about_Debuggers

Article
12/01/2022

Short description

Describes the PowerShell debugger.

Long description

Debugging is the process of examining a script while it is running to identify and correct errors in the script instructions. The PowerShell debugger can help you examine and identify errors and inefficiencies in your scripts, functions, commands, PowerShell Desired State Configuration (DSC) configurations, or expressions.

Starting in PowerShell 5.0, the PowerShell debugger has been updated to debug scripts, functions, commands, configurations, or expressions that are running in either the console or Windows PowerShell ISE on remote computers. You can run Enter-PSSession to start an interactive remote PowerShell session in which you can set breakpoints and debug script files and commands on the remote computer. Enter-PSSession functionality has been updated to let you reconnect to and enter a disconnected session that is running a script or command on a remote computer. If the running script hits a breakpoint, your client session automatically starts the debugger. If the disconnected session that is running a script has already hit a breakpoint, and is stopped at the breakpoint, Enter-PSSession automatically starts the command-line debugger, after you reconnect to the session.

You can use the features of the PowerShell debugger to examine a PowerShell script, function, command, or expression while it is running. The PowerShell debugger includes a set of cmdlets that let you set breakpoints, manage breakpoints, and view the call stack.

Debugger Cmdlets

The PowerShell debugger includes the following set of cmdlets:

Set-PSBreakpoint: Sets breakpoints on lines, variables, and commands.
Get-PSBreakpoint: Gets breakpoints in the current session.
Disable-PSBreakpoint: Turns off breakpoints in the current session.
Enable-PSBreakpoint: Re-enables breakpoints in the current session.
Remove-PSBreakpoint: Deletes breakpoints from the current session.
Get-PSCallStack: Displays the current call stack.

Starting and Stopping the Debugger

To start the debugger, set one or more breakpoints. Then, run the script, command, or function that you want to debug.

When you reach a breakpoint, execution stops, and control is turned over to the debugger.

To stop the debugger, run the script, command, or function until it is complete. Or, type stop or t.

Debugger Commands

When you use the debugger in the PowerShell console, use the following commands to control the execution. In Windows PowerShell ISE, use commands on the Debug menu.

Note: For information about how to use the debugger in other host applications, see the host application documentation.

s, StepInto: Executes the next statement and then stops.
v, StepOver: Executes the next statement, but skips functions and invocations. The skipped statements are executed, but not stepped through.
Ctrl+Break: (Break All in ISE) Breaks into a running script within either the PowerShell console, or Windows PowerShell ISE. Note that Ctrl+Break in Windows PowerShell 2.0, 3.0, and 4.0 closes the program. Break All works on both local and remote interactively-running scripts.
o, StepOut: Steps out of the current function; up one level if nested. If in the main body, it continues to the end or the next breakpoint. The skipped statements are executed, but not stepped through.
c, Continue: Continues to run until the script is complete or until the next breakpoint is reached. The skipped statements are executed, but not stepped through.
l, List: Displays the part of the script that is executing. By default, it displays the current line, five previous lines, and 10 subsequent lines. To continue listing the script, press ENTER.
l <m>, List: Displays 16 lines of the script beginning with the line number specified by <m>.
l <m> <n>, List: Displays <n> lines of the script, beginning with the line number specified by <m>.
q, Stop, Exit: Stops executing the script, and exits the debugger. If you are debugging a job by running the Debug-Job cmdlet, the Exit command detaches the debugger, and allows the job to continue running.
k, Get-PsCallStack: Displays the current call stack.
<Enter>: Repeats the last command if it was Step (s), StepOver (v), or List (l). Otherwise, represents a submit action.
?, h: Displays the debugger command Help.

To exit the debugger, you can use Stop (q).

Starting in PowerShell 5.0, you can run the Exit command to exit a nested debugging session that you started by running either Debug-Job or Debug-Runspace.

By using these debugger commands, you can run a script, stop on a point of concern, examine the values of variables and the state of the system, and continue running the script until you have identified a problem.

NOTE: If you step into a statement with a redirection operator, such as ">", the PowerShell debugger steps over all remaining statements in the script.

Displaying the Values of script Variables

While you are in the debugger, you can also enter commands, display the value of variables, use cmdlets, and run scripts at the command line.

You can display the current value of all variables in the script that is being debugged, except for the following automatic variables:

$_
$Args
$Input
$MyInvocation
$PSBoundParameters

If you try to display the value of any of these variables, you get the value of that variable for in an internal pipeline the debugger uses, not the value of the variable in the script.

To display the value these variables for the script that is being debugged, in the script, assign the value of the automatic variable to a new variable. Then you can display the value of the new variable.

For example,

$scriptArgs = $Args
$scriptArgs

In the example in this topic, the value of the $MyInvocation variable is reassigned as follows:

$scriptname = $MyInvocation.MyCommand.Path

The Debugger Environment

When you reach a breakpoint, you enter the debugger environment. The command prompt changes so that it begins with "[DBG]:".

For more information about customizing the prompt, see about_Prompts.

Also, in some host applications, such as the PowerShell console, (but not in Windows PowerShell Integrated Scripting Environment [ISE]), a nested prompt opens for debugging. You can detect the nested prompt by the repeating greater-than characters (ASCII 62) that appear at the command prompt.

For example, the following is the default debugging prompt in the PowerShell console:

[DBG]: PS (get-location)>>>

You can find the nesting level by using the $NestedPromptLevel automatic variable.

Additionally, an automatic variable, $PSDebugContext, is defined in the local scope. You can use the presence of the $PsDebugContext variable to determine whether you are in the debugger.

For example:

if ($PSDebugContext) {"Debugging"} else {"Not Debugging"}

You can use the value of the $PSDebugContext variable in your debugging.

[DBG]: PS>>> $PSDebugContext.InvocationInfo

Name   CommandLineParameters  UnboundArguments  Location
----   ---------------------  ----------------  --------
=      {}                     {}                C:\ps-test\vote.ps1 (1)

Debugging and Scope

Breaking into the debugger does not change the scope in which you are operating, but when you reach a breakpoint in a script, you move into the script scope. The script scope is a child of the scope in which you ran the debugger.

To find the variables and aliases that are defined in the script scope, use the Scope parameter of the Get-Alias or Get-Variable cmdlets.

For example, the following command gets the variables in the local (script) scope:

Get-Variable -scope 0

You can abbreviate the command as:

gv -s 0

This is a useful way to see only the variables that you defined in the script and that you defined while debugging.

Debugging at the Command Line

When you set a variable breakpoint or a command breakpoint, you can set the breakpoint only in a script file. However, by default, the breakpoint is set on anything that runs in the current session.

For example, if you set a breakpoint on the $name variable, the debugger breaks on any $name variable in any script, command, function, script cmdlet or expression that you run until you disable or remove the breakpoint.

This allows you to debug your scripts in a more realistic context in which they might be affected by functions, variables, and other scripts in the session and in the user's profile.

Line breakpoints are specific to script files, so they are set only in script files.

Debugging Functions

When you set a breakpoint on a function that has Begin, Process, and End sections, the debugger breaks at the first line of each section.

For example:

function test-cmdlet {
    begin {
        write-output "Begin"
    }
    process {
        write-output "Process"
    }
    end {
        write-output "End"
    }
}

C:\PS> Set-PSBreakpoint -command test-cmdlet

C:\PS> test-cmdlet

Begin
Entering debug mode. Use h or ? for help.

Hit Command breakpoint on 'prompt:test-cmdlet'

test-cmdlet

[DBG]: C:\PS> c
Process
Entering debug mode. Use h or ? for help.

Hit Command breakpoint on 'prompt:test-cmdlet'

test-cmdlet

[DBG]: C:\PS> c
End
Entering debug mode. Use h or ? for help.

Hit Command breakpoint on 'prompt:test-cmdlet'

test-cmdlet

# [DBG]: C:\PS>

Debugging Remote Scripts

Starting in PowerShell 5.0, you can run the PowerShell debugger in a remote session, in either the console, or Windows PowerShell ISE. Enter-PSSession functionality has been updated to let you reconnect to and enter a disconnected session that is running on a remote computer, and currently running a script. If the running script hits a breakpoint, your client session automatically starts the debugger.

The following is an example that shows how this works, with breakpoints set in a script at lines 6, 11, 22, and 25. Note that in the example, when the debugger starts, there are two identifying prompts: the name of the computer on which the session is running, and the DBG prompt that lets you know you are in debugging mode.

Enter-Pssession -Cn localhost
[localhost]: PS C:\psscripts> Set-PSBreakpoint .\ttest19.ps1 6,11,22,25

ID Script          Line     Command          Variable          Action
-- ------          ----     -------          --------          ------
0 ttest19.ps1          6
1 ttest19.ps1          11
2 ttest19.ps1          22
3 ttest19.ps1          25

[localhost]: PS C:\psscripts> .\ttest19.ps1
Hit Line breakpoint on 'C:\psscripts\ttest19.ps1:11'

At C:\psscripts\ttest19.ps1:11 char:1
+ $winRMName = "WinRM"
# + ~

[localhost]: [DBG]: PS C:\psscripts>> list

6:      1..5 | foreach { sleep 1; Write-Output "hello2day $_" }
7:  }
# 8:

9:  $count = 10
10:  $psName = "PowerShell"
11:* $winRMName = "WinRM"
12:  $myVar = 102
# 13:

14:  for ($i=0; $i -lt $count; $i++)
15:  {
16:      sleep 1
17:      Write-Output "Loop iteration is: $i"
18:      Write-Output "MyVar is $myVar"
# 19:

20:      hello2day
# 21:


[localhost]: [DBG]: PS C:\psscripts>> stepover
At C:\psscripts\ttest19.ps1:12 char:1
+ $myVar = 102
# + ~

[localhost]: [DBG]: PS C:\psscripts>> quit
[localhost]: PS C:\psscripts> Exit-PSSession
PS C:\psscripts>

Examples

This test script detects the version of the operating system and displays a system-appropriate message. It includes a function, a function call, and a variable.

The following command displays the contents of the test script file:

PS C:\PS-test>  Get-Content test.ps1

function psversion {
  "PowerShell " + $PSVersionTable.PSVersion
  if ($PSVersionTable.PSVersion.Major -lt 6) {
    "Upgrade to PowerShell 6.0!"
  }
  else {
    "Have you run a background job today (start-job)?"
  }
}

$scriptName = $MyInvocation.MyCommand.Path
psversion
"Done $scriptName."

To start, set a breakpoint at a point of interest in the script, such as a line, command, variable, or function.

Start by creating a line breakpoint on the first line of the Test.ps1 script in the current directory.

PS C:\ps-test> Set-PSBreakpoint -line 1 -script test.ps1

You can abbreviate this command as:

PS C:\ps-test> spb 1 -s test.ps1

The command returns a line-breakpoint object (System.Management.Automation.LineBreakpoint).

Column     : 0
Line       : 1
Action     :
Enabled    : True
HitCount   : 0
Id         : 0
Script     : C:\ps-test\test.ps1
ScriptName : C:\ps-test\test.ps1

Now, start the script.

PS C:\ps-test> .\test.ps1

When the script reaches the first breakpoint, the breakpoint message indicates that the debugger is active. It describes the breakpoint and previews the first line of the script, which is a function declaration. The command prompt also changes to indicate that the debugger has control.

The preview line includes the script name and the line number of the previewed command.

Entering debug mode. Use h or ? for help.

Hit Line breakpoint on 'C:\ps-test\test.ps1:1'

test.ps1:1   function psversion {
# DBG>

Use the Step command (s) to execute the first statement in the script and to preview the next statement. The next statement uses the $MyInvocation automatic variable to set the value of the $scriptName variable to the path and file name of the script file.

DBG> s
test.ps1:11  $scriptName = $MyInvocation.MyCommand.Path

At this point, the $scriptName variable is not populated, but you can verify the value of the variable by displaying its value. In this case, the value is $null.

DBG> $scriptname
# DBG>

Use another Step command (s) to execute the current statement and to preview the next statement in the script. The next statement calls the PsVersion function.

DBG> s
test.ps1:12  psversion

At this point, the $scriptName variable is populated, but you verify the value of the variable by displaying its value. In this case, the value is set to the script path.

DBG> $scriptName
C:\ps-test\test.ps1

Use another Step command to execute the function call. Press ENTER, or type "s" for Step.

DBG> s
test.ps1:2       "PowerShell " + $PSVersionTable.PSVersion

The debug message includes a preview of the statement in the function. To execute this statement and to preview the next statement in the function, you can use a Step command. But, in this case, use a StepOut command (o). It completes the execution of the function (unless it reaches a breakpoint) and steps to the next statement in the script.

DBG> o
Windows PowerShell 2.0
Have you run a background job today (start-job)?
test.ps1:13  "Done $scriptName"

Because we are on the last statement in the script, the Step, StepOut, and Continue commands have the same effect. In this case, use StepOut (o).

Done C:\ps-test\test.ps1
PS C:\ps-test>

The StepOut command executes the last command. The standard command prompt indicates that the debugger has exited and returned control to the command processor.

Now, run the debugger again. First, to delete the current breakpoint, use the Get-PSBreakpoint and Remove-PSBreakpoint cmdlets. (If you think you might reuse the breakpoint, use the Disable-PSBreakpoint cmdlet instead of Remove-PSBreakpoint.)

PS C:\ps-test> Get-PSBreakpoint| Remove-PSBreakpoint

You can abbreviate this command as:

PS C:\ps-test> gbp | rbp

Or, run the command by writing a function, such as the following function:

function delbr { gbp | rbp }

Now, create a breakpoint on the $scriptname variable.

PS C:\ps-test> Set-PSBreakpoint -variable scriptname -script test.ps1

You can abbreviate the command as:

PS C:\ps-test> sbp -v scriptname -s test.ps1

Now, start the script. The script reaches the variable breakpoint. The default mode is Write, so execution stops just before the statement that changes the value of the variable.

PS C:\ps-test> .\test.ps1
Hit Variable breakpoint on 'C:\ps-test\test.ps1:$scriptName'
(Write access)

test.ps1:11  $scriptName = $MyInvocation.MyCommand.Path
# DBG>

Display the current value of the $scriptName variable, which is $null.

DBG> $scriptName
# DBG>

Use a Step command (s) to execute the statement that populates the variable. Then, display the new value of the $scriptName variable.

DBG> $scriptName
C:\ps-test\test.ps1
```powershell

Use a Step command (s) to preview the next statement in the script.

```powershell
DBG> s
test.ps1:12  psversion

The next statement is a call to the PsVersion function. To skip the function but still execute it, use a StepOver command (v). If you are already in the function when you use StepOver, it is not effective. The function call is displayed, but it is not executed.

DBG> v
Windows PowerShell 2.0
Have you run a background job today (start-job)?
test.ps1:13  "Done $scriptName"

The StepOver command executes the function, and it previews the next statement in the script, which prints the final line.

Use a Stop command (t) to exit the debugger. The command prompt reverts to the standard command prompt.

C:\ps-test>

To delete the breakpoints, use the Get-PSBreakpoint and Remove-PSBreakpoint cmdlets.

PS C:\ps-test> Get-PSBreakpoint| Remove-PSBreakpoint

Create a new command breakpoint on the PsVersion function.

PS C:\ps-test> Set-PSBreakpoint -command psversion -script test.ps1

You can abbreviate this command to:

PS C:\ps-test> sbp -c psversion -s test.ps1

Now, run the script.

PS C:\ps-test> .\test.ps1
Hit Command breakpoint on 'C:\ps-test\test.ps1:psversion'

test.ps1:12  psversion
# DBG>

The script reaches the breakpoint at the function call. At this point, the function has not yet been called. This gives you the opportunity to use the Action parameter of Set-PSBreakpoint to set conditions for the execution of the breakpoint or to perform preparatory or diagnostic tasks, such as starting a log or invoking a diagnostic or security script.

To set an action, use a Continue command (c) to exit the script, and a Remove-PSBreakpoint command to delete the current breakpoint. (Breakpoints are read-only, so you cannot add an action to the current breakpoint.)

DBG> c
Windows PowerShell 2.0
Have you run a background job today (start-job)?
Done C:\ps-test\test.ps1

PS C:\ps-test> Get-PSBreakpoint| Remove-PSBreakpoint
PS C:\ps-test>

Now, create a new command breakpoint with an action. The following command sets a command breakpoint with an action that logs the value of the $scriptName variable when the function is called. Because the Break keyword is not used in the action, execution does not stop. (The backtick (`) is the line-continuation character.)

PS C:\ps-test> Set-PSBreakpoint -command psversion -script test.ps1  `
-action { add-content "The value of `$scriptName is $scriptName." `
-path action.log}

You can also add actions that set conditions for the breakpoint. In the following command, the command breakpoint is executed only if the execution policy is set to RemoteSigned, the most restrictive policy that still permits you to run scripts. (The backtick (`) is the continuation character.)

PS C:\ps-test> Set-PSBreakpoint -script test.ps1 -command psversion `
-action { if ((Get-ExecutionPolicy) -eq "RemoteSigned") { break }}

The Break keyword in the action directs the debugger to execute the breakpoint. You can also use the Continue keyword to direct the debugger to execute without breaking. Because the default keyword is Continue, you must specify Break to stop execution.

Now, run the script.

PS C:\ps-test> .\test.ps1
Hit Command breakpoint on 'C:\ps-test\test.ps1:psversion'

test.ps1:12  psversion

Because the execution policy is set to RemoteSigned, execution stops at the function call.

At this point, you might want to check the call stack. Use the Get-PsCallStack cmdlet or the Get-PsCallStack debugger command (k). The following command gets the current call stack.

DBG> k
2: prompt
1: .\test.ps1: $args=[]
0: prompt: $args=[]

This example demonstrates just a few of the many ways to use the PowerShell debugger.

For more information about the debugger cmdlets, type the following command:

help <cmdlet-name> -full

For example, type:

help Set-PSBreakpoint -full

Other Debugging Features in PowerShell

In addition to the PowerShell debugger, PowerShell includes several other features that you can use to debug scripts and functions.

Windows PowerShell ISE includes an interactive graphical debugger. For more information, start Windows PowerShell ISE and press F1.
The Set-PSDebug cmdlet offers very basic script debugging features, including stepping and tracing.
Use the Set-StrictMode cmdlet to detect references to uninitialized variables, to references to non-existent properties of an object, and to function syntax that is not valid.
Add diagnostic statements to a script, such as statements that display the value of variables, statements that read input from the command line, or statements that report the current instruction. Use the cmdlets that contain the Write verb for this task, such as Write-Host, Write-Debug, Write-Warning, and Write-Verbose.

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