PS error handling and debugging

PowerShell Error Handling and PowerShell Error Handling and DebuggingDebuggingDon JonesDon JonesSenior Partner & Principal TechnologistConcentrated Technology, LLC

This slide deck was used in one of our many conference presentations. We hope you enjoy it, and invite you to use it

within your own organization however you like.

For more information on our company, including information on private classes and upcoming conference appearances, please

visit our Web site, www.ConcentratedTech.com.

For links to newly-posted decks, follow us on Twitter:@concentrateddon or @concentratdgreg

This work is copyright ©Concentrated Technology, LLC

http://www.ConcentratedTech.com/

CAUTION: DEMOS AHEADCAUTION: DEMOS AHEAD

This is a demonstration-intensive session (very few slides)

I will capture a shell transcript and save all of my scripts

You can download these (in a week or so) from ConcentratedTech.com (there’s a “Conference Materials” link in the menu)

3 • Don Jones • ConcentratedTech.com

AgendaAgenda

Debugging: Why and How– Trace Code– Breakpoints

Error Handling– How-to– How-not-to


DebuggingDebugging

Two types of bugs:– Typos. The shell usually gives you an error,

with a location, for these.– Logic. This means no errors, but you don’t

get the behavior you want.


Logic Errors: The CauseLogic Errors: The Cause

99% of the time, because some property or variable contained something other than you thought it did

The solution:– Have a clear expectation (write it down)– Debug the script to see where reality differs

from your expectation

If you don’t have an expectation – you can’t debug.


Technique #1: Trace CodeTechnique #1: Trace Code

Use Write-Debug Output variables when their contents

change Output objects (there’s a trick to

doing so) to examine properties Enable debug output with

$DebugPreference Leave trace code in – just disable

output when you’re not debugging


Technique #2: BreakpointsTechnique #2: Breakpoints

Set a breakpoint (Set-PSBreakpoint) When the script “breaks,” manually

examine properties, variables, etc. Run Exit to resume execution of the

script from “break mode” Set breakpoints on:

– Line or line/column of a script file– When a variable is read, written, or either– When a command is executed


Technique #3: Step DebuggerTechnique #3: Step Debugger

A bit outdated now that we have breakpoints (in PowerShell v2)

Enable using Set-PSDebug –step Read that cmdlet’s help file for more

information Typically, stick with breakpoints


Remember!Remember!

No expectation = No debugging You can only debug when you detect

a difference between expectations and reality

Let’s walk through an example


Error HandlingError Handling

“Catch” an error and do something about it

Most cmdlets generate “non-terminating” errors – e.g., the cmdlet still tries to keep running. You can’t “catch” these.

Use –ErrorAction (-EA) parameter to change them to “terminating” exceptions

Catch them with a Try…Catch…Finally block

Grab the error in –ErrorVariable (-EV) if desired


DO NOTDO NOT

Do not set $ErrorActionPreference globally within a script

Doing so suppresses errors – even the ones telling you about a typo or something


Option #2Option #2

You can also define a Trap block to catch any otherwise-uncaught terminating exceptions

Exit Trap with Continue or Break– Continue tries to resume execution following

the error-causing line– Break exits and passes the error to

whatever called the script

Harder to work with – deeply involves PowerShell scope, which can be complex to follow


Final ExamFinal Exam

Let’s build a script, debug it, and include error handling – all from scratch


Final Notes…Final Notes…

Please be sure to submit a session evaluation form!

Download slides & materials from www.ConcentratedTech.com within one week!

Blog, URLs, and other information is also available at www.ConcentratedTech.com for your reference

More resources at www.ShellHub.com Thank you very much!




http://www.ShellHub.com/

This slide deck was used in one of our many conference presentations. We hope you enjoy it, and invite you to use it

within your own organization however you like.

For more information on our company, including information on private classes and upcoming conference appearances, please

visit our Web site, www.ConcentratedTech.com.

For links to newly-posted decks, follow us on Twitter:@concentrateddon or @concentratdgreg

This work is copyright ©Concentrated Technology, LLC
