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About Trap

JuanPablo Jofre|Last Updated: 11/17/2016
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2 Contributors

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_Scopes

about_Throw

about_Try_Catch_Finally

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