    
     default { $today.AddDays(-2).Date }
    }
 Get-ChildItem "Path to backup folder" -Recurse -Filter "*.bak" -Exclude "master*", "msdb*" -File | 
            Where-Object {($_.LastWriteTime -lt $rd)} | 
            Remove-Item
}

answered Dec 28, 2019 at 16:24

avarice

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Theo · Accepted Answer · 2019-12-28 14:58:48Z

Why do the manual abbreviations at all and not use the int number from the dayofweek
(0 --> Sunday, 6 --> Saturday)? This will make for shorter code AND you don't have to worry about locale weekday names.

# Determine day of week
$today   = Get-Date
$weekDay = [int]$today.DayOfWeek   # today's weekday: 0 --> Sunday, 6 --> Saturday

# Write date to log file to ensure sripts ran
Write-output ('{0:ddMMyyyy}' -f $today) | Out-file <path>\HouseKeepLog.txt -append
# If not a sunday or Monday then process
if ($weekDay -gt 1) {
    # use '.Date' to strip off the time part, effectively return the date at midnight
    $refDate = switch ($weekDay) {
        2 { $today.AddDays(-4).Date }        # Tuesday:   delete files greater than 4 days old
        default { $today.AddDays(-2).Date }  # Wed..Sat:  delete files greater than 2 days old
    }
    Get-ChildItem –Path '<Path to backup folder>' -Recurse -Filter '*.bak' -Exclude 'master*', 'msdb*' -File | 
        Where-Object {($_.LastWriteTime -lt $refDate)} | 
        Remove-Item -WhatIf
}

^{P.S. remove or comment out the -WhatIf switch if you are satisfied with the results shown in the console}

js2010 · Accepted Answer · 2019-12-28 15:27:39Z

0

"Tu" not "Tue"? Btw you can compare enums with strings:

(get-date).dayofweek -eq 'saturday'
True

'saturday' -eq (get-date).dayofweek
True

Fyi, here's how the dayofweek enum is laid out:

(get-date).dayofweek.gettype()               

IsPublic IsSerial Name                                     BaseType
-------- -------- ----                                     --------
True     True     DayOfWeek                                System.Enum

0..6 | foreach { [system.dayofweek]$_ } 

Sunday
Monday
Tuesday
Wednesday
Thursday
Friday
Saturday

edited Dec 28, 2019 at 15:27

answered Dec 28, 2019 at 15:19

js2010

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