Introduction to Windows PowerShell - Type of Bracket
Did you ever buy a light bulb with a bayonet socket, only to discover that you really needed a bulb with a screw fitting? Well, PowerShell brackets require similar attention to detail otherwise
your script will also remain in the dark.
Windows PowerShell employs four types of
bracket, (parenthesis, also called curved) {braces, sometimes called curly} and [square]. Occasionally you may also see <angle> brackets. My point is that when you
use PowerShell commands, each type of bracket has a particular meaning and significance.
At first sight, one type of bracket seems much like another, but as you gain experience with a variety of
PowerShell constructions, you begin to tune-in to the differences. Eventually, you reach a level of expertise where it seems that the very type of PowerShell bracket is trying to tell you something
of significance. The bottom line is that if we employ the wrong type of bracket, instead of executing our commands, PowerShell presents us with an error message.
When a PowerShell construction requires multiple sets of brackets, the parenthesis (style) usually comes first. Parenthesis brackets
also come first in importance because these (curved) brackets are used for
what Guy calls compulsory arguments. Experts call these control
structures.
Let us take a foreach loop as an example. The (input definition) is the most important element;
it comes first and is enclosed by parenthesis. Observe that the {braces} style of bracket, comes second and inside these braces is a {Statement Block}, which dictates the code to execute.
Example 1: To demonstrate (parenthesis) and {braces} brackets
Note 1: Observe how the parenthesis brackets enclose the compulsory statement,
or control element ($objItem in $colItems), which starts the foreach loop.
Note 2: Find the
statement block,
enclosed by {braces}. This clause is important because it determines what to do inside each
loop. Where the statement block contains multiple lines, each half of the bracket has its own separate line, thus emphasising the action section of the script.
Note 3: While arrays are not covered here, please
note that the parentheses style of bracket would be used to declare an
array. For example, @() always return arrays.
Guy
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Employ braces, or curly brackets whenever you want to store a statement
block within your script. A common place to
pay attention to the braces style of PowerShell bracket is when you initiate a 'Where' clause.
# PowerShell curly bracket example Get-Wmiobject -list | where {$_.name -match "win32*"}
Note 1: Remember to the use of the | pipeline, because this streams the output of 'Get-Wmiobject -list', and passes it as input to the 'where' clause.
Guy thinks that the word which best sums up PowerShell's square bracket is: optional. Anytime you want to tag on an optional command or parameter, select the [square]
bracket.
However, experts think of square brackets as providing access to arrays. To
the expert, they introduce type names [2], and as character classes for
regular expressions. Another instance of the square bracket is for the
simpler 'like' operator, which is used for filename globbing.
Example 3a
This is one of the most useful application of PowerShell's square
bracket. The purpose is to filter out those properties beginning
with __
# Example of PowerShell Square Bracket
Clear-Host Get-WmiObject
Win32_Computersystem | Get-Member -Membertype property [a-z]*
Example 3b
List all the processes beginning with the letter 's'
Get-Process [s]*
Example 3c
Wildcards in square bracket can produce unexpected results. It's just a matter of trial and error and also you need to adjust to PowerShell's logic; [s-t] means beginning with 's', or
beginning 't'. '[SVC]' means beginning with 'S' or 'V' or 'C' and not beginning with specifically 'SVC....'.
Get-Process [r-s]*
Note 1: Experiment with different letter combinations, thus become expert at using the hyphen-filter.
Note 2: Pay attention to the wildcard asterisk*.
See what happens if you omit the *. Try: Get-Process [r*-s], or Get-Process [r-s]*. Did they produce what you expected? I was disappointed with [r*-s], but on reflection perhaps it
was a foolish request.
In a nutshell, the type of bracket is highly significant in Windows PowerShell. Take the time to tune-in to the personality of each style of bracket.
Parenthesis () come first, both in sequence and in
importance. Braces {} play a specific role in 'Where' statements and also
anywhere you need to employ a statement block. Lastly, Guy thinks of PowerShell's square [] brackets
as controlling optional parameters. Experts think of square brackets
as providing access to an array.
The most important
lesson is that each type of PowerShell bracket has a particular role; you must choose the correct bracket for the particular scripting task.
Guy would like to acknowledge Luke Breuer's help in providing deeper
insights into PowerShell's brackets.
If you like this page then please share it with your friends
Please email me if you have a better example script. Also please report any factual mistakes, grammatical errors or broken links, I will be happy to correct the fault.
Windows Management Instrumentation (WMI) is one of the hidden
treasures of Microsoft operating systems.
Fortunately, Solarwinds
have created the
Free WMI Monitor so that you can actually see and understand these gems of
performance information. Take the guess work out of which
WMI counters to use for applications like Microsoft Active Directory,
SQL or Exchange Server.
