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