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>> i is i is i is i is i is
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Now $i is equal to four, so we hit the breakpoint code. As in the stepping case, we can examine and change the state of the interpreter,
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*break* 1>> PS (2) > $i 4 1>> PS (3) > $i=8
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and use exit to resume the top-level execution thread.
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1>> PS (4) > exit i is 9 PS (6) >
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Now let s look at how we can use this feature to create a breakpoint command. Once again, we ll take advantage of scriptblocks to add a way to trigger the breakpoint based on a particular condition.
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PS (1) > function bp ([scriptblock] $condition) >> { >> if ($condition) >> { >> if (. $condition) >> {
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If the $condition parameter to bp is not null, evaluate it. If it evaluates to $true, then execute the breakpoint and enter a nested shell level.
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>> >> >> >> >> >> >> >> } >> PS (2) >> { >> $host.UI.WriteLine("*break*") $host.EnterNestedPrompt() } } else { $host.UI.WriteLine("*break*") $host.EnterNestedPrompt() }
> for ($i=0; $i -lt 10; $i++) . bp {$i -eq 5}
Here we re inserting a breakpoint that will cause execution to break when $i is equal to 5. Note that we re dotting the bp function. This is because we want it to be executed in the current scope, allowing us to change the state of the loop variable.
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>> "`$i is $i" >> } >> $i is 0 $i is 1 $i is 2 $i is 3 $i is 4 *break*
We hit the breakpoint. Increment the loop variable so that 5 is never displayed, and exit the nested prompt level and resume execution.
1>> PS (3) > $i++ 1>> PS (4) > exit $i is 6 $i is 7 $i is 8 $i is 9 PS (5) >
The loop exits, never having printed 5. This bp function is a handy tool to keep in your script debugging toolbox. You must modify your scripts to use the bp function, but it can really help when debugging complex scripts. 9.4.3 The script call stack, or How did I get here The bp function is a handy little debugging tool. From within a nested prompt, you can examine and change the state of interpreter. Another tool that would be nice would be one that tells you where you are and how you got there. In other words, we need a way to dump the script call stack. How to do this in PowerShell is not obvious, but it is possible. The key is to use the $MyInvocation variable. This variable is set to a new value every time you call a function or script. It provides you with a lot of information about what kind of command is running, where it was defined, and where it was called from. We can write a short script to illustrate this. We ll use redirection to save it into a file myinfo.ps1.
PS >> >> >> >> >> >> >> >> >> (1) > @' function showit { "Called from:" + $myinvocation.scriptname + ":" + $myinvocation.scriptlinenumber } showit '@ > myinfo.ps1
This script defines a function that prints out the information about where it was called from, and then calls the function it defined:
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PS (2) > ./myinfo Called from:C:\Temp\myinfo.ps1:6 PS (3) >
We can see that the function faithfully reported where it was called from.
AUTHOR S NOTE
There are a number of interesting properties on the InvocationInfo object in $MyInvocation. We won t be covering them here, so take some time to explore it using Get-Member. The PowerShell software developer s kit also documents this object.
We can figure out where the current function was called from. The trick is to find out where its parent was called from. This requires looking up the call stack and seeing what $MyInvocation was set to for the parent. The way to do that is to use the Get-Variable cmdlet. This cmdlet has a parameter that s used to specify what scope you want to get the variable from. If you specify scope 0, it looks the variable up in the current scope. If you specify 1 then the parent scope is searched, and so on. We ll take these two features and combine them in a function called gcs that will display the script call stack. Here s what that function looks like:
function { trap { 0..100 (gv } } gcs continue } | % { -scope $_ myinvocation).value.positionmessage -replace "`n"
This is a pretty simple function. It walks up the call stack, starting at 0 (current scope), until an error occurs. At that point, the trap statement catches the error and uses the continue statement to quietly exit the function. For each scope level, we print out a message displaying where it was called from. At this point we create a test script with a grouping of functions that call each other:
PS >> >> >> >> >> >> >> >> (1) > @' function a { b } function b { c } function c { d } function d { e } function e { gcs } a '@ > showstack.ps1
In this function, a calls b, which calls c, and so on until finally e calls our gcs function. Let s run it:
PS At At At (2) > ./showstack C:\Temp\showstack.ps1:5 char:17+ function e { gss <<<< } C:\Temp\showstack.ps1:4 char:15+ function d { e <<<< } C:\Temp\showstack.ps1:3 char:15+ function c { d <<<< }
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