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.
Types of PowerShell Bracket
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1) Parenthesis Bracket ()
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.
Another way of looking at parenthesis is they resolve problems of ambiguity; if PowerShell cannot decide which order to process components, clarify your instructions with parentheses.
Example 1: To Demonstrate (parenthesis) and {braces} Brackets
While this example lists your network adapters, my purpose is introduce the brackets in the control structure.
# PowerShell cmdlet to demonstrate brackets
$WmiClass = "Win32_NetworkAdapterConfiguration"
$colItems = Get-WmiObject -class $WmiClass
ForEach ($objItem in $colItems)
{
Write-Host $objItem.caption
Write-Host $objItem.ipaddress
Write-Host $objItem.macAddress
}
Note 1: Observe how the parenthesis brackets enclose the compulsory statement, or control element ($objItem in $colItems), which starts the foreach loop.
Note 2: Find the statement block, enclosed by {braces}. This clause is important because it determines what to do inside each loop. Where the statement block contains multiple lines, each half of the bracket has its own separate line, thus emphasising the action section of the script.
Note 3: While arrays are not covered here, please note that the parentheses style of bracket would be used to declare an array. For example, @() always return arrays.
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Example 2: PowerShell Braces Bracket { }
Employ braces, or curly brackets whenever you want to store a statement block within your script. A common place to pay attention to the braces style of PowerShell bracket is when you initiate a 'Where' clause.
# PowerShell curly bracket example
Get-WmiObject -List | Where-Object{$_.name -Match "win32*"}
Note 4: Remember to the use of the | pipeline, because this takes the output of ‘Get-WmiObject -List’, and passes it as input to the ‘where’ clause.
PowerShell ‘If’ Statement – Braces Bracket {}
Remember that with 'If' the (parenthesis are for the condition), whereas {braces are for the block command}.
If (condition) {block command}
# Another explanation would be
If (test) {
"Execute payload when test is true"
}
See More on PowerShell's If Statement »
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Example 3: PowerShell's Square Bracket [ ]
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 5: Experiment with different letter combinations, thus become expert at using the hyphen-filter.
Note 6: 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.
Conclusion About PowerShell's Brackets
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.
See More on PowerShell's Syntax »
Summary of PowerShell’s Brackets
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.
Footnote:
Guy would like to acknowledge Luke Breuer's help in providing deeper insights into PowerShell's brackets.
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See more Windows PowerShell flow control examples
• PowerShell Home • PowerShell If Statement • PowerShell ElseIf • Free Permissions Analyzer
• Conditional Operators • PowerShell -Match • PowerShell -Like • PowerShell -Contains
• PowerShell Comparison Operators • PowerShell Syntax • Where Filter • PowerShell Else
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.
