about_Trap

TOPIC
	about_Trap

SHORT DESCRIPTION
	Describes a keyword that handles a terminating error.


LONG DESCRIPTION
	A terminating error stops a statement from running. If Windows PowerShell 
	does not handle a terminating error in some way, Windows PowerShell also 
	stops running the function or script in the current pipeline. In other 
	languages, such as C#, terminating errors are referred to as exceptions.


	The Trap keyword specifies a list of statements to run when a terminating 
	error occurs. Trap statements handle the terminating errors and allow 
	execution of the script or function to continue instead of stopping.


  Syntax

	The Trap statement has the following syntax:

		trap [[<error type>]] {<statement list>}


	The Trap statement includes a list of statements to run when a 
	terminating error occurs. The Trap keyword can optionally specify an 
	error type. An error type requires brackets. 


	A script or command can have multiple Trap statements. Trap statements 
	can appear anywhere in the script or command. 


  Trapping All Terminating Errors

	When a terminating error occurs that is not handled in another way in a 
	script or command, Windows PowerShell checks for a Trap statement that 
	handles the error. If a Trap statement is present, Windows PowerShell 
	continues running the script or command in the Trap statement.
 

	The following example is a very simple Trap statement:

		trap {"Error found."}


	This Trap statement traps any terminating error. The following example is 
	a function that contains this Trap statement:

		function TrapTest {
			trap {"Error found."}
			nonsenseString
		}


	This function includes a nonsense string that causes an error. Running 
	this function returns the following:

		C:\PS> TrapTest
		Error found.


	The following example includes a Trap statement that displays the error 
	by using the $_ automatic variable:

		function TrapTest {
			trap {"Error found: $_"}
			nonsenseString
		}


	Running this version of the function returns the following:

		C:\PS> TrapTest
		Error found: The term 'nonsenseString' is not recognized as the name 
		of a cmdlet, function, script file, or operable program. Check the 
		spelling of the name, or if a path was included verify that the path 
		is correct, and then try again. 


	Trap statements can also be more complex. A Trap statement can include 
	multiple conditions or function calls. It can log, test, or even run 
	another program.


   Trapping Specified Terminating Errors

	The following example is a Trap statement that traps the 
	CommandNotFoundException error type:

		trap [System.Management.Automation.CommandNotFoundException] 
			{"Command error trapped"}


	When a function or script encounters a string that does not match a known 
	command, this Trap statement displays the "Command error trapped" string. 
	After running any statements in the Trap statement list, Windows 
	PowerShell writes the error object to the error stream and then continues 
	the script.


	Windows PowerShell uses the Microsoft .NET Framework exception types. The 
	following example specifies the System.Exception error type:

		trap [System.Exception] {"An error trapped"}


	The CommandNotFoundException error type inherits from the 
	System.Exception type. This statement traps an error that is created by 
	an unknown command. It also traps other error types.


	You can have more than one Trap statement in a script. Each error can be 
	trapped by only one Trap statement. If an error occurs, and more than one 
	Trap statement is available, Windows PowerShell uses the Trap statement 
	with the most specific error type that matches the error. 


	The following script example contains an error. The script includes a 
	general Trap statement that traps any terminating error and a specific 
	Trap statement that specifies the CommandNotFoundException type.

		trap {"Other terminating error trapped" }
		trap [System.Management.Automation.CommandNotFoundException] {"Command error trapped"}
		nonsenseString


	Running this script produces the following result:

		Command  error trapped
		The term 'nonsenseString' is not recognized as the name of a cmdlet, 
		function, script file, or operable program. Check the spelling of 
		the name, or if a path was included verify that the path is correct,
		and then try again.
		At C:\PS>testScript1.ps1:3 char:19
		+	 nonsenseString <<<< 


	Because Windows PowerShell does not recognize "nonsenseString" as a 
	cmdlet or other item, it returns a CommandNotFoundException error. This 
	terminating error is trapped by the specific Trap statement.


	The following script example contains the same Trap statements with a 
	different error:

		trap {"Other terminating error trapped" }
		trap [System.Management.Automation.CommandNotFoundException] 
			{"Command error trapped"}
		1/$null


	Running this script produces the following result:

		Other terminating error trapped
		Attempted to divide by zero.
		At C:PS> errorX.ps1:3 char:7
		+	 1/ <<<< $null


	The attempt to divide by zero does not create a CommandNotFoundException 
	error. Instead, that error is trapped by the other Trap statement, which 
	traps any terminating error. 


  Trapping Errors and Scope

	If a terminating error occurs in the same scope as the Trap statement, 
	after running the Trap statements, Windows PowerShell continues at the 
	statement after the error. If the Trap statement is in a different scope 
	from the error, execution continues at the next statement that is in the 
	same scope as the Trap statement.
 

	For instance, if an error occurs in a function, and the Trap statement is 
	in the function, the script continues at the next statement. For example, 
	the following script contains an error and a Trap statement:

		function function1 {
			trap { "An error: " }
			NonsenseString
			"function1 was completed"
		}


	Later in the script, running the Function1 function produces the 
	following result: 

		function1
		An error: 
		The term 'NonsenseString' is not recognized as the name of a cmdlet, 
		function, script file, or operable program. Check the spelling of the 
		name, or if a path was included verify that the path is correct, and 
		then try again.
		At C:\PS>TestScript1.ps1:3 char:19
		+	 NonsenseString <<<<

		function1 was completed  
 

	The Trap statement in the function traps the error. After displaying the 
	message, Windows PowerShell resumes running the function. Note that 
	Function1 was completed.


	Compare this with the following example, which has the same error and 
	Trap statement. In this example, the Trap statement occurs outside the 
	function:

		function function2 {
			NonsenseString
			"function2 was completed"
		}

		trap { "An error: " }
			. . .
		function2


	Later in the script, running the Function2 function produces the 
	following result:

		An error: 
		The term 'NonsenseString' is not recognized as the name of a cmdlet, 
		function, script file, or operable program. Check the spelling of the 
		name, or if a path was included verify that the path is correct, and 
		then try again.
		At C:\PS>TestScript2.ps1:4 char:19
		+	 NonsenseString <<<<


	In this example, the "function2 was completed" command was not run. 
	Although both terminating errors occur within a function, if the Trap 
	statement is outside the function, Windows PowerShell does not go back 
	into the function after the Trap statement runs.


  Using the Break and Continue Keywords

	You can use the Break and Continue keywords in a Trap statement to 
	determine whether a script or command continues to run after a 
	terminating error. 


	If you include a Break statement in a Trap statement list, Windows 
	PowerShell stops the function or script. The following sample function 
	uses the Break keyword in a Trap statement:

		C:\PS> function break_example {
			trap {"Error trapped"; break;}
			1/$null
			"Function completed."
		}

		C:\PS> break_example
		Error trapped
		Attempted to divide by zero.
		At line:4 char:7


	Because the Trap statement included the Break keyword, the function does 
	not continue to run, and the "Function completed" line is not run.


	If you include a Continue statement in a Trap statement, Windows 
	PowerShell resumes after the statement that caused the error, just as it 
	would without Break or Continue. With the Continue keyword, however, 
	Windows PowerShell does not write an error to the error stream. 


	The following sample function uses the Continue keyword in a Trap 
	statement:

		C:\PS> function continue_example {
			trap {"Error trapped"; continue;}
			1/$null
			"Function completed."}

		C:\PS> continue_example
		Error trapped
		Function completed. 


	The function resumes after the error is trapped, and the "Function 
	completed" statement runs. No error is written to the error stream. 


SEE ALSO
	about_Break
	about_Continue
	about_Throw
	about_Try_Catch_Finally
	about_Scopes
	about_Try_Catch_Finally