Negation in cmd and powershell

Question

Is there a way to do negation in cmd or powershell? In other words, what I want is to find all files that do not meet a certain criteria (other than attribute where negation is specified as "-") say in the name. It would be helpful if there was a generic negation that could be used in other contexts as well. In addition, for powershell, is there a way to get the list of file names and then store it as an array that can be sorted etc?

Apologies for asking something that seems so basic.

Answer 1

With PowerShell there are many ways to negate a set of criteria, but the best way depends on the situation. Using a single negation method in every situation could be very inefficient at times. If you wanted to return all items that were not DLLs older than 05/01/2011, you could run:

#This will collect the files/directories to negate
$NotWanted = Get-ChildItem *.dll| Where-Object {$_.CreationTime -lt '05/01/2011'}
#This will negate the collection of items
Get-ChildItem | Where-Object {$NotWanted -notcontains $_}

This could be very inefficient since every item passing through the pipeline will be compared to another set of items. A more efficient way to get the same result would be to do:

Get-ChildItem | 
  Where-Object {($_.Name -notlike *.dll) -or ($_.CreationTime -ge '05/01/2011')}

As @riknik said, check out:

get-help about_operators
get-help about_comparison_operators

Also, many cmdlets have an "Exclude" parameter.

# This returns items that do not begin with "old"
Get-ChildItem -Exclude Old*

To store the results in an array that you can sort, filter, reuse, etc.:

# Wrapping the command in "@()" ensures that an array is returned
# in the event that only one item is returned.
$NotOld = @(Get-ChildItem -Exclude Old*)

# Sort by name
$NotOld| Sort-Object
# Sort by LastWriteTime
$NotOld| Sort-Object LastWriteTime

# Index into the array
$NotOld[0]