James O'Neill's Blog

August 21, 2019

Exit, Throw, Return, Break and Continue. A Round up.

Filed under: Powershell — jamesone111 @ 2:14 pm

PowerShell recognises all the words in the title. I’ve previously written about the problem with assuming that throw will exit from a script or function. Normally it will, but if the error action preference has been changed it might not. So I now put return after throw to prevent execution running on.

Making exceptions for exceptions

Return is one of those Powershell commands that people get upset about; we can break normal rules to handle exceptions, I tend to avoid using throw, unless I expect something to want to catch the error. I prefer this:

Write-Warning "Couldn't do what you wanted with $parameter." ; return
to this:
Throw "$parameter is Evil" ; return

But when it isn’t really an exception… I was taught that this:
if ($result) {return}
Nextcommand
etc
FinalCommand

is wrong – I can hear “that’s just ‘goto end’, and you know `goto` is evil” and the right way is:
if (-not $result) {
  Nextcommand
  etc
  FinalCommand

}

However when the “etc” part of that code goes on for a whole screen it gets hard to see that nothing else happens if $result evaluated as true; in that case  putting in a comment (“If result was set, the work is complete”) and using return can make things clearer. I’ll come back to this later.

Break and stopping with “Continue”

There are two other sort-of “goto” commands which can be allowed: Continue has one use if you use trap instead of try/catch, which is to resume execution at the line after the error. It has another use in loops; to save doing the job of an if which runs a large amount of the script conditionally, continue says “Skip the rest of the work for this item and Continue with the next one.”, it has a companion, break which says “Skip the rest of the work on for this item, and don’t bother with any remaining ones.”  Here’s a slightly contrived example, finding primes with the sieve of Eratosthenes


$Primes = @()
foreach ($i in 2..100) {
    $isPrime = $true
    foreach ($p in $Primes) {
        if ($i % $p -eq 0 ) {$isPrime = $false; break}
    }
    if (-not $isPrime) {continue}
    Write-Verbose "$i is prime" -Verbose
    $primes += $i
}

There are two nested loops. The inner one looks at each of the primes already found and sees if the number being looked at divides by any of them. Once we have found one we don’t need to look at any of the others, break gets us out of that for loop, but not out of the Outer loop or the script or function that contains it. 
The outer loop does something if the number IS prime, but it uses continue to go on to the next number – I said the example was contrived, the if would normally be written the other way round without using continue.

Switch Statements

Both Break and continue work in a switch statement if you are using switch against a file, break stops looking and ignores the rest of the file, and Continue stops for the current line and continues for the next. I’ve saved the fragment below as deleteMe.Ps1 so it reads itself …

Switch -Regex -file .\deleteme.ps1 {
    "w" {"Contains W" ; break}
    "o" {"Contains o" ; break}
    default {"Default msg"}
}

The first line matches on the W in switch so it outputs “Contains W” and the statement stops.
If I replace each break with continue I get
“Contains W”, “Contains W”, “Contains O”, “Default”, and “Default”.
I.e. each line line is processed for a maximum of one match; And if I remove break/continue , the second line matches both W in the quotation marks and the O  in “contains” so I get
“Contains W”, “Contains W”, “Contains O”,“Contains O”, “Default”, and “Default”.

But that is the less common way to use switch this is more usual:

$s = "Hello World"
Switch -Regex ($s) {
    "w" {"Contains W" ; }
    "o" {"Contains o" ; }
    default {"Default msg"}
}

Here, without break or continue the value matches two values and outputs “Contains W”, “Contains O”, because only one value is examined, both break or continue have the same effect. Default only gets run if nothing matches.  Often I’ll see something a switch statement that could be written like this: 

Switch ($birthday.tostring("dddd")) {
   "Monday"      {"fair of face"}
   "Tuesday"     {"full of grace"}
   "Wednesday"   {"full of woe"}
   #etc
}

and although the values don’t overlap and all there isn’t an “Output this for ‘none of the above’”  (we’ve written the case for each of the days) the writer has carefully added Break or Continue to each of the blocks and a default block which only contains Continue. Does putting these things in and being absolutely explicit make things clearer ? I don’t think so. Putting in an empty “else” is just more to read; and the continue is a stylistic tick – because it is needed sometimes, and it is harmless when it is not needed why not put it in always?  It’s more typing, more to read and some people will focus on the tick. 

Break and Continue work anywhere. Switch, while, for, and foreach statements handle them as “exit from this statement”, other commands (including if and the ForEach-Object cmdlet) treat them as “exit from where this is running”  So I could write this:
Write-Warning "Couldn't do what you wanted with $parameter." ; break
or this:
Write-Warning "Couldn't do what you wanted with $parameter." ; continue

But using “Continue” to exit from a function or script is showing off in a “I know a trick that I bet you don’t” kind of way. It hinders when someone else is dealing with my code. Lately I find I keep repeating the importance of clarity, some people like to say “Imagine the next person who looks at this is an axe wielding maniac who knows where you live”; I imagine that the next person will be me, and people will be screaming that something is broken, it’s late, I’m tired and I have forgotten ever writing the script.    

Since I’ve returned to the example that used return, it’s worth taking a moment to mention that I have written about implicit or explicit return before.
Some people habitually write return $result as the last line of their function / script ; which sets other people’s teeth on edge. I tend to only write that if, somewhere before the end of a function / script I would otherwise write
$result
return

As I said at the start my computer science training would tell me that I should write this way : 
#try quick way
$result = simpleCommand
if (-not $result) {
    complex | pipeline -of "commands"
}
else {$result}


But I think the return in the next example is OK: it is clearer so say “if we got a result return it, otherwise do X, Y and z;” than to write it the other way around “if we didn’t get the result do X,Y and Z, otherwise return the result”. (If one clause is simple and one is complex, put the simple clause in the IF ).   
#try quick way
$result = simpleCommand
if ($result) {return $result}       
complex | pipeline -of "commands"  

but this next return is unnecessary

$Result = complex | pipeline -of "commands"     
return $result     

And finally … Exit

And then there is Exit. Exit says “Leave what you are running” at the PowerShell prompt it is Leave PowerShell, in a script it is Leave the script. Exit can return an exit code. If a script wants to tell another script or PowerShell itself what happened it should really send output or throw errors; some people really don’t like seeing Exit in a script and often it’s just old habits refusing to die. Codes don’t help fix a problem “Error 4096 occurred” doesn’t help users understand what did go wrong but makes them feel worse for not knowing 4095 other things that might have gone wrong.  But sometimes an error code is the only way to to tell something which called the script what happened.

However in a script exit doesn’t always behave as people expect:  
PowerShell "something" is treated as PowerShell –Command "something" which  works like this:

  • It starts PowerShell,
  • It runs the command and returns any output
  • Because -command was specified and –NoExit wasn’t, PowerShell exits. If the last command ran to completion the exit code is 0; if the last command threw a terminating error the exit code is 1.

So

  • If I run PowerShell –Command "1/0" from an existing instance of PowerShell $LASTEXITCODE is 1 ;
  • If I run PowerShell –Command "1/0 ; hostname"  it is 0 because the last command ran to completion and past errors are forgotten.
  • If I run PowerShell –Command "MyScript.ps1" the rules don’t change. PowerShell returns an exit code of 1 or 0 depending on whether the last (only) command threw an error.   If the script ends with exit 123 then $LASTEXITCODE is 123 in that instance of PowerShell and in that instance something else can see that exit code. 
    Then when that instance of PowerShell exits, it follows the standard rules – the Exit code from the script is lost.

I’m sure someone must want this behaviour, but there are multiple ways to get the script’s exit code back one is to make a command which runs the script, and explicitly exits with the code it returns, like this:
powershell 'MyScript.ps1; exit $lastexitcode'  
A better way is to tell PowerShell this is not a command, but a file. That does return the the error code from the script. 
Don’t run  PowerShell 'MyScript.ps1' instead run  
PowerShell –File 'MyScript.ps1'.

In PowerShell [core] 6 and later the behaviour is reversed so pwsh –command 'hostname' needs the explicit –command but Pwsh 'MyScript.ps1' doesn’t need the explicit –File.  Getting in the habit of being explicit with the switches in either environments means if/when a script/command it should do what you expect.

The other way  is to put
$host.setshouldExit(123)

in the script. This time when PowerShell exits it has been primed to leave with a specific code. Which is better ? Of course, it depends. It you want to test the script by running it in PowerShell and looking at $lastExitCode using exit in the script might be better, but it relies on others not just running powershell MyScript  The second way (with information written to error or verbose) avoids that, and as bonus lets you set what code should go back from PowerShell to the caller if a terminating error happens in specific section of code, and then change the code for the next section and so on.

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