TOPIC
about_Debuggers
SHORT DESCRIPTION
Describes the Windows PowerShell debugger.
LONG DESCRIPTION
Debugging is the process of examining a script while it is running in
order to identify and correct errors in the script instructions. The
Windows PowerShell debugger is designed to help you examine and identify
errors and inefficiencies in your scripts.
Note: The Windows PowerShell debugger does not run remotely. To debug
a script on a remote computer, copy the script to the local
computer.
You can use the features of the Windows PowerShell debugger to examine a
Windows PowerShell script, function, command, or expression while it is
running. The Windows PowerShell debugger includes a set of cmdlets that
let you set breakpoints, manage breakpoints, and view the call stack.
Debugger Cmdlets
The Windows 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 Windows PowerShell console, use the
following commands to control the execution.
Note: For information about how to use the debugger in other host
applications, see the host application documentation.
s, Step-into Executes the next statement and then stops.
v, Step-over Executes the next statement, but skips functions
and invocations. The skipped statements are
executed, but not stepped through.
o, Step-out 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 Stops executing the script, and exits the debugger.
k, Get-PsCallStack Displays the current call stack.
<Enter> Repeats the last command if it was Step (s),
Step-over (v), or List (l). Otherwise, represents a
submit action.
?, h Displays the debugger command Help.
To exit the debugger, use Stop (q).
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 Windows 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]:". You can customize
the prompt.
Also, in some host applications, such as the Windows 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
Windows 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
You cannot run the Windows PowerShell debugger in a remote session. To
debug a script on a remote computer, copy the script to the local
computer.
The following command copies the Test.ps1 script from the Server01 remote
computer to the local computer:
invoke-command -computername Server01 `
{get-content c:\ps-test\test.ps1} | set-content c:\ps-test\test.ps1
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:
c:>\PS-test> get-content test.ps1
function psversion {
"Windows Powershell " + $psversiontable.psversion
if ($psversiontable.psversion.major -lt 2) {
"Upgrade to Windows PowerShell 2.0!"
}
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 "Windows 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 Step-Out
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, Step-Out,
and Continue commands have the same effect. In this case, use
Step-Out (o).
Done C:\ps-test\test.ps1
PS C:\ps-test>
The Step-Out 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
Use a Step command (s) to preview the next statement in the script.
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 Step-Over command (v). If you are
already in the function when you use Step-Over, 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 Step-Over 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 Windows
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 Windows PowerShell
In addition to the Windows PowerShell debugger, Windows PowerShell includes
several other features that you can use to debug scripts and functions.
-- Windows PowerShell Interactive Scripting Environment (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.
SEE ALSO
Disable-PsBreakpoint
Enable-PsBreakpoint
Get-PsBreakpoint
Get-PsCallStack
Remove-PsBreakpoint
Set-PsBreakpoint
Set-PsDebug
Set-Strictmode
Write-Debug
Write-Verbose
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