Ezine 178 - Filter Data with PowerShell's -Match and -Like
How to Filter Data with PowerShell's -Match and -Like
As with life, scripts often give us too much information. In both
settings the answer is a suitable filter. PowerShell provides a rich
variety of tools to help us select just the
data we need. My favourite
method, as we will see, is the 'Where' statement with the '-match' comparitor.
Topics for the Comparators -Match and -Like
♣
In the case of spreadsheet formulae I admit a fixation with the 'If'
statement. Often times it would have been more efficient to use
'Lookup'. With PowerShell scripts, although again I could use the 'If' construction, my
preference now is for the 'where' clause. My point is that
whether you are designing a spreadsheet or a script, there are numerous ways of achieving
the same result. And while some methods are more
efficient than others, it's often just a matter of taste which method you
choose.
This Week's Mission is relatively straightforward; I want to show you
PowerShell techniques which filter data. The scenario is that we want research WmiObjects
in general and 'system' classes in particular. The problem is we are swamped with
names of hundreds of
classes.
Our solution will be to use the parent construction of a 'Where-Object' clause,
with a comparitor of -Match or -Like.
-Match and -Like
We can distinguish between these two parameter in several ways. In
terms of remembering the difference I think of '-Match' as meaning a pure pattern
match. Whereas to me, '-Like' is a more fuzzy concept that is going
to need the wildcard* symbol. A more technical definition is as
follows: -Like is just a wildcard comparison, while -Match is a regular
expression, and a superset of -Like.
WmiObject is our vehicle for this example. The where-Object
statement employs a comparitor to find the pattern "system".
Because we use -Match, "system" can be anywhere in the
WMI object's name. Incidentally, this combination of 'Where' with
'-Match' is my favourite method of filtering data.
# PowerShell example to list demonstrate -Match
# Author: Guy Thomas
#
May 2010 tested on PowerShell v 1.0 and 2.0 clear-Host
$WMI = get-WmiObject -list | where-Object {$_.name
-match "system"}
$WMI
Write-Host
$WMI.count "WMI objects contain the word system"
Learning Points
Note 1: Take special note of the
important construction $_. This means 'in this data stream'.
Note 2: See how I have employed PowerShell's
signature tune the (|) pipe. The purpose of the pipe is to make the output
of the list,
the input into the where
clause.
Challenge 1: (Easy) remove the -Object from the where clause.
The script should run just the same.
Challenge 2: Substitute the question mark (?) for where-Object.
Guy does not like this particular short-cut but many script writers prefer the tighter code
which results from using this alias.
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The only two changes compared with Example 1 are substituting -Like for
-Match and adding a wildcard* in front of system. -Like says to me
'sort of', 'fuzzy', or 'around about'.
# PowerShell example to demonstrate the comparitor -Like
# Author: Guy Thomas
# Version 1.3
December 2008 tested on PowerShell v 1.0
clear-Host
$WMI = get-WmiObject -list | where-Object
{$_.name -like "*system"}
$WMI
Write-Host $WMI.count "WMI
objects contain the word system"
Learning Points
Note 1: Introducing $WMI.count helps us to compare these results with example 1;
you should see that -Like produces fewer wmiObjects
then -Match. To make -Like show all the "system" object you need
to add another
wildcard: "*system*".
To give you perspective, I have introduced
variations, for example employing an 'If' construction instead of the
'Where' in example 1 and 2.
clear-Host
$i=0
get-WmiObject -list | foreach {
if($_.name -match "TCP") {
write-Host $($_.name) ;
$i++
}
}
"There are $i objects containing
TCP"
Learning Points
Note 1: Unlike 'Where', the 'If' statement needs
the additional command 'foreach' to kick off the loop through the $_.name.
Note 2: The syntax of the 'If' construction is
(test) then {script block for matching items}
Note 3: In the line: "There are $i objects
containing TCP"
admire how PowerShell interprets the counting variable
($i) in the middle of that text string. This is called expanding a
variable, and only works with "Double quotes".
The most obvious comparitor is 'equals'. You code this in PowerShell
with
-eq (not =). However in the present examples, to use -eq
effectively we would need to know the name of the specific class.
Whereas in this context we want broader research into possible names
containing 'Win32'.
Thus let us look at alternatives to -eq. It is surprising how often the negative -NotMatch
produces a neat solution to a scripting problem. For instance there
are many WmiObjects beginning
with CIM, thus one way of excluding them would be to use -NotMatch "CIM".
Just one more point, using multiple criteria for your filter is easy once you master the
deceptively simple '-And' syntax.
# PowerShell example to list demonstrate -NotMatch and -and
#
Author: Guy Thomas
# Version 1.2 December 2008 tested on PowerShell v
1.0
clear-PSAhost
$WMI = get-WmiObject -list `
| where-Object
{$_.name -NotMatch "CIM" -And $_.name -NotMatch "__"}
$WMI
Write-Host $WMI.count "WMI objects not contain CIM or __"
Learning Points
Note 1: It would make my day if you experimented with
different filters. Substitute your ideas for "CIM", and "__".
Perhaps best of all would be to combine -NotMatch and -Match.
Follow-up
As usual with my scripts, the mission is to get you started. If
you want to know more about -Match, -Like and their relatives, then start
with PowerShell's own help thus:
get-help about_comparison_operators.
(Or try get-help about*)
These help files introduce a whole world of specific terms, for example, 'regular expression
comparisons' and 'wildcard comparison'. Once you need and understand
such extra information, then I have succeeded in my mission of introducing
you to -Match and -Like.
If you are looking for handy network utilities, try some of the free downloads at
Tools4Ever
If you like this page then please share it with your friends
See more Microsoft PowerShell tutorials
• PowerShell Tutorials •
Methods • Cmdlets
• PS Snapin •
Profile.ps1 • Exchange 2007
•
Command & Expression Mode •
PowerShell pipeline (|) •
PowerShell 'where' •
PowerShell 'Sort'
If you see an error of any kind, do let me know. Please report any factual mistakes, grammatical errors or broken links, I will be happy to not only to correct the fault, but also to give you credit.
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