0xC0Lesson 13 of 17

Handling errors

Tell terminating from non-terminating errors, catch them with try/catch/finally, and raise your own.

28 min 7-question quiz 2 code exercises

By the end of this lesson you can

Explain the difference between non-terminating and terminating errors
Use -ErrorAction and $ErrorActionPreference to control what happens
Handle errors with try/catch/finally, typed catches and throw

PowerShell has two kinds of errors:

Non-terminating errors are reported and then the command carries on. Ask Get-Item for five files when two are missing and you get three files plus two red error messages.
Terminating errors stop the current statement - and the script, unless something catches them. throw, .NET exceptions (like dividing by zero) and parameter validation failures are terminating.

This matters because try/catch only catches terminating errors. To catch a cmdlet’s non-terminating error, turn it into a terminating one with -ErrorAction Stop - or set $ErrorActionPreference = 'Stop' for the whole script.

two-kinds.ps1

1Set-Location ([IO.Path]::GetTempPath())
2try {
3  Get-Item 'no-such-dragon.txt' -ErrorAction SilentlyContinue
4  'still running after a non-terminating error'
5  Get-Item 'no-such-dragon.txt' -ErrorAction Stop
6  'never printed'
7} catch {
8  "caught: $($_.Exception.GetType().Name)"
9}
10try {
11  $meals = 0
12  12 / $meals
13} catch [System.DivideByZeroException] {
14  'caught a division by zero'
15} finally {
16  'finally always runs'
17}

Output

still running after a non-terminating error
caught: ItemNotFoundException
caught a division by zero
finally always runs

`-ErrorAction` value	What happens
`Continue` (default)	show the error, carry on
`SilentlyContinue`	hide it (it’s still recorded in `$Error`), carry on
`Ignore`	hide it and don’t record it
`Stop`	make it terminating, so `catch` can handle it
`Inquire`	ask the user

Inside catch, $_ is the error record: $_.Exception.Message is the message, $_.Exception the .NET exception, $_.CategoryInfo and $_.InvocationInfo say what failed and where. Put the most specific catch [Type] blocks first and a plain catch last. A finally block runs whether or not anything failed - perfect for closing files and connections.

Raising errors

throw 'message' raises a terminating error. Use it when your function can’t possibly continue.
Write-Error 'message' writes a non-terminating error - right when one item of many failed but the rest can go on.
For programs that aren’t PowerShell (git, ping, python...), check $LASTEXITCODE (their exit code) or $? (did the last command succeed?).

throw.ps1

1function Get-Ration {
2  param([int]$Kilograms, [int]$Dragons)
3  if ($Dragons -le 0) { throw "No dragons to feed with $Kilograms kg" }
4  $Kilograms / $Dragons
5}
6foreach ($count in 4, 0, 5) {
7  try {
8    "ration: $(Get-Ration -Kilograms 60 -Dragons $count) kg"
9  } catch {
10    "skipped: $($_.Exception.Message)"
11  }
12}

Output

ration: 15 kg
skipped: No dragons to feed with 60 kg
ration: 12 kg

Try it

Review the night-feed script

A keeper wrote this script to feed the night shift. Click every line that handles errors badly, then check.

night-feed.ps1 (1 of 1)Flags found 0/0

Click every part that looks suspicious. There are 4.

$ErrorActionPreference = 'SilentlyContinue' try { $log = Get-Content feeding.csv } catch { } $meals = $log | ConvertFrom-Csv Remove-Item feeding.csv -Force "Night feed done"

Key takeaways

Non-terminating errors report and continue; terminating errors stop the statement.
try/catch only catches terminating errors - add -ErrorAction Stop to cmdlets inside try.
In catch, $_ is the error record; $_.Exception.Message is its message. finally always runs.
throw when you can’t continue, Write-Error when one item failed; check $LASTEXITCODE for native programs.

Lesson quiz

7 questions · pass with 5 correct · up to 50 XP

Passing this quiz completes the lesson and keeps your streak going. Questions you miss come back in review sessions later.

Practice: write PowerShell scripts

Write a script in the editor and run it for real against sample input, which is piped into your script as $input. Scripts run on PowerShell 6.2 through Try It Online (tio.run), a free public service, so these exercises avoid PowerShell 7-only syntax; your script and test input are sent there.

Exercise 1

Ration planner

+25 XP

Each input line is kilograms dragons. Print the ration per dragon, rounded to one decimal place with [math]::Round(x, 1), like 60 kg / 4 dragons = 15 kg each. If there are no dragons, the division throws - catch it and print 60 kg / 0 dragons = nobody to feed. Use try/catch, not an if.

Three shifts

script.ps1

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Lessons teach PowerShell 7; your script runs on PowerShell 6.2 via Try It Online (tio.run), a free public service, so stick to syntax that works there. Test input is piped into your script as $input. Your script and test input are sent to that service.

Exercise 2

Enclosure check

+25 XP

The starter creates notes for some enclosures. Each input line is an enclosure name. For each, read <name>.txt with Get-Content and print volcano: <its contents>; if the file is missing print volcano: MISSING - by catching the error. Finally print checked N, missing M. No red errors may appear.

Three enclosures
All missing

script.ps1

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