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about_Trap

Updated: August 9, 2012

Applies To: Windows PowerShell 2.0, Windows PowerShell 3.0

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_Scopes
    about_Throw
    about_Try_Catch_Finally




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