MicrosoftВ® Windows PowerShell(TM) Step By Step (Step By Step (Microsoft))

Understanding the use of variables and constants in Windows PowerShell is fundamental to much of the flexibility of the PowerShell scripting language. Variables are used to hold information for use later in the script. Variables can hold any type of data, including text, numbers, and even objects.

Use of Variables

By default when working with Windows PowerShell, you do not need to declare variables before use. When you use the variable to hold data, it is declared. All variable names must be preceded with a dollar sign ($). There are a number of special variables in Windows PowerShell. These variables are created automatically and have a special meaning. A listing of the special variables and their associated meaning appears in Table 4-2.

In the  ReadUserInfoFromReg.ps1 script, there are five variables used. These are listed in Table 4-3.

The  ReadUserInfoFromReg.ps1 script uses the Set-Location cmdlet to change to the HKCU PSDrive. This makes it easier to work with the registry. After the location has been set to the HKCU drive, the script uses the Get-ItemProperty cmdlet to retrieve the data stored in the specified registry key. The Get-ItemProperty cmdlet needs two arguments to be supplied: path and name. The path argument receives the registry path that is stored in the $strUserPath variable, whereas the name argument receives the string stored in the $strUserName variable.

After the value is retrieved from the registry, the object is pipelined to the Format-List cmdlet, which once again uses the string contained in the $strUserName variable as the property to display.

The really powerful aspect of the  ReadUserInfoFromReg.ps1 script is that it uses the array of strings contained in the $strName variable. To read the values out of the registry, we need to “singularize” the strings contained within the $strName variable. To do this, we use the ForEach-Object cmdlet (however, we reference it by the alias foreach). After we have an individual value from the $strName array, we store the string in a variable called $i. The Get-ItemProperty cmdlet is used in exactly the same manner as it was used earlier. However, this time, we use the string contained in the $strPath variable, and the name of the registry key to read is contained in the $i variable, whose value will change four times with the execution of each pass through the array.

When the  ReadUserInfoFromReg.ps1 script is run, it reads five pieces of information from the registry: the logon user name, the logon server name, the user’s home path location, the user’s application data store, and the user’s home drive mapping. The  ReadUserInfoFromReg.ps1 script is shown here:

ReadUserInfoFromReg.ps1 $strUserPath = "\Software\Microsoft\Windows\CurrentVersion\" ` + "Explorer" $strUserName = "Logon User Name" $strPath = "\Volatile Environment" $strName = "LOGONSERVER","HOMEPATH", "APPDATA","HOMEDRIVE" Set-Location HKCU:\ Get-ItemProperty -path $strUserPath -name $strUserName | Format-List $strUserName foreach ($i in $strName) {Get-ItemProperty -path $strPath -name $i | Format-List $i}

Q. To read a value from the registry, which provider is used?

A. The registry provider is used to read from the registry.

Q. Which cmdlet is used to retrieve a registry key value from the registry?

A. The Get-ItemProperty cmdlet is used to retrieve a registry key value from the registry.

Q. How do you concatenate two string values?

A. You can use the plus symbol (+) to concatenate two string values together.

Exploring strings

1. Open Windows PowerShell.

2. Create a variable called $a and assign the value "this is the beginning" to it. The code for this is shown here:

   $a = "this is the beginning"

3. Create a variable called $b and assign the number 22 to it. The code for this is shown here:

   $b = 22

4. Create a variable called $c and make it equal to $a + $b. The code for this is shown here:

   $c = $a + $b

5. Print out the value of $c. The code for this is shown here:

   $c

6. The results of printing out c$ are shown here:

   this is the beginning22

7. Modify the value of $a. Assign the string "this is a string" to the variable $a. This is shown here:

   $a = "this is a string"

8. Use the up arrow and retrieve the $c = $a + $b. This command is shown here:

   $c = $a + $b

9. Now print out the value of $c. The command to do this is shown here:

   $c

10. Assign the string "this is a number" to the variable $b. The code to do this is shown here:

    $b = "this is a number"

11. Use the up arrow to retrieve the $c = $a + $b command. This will cause Windows PowerShell to re-evaluate the value of $c. This command is shown here:

    $c = $a + $b

12. Print out the value of $c. This command is shown here:

    $c

13. Change the $b variable so that it can only contain an integer. (Data type aliases are seen in Table 4-4.) Use the $b variable to hold the number 5. This command is shown here:

    [int]$b = 5

14. Use the up arrow to retrieve the $c = $a + $b command. This command is shown here:

    $c = $a + $b

15. Print out the value contained in the $c variable, as shown here:

    $c

16. Assign the string "this is a string" to the $b variable. This command is shown here:

    $b = "this is a string"

17. Attempting to assign a string to a variable that has a [int] constraint placed on it results in the error shown here (these results are wrapped for readability):

    Cannot convert value "this is a number" to type "System.Int32". Error: "Input string was not in a correct format." At line:1 char:3 + $b <<<< = "this is a string"

18. This concludes this procedure.

These commands are found in the  ExploringStrings.txt script in the scripts folder for this chapter.

Use of Constants

Constants in Windows PowerShell are like variables-with two important exceptions: their value never changes, and they cannot be deleted. Constants are created by using the Set-Variable cmdlet and specifying the option argument to be equal to constant.

In the  GetHardDiskDetails.ps1 script, we create a constant called $intDriveType and assign the value of 3 to it because the WIN32_LogicalDisk WMI class uses a value of 3 in the disktype property to describe a local fixed disk. Because we are not interested in network drives, removable drives, or ram drives, we use the Where-Object to return only items that have a drivetype of 3.

Q. How do you create a constant in a script?

A. You create a constant in a script by using the Set-Variable and specifying the value of constant to the option argument.

Q. How do you indicate that a variable will only hold integers?

A. To indicate that a variable will only contain integers, use the [int] in front of the variable name when assigning a value to the variable.

In looking at the  GetHardDiskDetails.ps1 script, the value of $intDriveType is never changed. It is assigned the value of 3 on the Set-Variable line. The $intDriveType constant is only used with the Where filter line. The value of $strComputer, however, will change once for each computer name that is specified in the array $aryComputers. In this script, it will change twice. The first time through the loop, it will be equal to loopback, and the second time through the loop, it will be equal to localhost. However, if you added 250 different computer names, the effect would be the same-the value of $strComputer would change each time through the loop.

 GetHardDiskDetails.ps1

$aryComputers = "loopback", "localhost" Set-Variable -name intDriveType -value 3 -option constant foreach ($strComputer in $aryComputers) {"Hard drives on: " + $strComputer Get-WmiObject -class win32_logicaldisk -computername $strComputer| Where {$_.drivetype -eq $intDriveType}}