About Trap

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Short description

Describes a keyword that handles a terminating error.

Long description

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

Trap statements can also be more complex. The statement list of the trap can include multiple conditions or function calls. A trap can write logs, test conditions, or even run another program.

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. A Trap statement consists of the trap keyword, optionally followed by a type expression, and the statement block containing the list of statements to run when an error is trapped. The type expression refines the types of errors the trap catches.

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, PowerShell checks for a Trap statement that handles the error. If a Trap statement is present, 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.

In the following example, the function includes a nonsense string that causes a runtime error.

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

TrapTest

Running this function returns the following:

Error found.

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

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

TrapTest

Running this version of the function returns the following:

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.

Important

Trap statements may be defined anywhere within a given scope, but always apply to all statements in that scope. At runtime, traps in a block are defined before any other statements are executed. In JavaScript, this is known as hoisting. This means that traps apply to all statements in that block even if execution has not advanced past the point at which they are defined. For example, defining a trap at the end of a script and throwing an error in the first statement still triggers that trap.

Trapping specific errors

A script or command can have multiple Trap statements. Traps can be defined to handle specific errors.

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

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 the Trap statement list, PowerShell writes the error object to the error stream and then continues the script.

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 type can be trapped by only one Trap statement. When a terminating error occurs, PowerShell searches for the trap with the most specific match, starting in the current scope of execution.

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
nonsenseString : 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 try again.
At C:\temp\test\traptest.ps1:5 char:1
+ nonsenseString
+ ~~~~~~~~~~~~~~
+ CategoryInfo          : ObjectNotFound: (nonsenseString:String) [], CommandNotFoundException
+ FullyQualifiedErrorId : CommandNotFoundException

Because 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:\temp\test\traptest.ps1:5 char:1
+ 1/$null
+ ~~~~~~~
+ CategoryInfo          : NotSpecified: (:) [], RuntimeException
+ FullyQualifiedErrorId : RuntimeException

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, PowerShell runs the list of statements defined by the trap. Execution 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 example, if an error occurs in a function, and the Trap statement is in the function, the script continues at the next statement. 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, 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

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. In both examples, the terminating error occurs within the function. In this example, however, the Trap statement is outside the function. PowerShell does not go back into the function after the Trap statement runs.

Caution

When multiple traps are defined for the same error condition, the first trap defined lexically (highest in the scope) is used.

In the following example, only the trap with "whoops 1" is run.

Remove-Item -ErrorAction Stop ThisFileDoesNotExist
trap { "whoops 1"; continue }
trap { "whoops 2"; continue }

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, PowerShell stops the function or script. The following sample function uses the Break keyword in a Trap statement:

function break_example {
    trap {
        "Error trapped"
        break
    }
    1/$null
    "Function completed."
}

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, PowerShell resumes after the statement that caused the error, just as it would without Break or Continue. With the Continue keyword, however, PowerShell does not write an error to the error stream.

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

function continue_example {
    trap {
        "Error trapped"
        continue
    }
    1/$null
    "Function completed."
}

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.

Notes

Trap statements provide a simple way to broadly ensure all terminating errors within a scope are handled. For more finer-grained error handling, use try/catch blocks where traps are defined using Catch statements. The Catch statements only apply to the code inside the associated Try statement. For more information, see about_Try_Catch_Finally.

See also

about_Break

about_Continue

about_Scopes

about_Throw

about_Try_Catch_Finally