Powershell Sort-Object by [DateTime]::ParseExact

Question

I'm trying to sort the lines returned by a console program by date, using PowerShell.

The dates are formated in MM/dd/yyyy format, so they have to be converted to DateTime objects to be in a sortable format.

To parse the dates, I use:

$dates = %{ "10/24/2010", "02/03/2010" }
$dates | %{ [System.DateTime]::ParseExact($_, "MM/dd/yyyy", $null) }

This parses the dates into System.DateTime objects and displays their default ToString() representation, but it also shows an additional blank line at the beginning.

Now, if I try to sort the dates with Sort-Object, I get an error message, and I guess the error comes from the additional blank line:

$sortedDates = $dates | Sort-Object [System.DateTime]::ParseExact($_, "MM/dd/yyyy", $null)

Error message:

"Sort-Object : A positional parameter cannot be found that accepts argument 'System.Object[]'."

Where is the extra blank line coming from? Am I doing something wrong to parse the dates, or to sort them?

Mike Shepard · Accepted Answer · 2011-04-28 16:30:00Z

9

I think part of the problem is in the first line.

%{ } means foreach-object { }.

I think you meant @( , ). You don't really even need the @( ).

$dates= "10/24/2010", "02/03/2010"

works fine.
To sort by a "derived field", use a scriptblock.

$sortedDates = $dates | Sort-Object {[System.DateTime]::ParseExact($_, "MM/dd/yyyy", $null)}

answered Apr 28, 2011 at 16:30

Mike Shepard

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ckarras · Accepted Answer · 2011-04-28 17:26:48Z

0

For reference, if anyone has a similar problem, here is my complete solution (based on the accepted answer)

I'm sorting the output of tf.exe labels (TFS command line) by date, and then getting the most recent label:

$lines = tf labels | Select-Object -skip 2 # Skip the header lines
$sortedLines = $lines | Sort-Object { [DateTime]::ParseExact(($_ -split '\s+')[2],"M/d/yyyy",$null) }

$labels = $sortedLines | %{ ($_ -split '\s+')[0] } # get the first column

$latestLabel = $labels[-1] # Last item in the list

answered Apr 28, 2011 at 17:26

ckarras

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