Parsing

 

SHORT DESCRIPTION

How the Windows PowerShell parses commands

 

LONG DESCRIPTION

When you enter a command at the command prompt, PowerShell breaks the command text into a series of segments called tokens and then determines how to interpret each one. For example, PowerShell breaks the following command into two tokens:

 

 

The first token is "Write-Host," and the second token is "book." PowerShell interprets each token separately, within the context of the command as a whole.

 

When processing a command, the PowerShell parser operates in one of two modes:

 

·         Expression mode - Character string values must be contained in quotation marks. Numbers not enclosed in quotation marks are treated as numerical values (rather than as a series of characters).

·         Argument mode - Each value is treated as an expandable string unless it begins with one of the following special characters: dollar sign ($), at symbol (@), single quote (?), double quote ("), or an opening parenthesis ((). If preceded by one of these characters, the value is   treated as a value expression.

 

The following table provides several examples of commands processed in expression mode and argument mode and the results produced by those commands:

 

Example	Mode	Result
2+2	expression	numerical value of "4"
Write-Output 2+2	argument	string value of "2+2"
Write-Output (2+2)	expression	numerical value of "4"
$a = 2+2	expression	numerical value of "4"  (assigned to $a)
Write-Output $a	expression	numerical value of "4"
Write-Output $a/H	argument	string value of "4/H"

 

Every token can be interpreted as some kind of object type, such as boolean or string. PowerShell attempts to determine the object type from the expression. The object type depends on the type of parameter a command expects and on whether PowerShell knows how to convert the argument to the correct type. The following table shows several examples of the types assigned to values returned by the expressions:

 

Example	Mode	Result
Write-Output !1	argument	string value of "!1"
Write-Output (!1)	expression	boolean value of "false"
Write-Output (2)	expression	integer value of "2"

 

When processing cmdlet parameters, PowerShell recognizes strings contained in quotation marks as being different from strings not enclosed in quotation marks. For example, in the following command, -type is treated as a parameter name:

 

 

In this case, "cmdlet" is the argument passed to the -type parameter. However, in the next example, -type is treated as an argument:

 

 

PowerShell will attempt to use the "-type" value as an argument to the first positional parameter, while the "cmdlet" argument will be treated as a parameter name, causing the command to fail.

 

SEE ALSO

For information about command syntax, enter the following command at the PowerShell command prompt:

 

 

For information about quotation rules, enter the following command: