home

PowerShell, batch files, and exit codes. Recipes & Secrets.

23 Jun 2012

TL;DR;

Update: If you want to save some time, skip reading this and just use my PowerShell Script Boilerplate. It includes an excellent batch file wrapper, argument escaping, and error code bubbling.

PowerShell.exe doesn't return correct exit codes when using the -File option. Use -Command instead. (Vote for this issue on Microsoft Connect.)

This is a batch file wrapper for executing PowerShell scripts. It forwards arguments to PowerShell and correctly bubbles up the exit code (when it can).

PowerShell.exe still returns a passing (0) exit code when a ParserError is thrown. Even when using -Command. I haven't found a workaround for this. (Vote for this issue on Microsoft Connect.)

You can use black magic to include spaces and quotes in the arguments you pass through the batch file wrapper to PowerShell.

Want to learn PowerShell? Check out Windows PowerShell in Action by Bruce Payette. This is the book that got me started and it's one of the best tech books I've read.

PowerShell

PowerShell is a great scripting environment, and it is my preferred tool for writing build scripts for .NET apps. Exit codes are vital in build scripts because they are how your Continuous Integration server knows whether the build passed or failed.

This is a quick tour of working with exit codes in PowerShell scripts and batch files. I'm including batch files because they are often necessary to wrap the execution of your PowerShell scripts.

Let's start easy. Say you need to run a command line app or batch file from your PowerShell script. How can you check the exit code of that process?

# script.ps1

cmd /C exit 1
Write-Host $LastExitCode    # 1

$LastExitCode is a special variable that holds the exit code of the last Windows based program that was run. So says the documentation.

Remember though, $LastExitCode doesn't do squat for PowerShell commands. Use $? for that.

# script.ps1

Get-ChildItem "C:\"
Write-Host $?    # True

Get-ChildItem "Z:\some\non-existant\path"
Write-Host $?    # False

Anytime you run an external command like this, you need to check the exit code and throw an exception if needed. Otherwise the PowerShell script will keep right on trucking after a failure.

# script.ps1

cmd /C exit 1
if ($LastExitCode -ne 0) {
    throw "Command failed with exit code $LastExitCode."
}
Write-Host "You'll never see this."

Writing these assertions all the time will get old. Fortunately you can use a helper function, like this one found in the excellent psake project.

# script.ps1

function Exec
{
    [CmdletBinding()]
    param (
        [Parameter(Position=0, Mandatory=1)]
        [scriptblock]$Command,
        [Parameter(Position=1, Mandatory=0)]
        [string]$ErrorMessage = "Execution of command failed.`n$Command"
    )
    & $Command
    if ($LastExitCode -ne 0) {
        throw "Exec: $ErrorMessage"
    }
}

Exec { cmd /C exit 1 }
Write-Host "You'll never see this."

Throwing & exit codes

The throw keyword is how you generate a terminating error in PowerShell. It will, sometimes, cause your PowerShell script to return a failing exit code (1). Wait, when does it not cause a failing exit code, you ask? This is where PowerShell's warts start to show. Let me demonstrate some scenarios.

# broken.ps1

throw "I'm broken"

From the PowerShell command prompt:

PS> .\broken.ps1
I'm broken.
At C:\broken.ps1:1 char:6
+ throw <<<<  "I'm broken."
    + CategoryInfo          : OperationStopped: (I'm broken.:String) [], RuntimeException
    + FullyQualifiedErrorId : I'm broken.
    
PS> $LastExitCode
1

From the Windows command prompt:

> PowerShell.exe -NoProfile -NonInteractive -ExecutionPolicy unrestricted -Command ".\broken.ps1"
I'm broken.
At C:\broken.ps1:1 char:6
+ throw <<<<  "I'm broken."
    + CategoryInfo          : OperationStopped: (I'm broken.:String) [], RuntimeException
    + FullyQualifiedErrorId : I'm broken.
    
> echo %errorlevel%
1

That worked, too. Good.

Again, from the Windows command prompt:

> PowerShell.exe -NoProfile -NonInteractive -ExecutionPolicy unrestricted -File ".\broken.ps1"
I'm broken.
At C:\broken.ps1:1 char:6
+ throw <<<<  "I'm broken."
    + CategoryInfo          : OperationStopped: (I'm broken.:String) [], RuntimeException
    + FullyQualifiedErrorId : I'm broken.

> echo %errorlevel%
0

Whoa! We still saw the error, but PowerShell returned a passing exit code. What the heck?! Yes, this is the wart.

A workaround for -File

-File allows you to pass in a script for PowerShell to execute, however terminating errors in the script will not cause PowerShell to return a failing exit code. I have no idea why this is the case. If you know why, please share!

A workaround is to add a trap statement to the top of your PowerShell script. (Thanks, Chris Oldwood, for pointing this out!)

# broken.ps1

trap
{
    Write-Error $_
    exit 1
}
throw "I'm broken."

From the Windows command prompt:

> PowerShell.exe -NoProfile -NonInteractive -ExecutionPolicy unrestricted -File ".\script.ps1"
C:\broken.ps1 : I'm broken.
    + CategoryInfo          : NotSpecified: (:) [Write-Error], WriteErrorException
    + FullyQualifiedErrorId : Microsoft.PowerShell.Commands.WriteErrorException,broken.ps1

> echo %errorlevel%
1

Notice that we got the correct exit code this time, but our error output didn't include as much detail. Specifically, we didn't get the line number of the error like we were getting in the previous tests. So it isn't a perfect workaround.

Remember. Use -Command instead of -File whenever possible. If for some reason you must use -File or your script needs to support being run that way, then use the trap workaround above.

-Command can still fail

I've discovered that PowerShell will still exit with a success code (0) when a ParserError is thrown. Even when using -Command.

From the Windows command prompt:

> PowerShell.exe -NoProfile -NonInteractive -Command "Write-Host 'You will never see this.'" "\"
The string starting:
At line:1 char:39
+ Write-Host 'You will never see this.'  <<<< "
is missing the terminator: ".
At line:1 char:40
+ Write-Host 'You will never see this.' " <<<<
    + CategoryInfo          : ParserError: (:String) [], ParentContainsErrorRecordException
    + FullyQualifiedErrorId : TerminatorExpectedAtEndOfString

> echo %errorlevel%
0

I'm not aware of any workaround for this behavior. This is very disturbing, because these parser errors can be caused by arguments (as I demonstrated above). This means there is no way to guarantee your script will exit with the correct code when it fails.

Note: This was tested in PowerShell v2, on Windows 7 (x64).

There are other known bugs with PowerShell's exit codes. Beware.

Batch files

I mentioned early on that it is often necessary to wrap the execution of your PowerShell script in a batch file. Some common reasons for this might be:

Whatever the reason, writing a batch file wrapper for a PowerShell script is easy. You just need to make sure that your batch file properly returns the exit code from PowerShell. Otherwise, your PowerShell script might fail and your batch file would return a successful exit code (0).

This is a safe template for you to use. Bookmark it.

Update: I've created a much better batch file wrapper for my PowerShell scripts. I recommend you ignore the one below and use my new one instead.

:: script.bat

@ECHO OFF
PowerShell.exe -NoProfile -NonInteractive -ExecutionPolicy unrestricted -Command "& %~d0%~p0%~n0.ps1" %*
EXIT /B %errorlevel%

This wrapper will execute the PowerShell script with the same file name (i.e., script.ps1 if the batch file is named script.bat), and then exit with the same code that PowerShell exited with. It will also forward any arguments passed to the batch file, to the PowerShell script.

Let's test it out.

# script.ps1

param($Arg1, $Arg2)
Write-Host "Arg 1: $Arg1"
Write-Host "Arg 2: $Arg2"

From the Windows command prompt:

> script.bat happy scripting
Arg 1: happy
Arg 2: scripting

What if we want "happy scripting" to be passed as a single argument?

> script.bat "happy scripting"
Arg 1: happy
Arg 2: scripting

Well that didn't work at all. This is the secret recipe.

> script.bat "'Happy scripting with single '' and double \" quotes!'"
Arg 1: Happy scripting with single ' and double " quotes!
Arg 2:

Please don't ask me to explain this black magic, I only know that it works. Much credit to this StackOverflow question for helping me solve this!

For comparison, here is how you would do it if you were executing the script from PowerShell, without using the batch file wrapper.

From the PowerShell command prompt:

PS> .\script.ps1 happy scripting
Arg 1: happy
Arg 2: scripting

PS> .\script.ps1 "Happy scripting with single ' and double `" quotes included!"
Arg 1: Happy scripting with single ' and double " quotes included!
Arg 2: 

That's all folks!

blog comments powered by Disqus