Windows PowerShell's Syntax
Introduction to PowerShell's SyntaxThe fact that you
almost don't need this page is a testament to the intuitive nature of PowerShell. Yet for those who wish to save time fumbling with the
PowerShell syntax, it may pay to have a refresher of these rules of
scripting grammar. Windows PowerShell Syntax Topics
♣
For many years a bad attitude to syntax hindered me. My
breakthrough was realizing that punctuation marks are there to aid the
readers' understanding; my mistake was thinking syntax rules were designed
by my English teacher as a way of finding new ways to tell me off.
With PowerShell's syntax the comma is frequently used to separate items on a list.
Whereas the semi-colon is used to split separate ideas. Let us study this
example:
# Eventlog example script to illustrate PowerShell's syntax.
Clear-host
$i=0
$Log = Get-EventLog -List
ForEach ($Item in
$Log) {
"{0,-30} {1,-20} {2,13}" -f `
$Item.Log, $Item.OverflowAction,
$Item.MaximumKilobytes
}
Note 1: Each $Item is separated by a comma. No
sign of the semi-colon, yet.
Note 2: The comma is also used to separate items in an
array {0,-30}
Suppose we want to count the number of eventlogs. Let us introduce
a variable $i
Clear-host
$i=0
$Log = Get-EventLog -List
ForEach ($Item in
$Log) {
"{0,-30} {1,-20} {2,13}" -f `
$Item.Log, $Item.OverflowAction,
$Item.MaximumKilobytes; $i++
}
"There are $i eventlogs"
Note 3: The counter variable, $i++ is new element, which
is not connected to the list; time for a semi-colon before the counter
variable.
The equals sign (=) behaves just as expected. As usual,
'=' tests for equivalence, my main use for equals sign is to sets a variable to
= a certain value.
The equals sign has a counterpart ! (Exclamation
mark) meaning, 'not equal'. You may also employ -Not instead of ! I just include these
two basic operators, '=' and ! for completeness.
PowerShell -eq
PowerShell has a family of conditional operators
- -eq meaning equals
- -ne in the negative, not equal to...
Note: there is no
-neq operator; just use the two letters -ne.
- -gt and also -ge (greater than or equal)
- -lt and also -le (less than or equal)
Here is how you would use the most famous member -eq
Clear-host
Get-Service | Where-Object {$_.Status -eq
"Running"}
Note 4: Don't be tempted to use the "=" sign here, that
would be a big mistake.
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PowerShell is fundamentally case insensitive.
Every object and every cmdlet is case insensitive. Set-Location
performs exactly the same action as set-location. However, where your data has case sensitive values, there are PowerShell operators to deal with
'case'. For example, -gt means greater than, -Match means
contains a particular string value.
However, you can force these and similar operators to be case sensitive by prefixing
hem with a 'C'. -CMatch, or -CGt mean that the comparison will be case sensitive.
See the
rest of PowerShell's conditional operators
»
When I wanted to join text and numbers, I spent time looking for PowerShell's concatenator. Silly me, all
I need is the simple + plus sign. Where other languages use + for
adding numbers, PowerShell uses ' + ' for
joining strings, or even for combining text with numbers:
#PowerShell + concatenator
$Total = 180
"My total is " + $Total
Some people call this symbol (-) minus, others a refer to this sign as a dash, I mostly call it a hyphen. Let me be clear, this character maps to ASCII 45, to see the character, hold down ALT key, type 45 on numeric keypad, now let go of ALT key. PowerShell uses this - symbol for two
purposes. Firstly, to join verb-Noun pairs, for example Out-File guy.txt. Secondly, this minus sign is also used for
parameters, modifiers, or filters such as -List; as in Get-Eventlog -List. The
trap I fall into is to put a space between the minus and the modifier. get -Eventlog is clearly wrong,
because there is a space between get and -. The correct format is, Get-Eventlog,
with no space.
Guy
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The ability to pipe the output of one command, so that it becomes the input
of the second command is PowerShell's signature tune. Thus it is
important to be clear about this | symbol.
When typed in notepad, the pipeline symbol looks like this: | but when typed in the Microsoft Shell
it looks like ¦. On my keyboard the key I am using this symbol is next to the z, however I have seen keyboards where the
pipeline key is next
to numeric 1 on the top row. Once you find, then type the key, you get a pipe symbol
(|). To be crystal clear this pipeline symbol corresponds to ASCII 124. N.B this not ASCI 0166.
Test by holding down the Alt key and typing the number (124 or 0166) on the numeric pad, then letting go of the Alt key.
In PowerShell syntax the pipeline symbol (|) has three roles.
- Think of the pipeline as a method for joining two commands.
Get-Eventlog system |
Format-List
You could even have two pipelines in one statement. - PowerShell deploys Pipeline to introduce
a 'Where' clause.
Get-Eventlog security |where {$_.Eventid -eq "540"}
-
Pipeline is similar to 'more' in DOS
Get-Eventlog system | more
...
See more about $_.
If I had to choose one element of PowerShell's syntax to master it would
be the bracket. I love the logic of an 'If' statement; however, to get
the command to work you have to understand If (parenthesis for condition)
{curly brackets for payload}.
At first PowerShell's brackets surprised me. Each type has a
specific role, the wrong bracket will cause an otherwise sound command, to
fail miserably. The message is clear, you have to understand your
brackets. Let us see how each of these (), {} or [] has a different
purpose. 1) () Parenthesis or Curved brackets are
used for required options in the foreach loop
# PowerShell syntax - types of bracket
Clear-host
$disk=
WmiObject Win32_LogicalDisk
"Drive Letter Size GB "
Foreach
($drive in $disk ) {"Drive = " +
$drive.Name}
2) {} Braces or 'curly' brackets are required for
block expressions within a command, for instance, the 'where' or 'Where-Object' command. Example: Get-Service |
Where-Object {$_.status
-eq "stopped" }
| 
