What is `$?` in Powershell?

It returns true if the last command was successful, else false.

However, there are a number of caveats and non-obvious behaviour (e.g. what exactly is meant by "success"). I strongly recommend reading this article for a fuller treatment.

For example, consider calling Get-ChildItem.

PS> Get-ChildItem 

PS> $? 
    True

$? will return True as the call to Get-ChildItem succeeded.

However, if you call Get-ChildItem on a directory which does not exist it will return an error.

PS> Get-ChildItem \Some\Directory\Which\Does\Not\Exist
    Get-ChildItem : Cannot find path 'C:\Some\Directory\Which\Does\Not\Exist' because it does not exist.

PS> $?
    False

$? will return False here, as the previous command was not successful.

You can also access last commands exit code using $LastExitCode parameter.

# run some command
# ...
if ((! $?) -and $ErrorAction -eq "Stop") { exit $LastExitCode }

$? will contain $false if the last command resulted in an error. It will contain $true if it did not. In the PowerShell v1 days, this was a common way to do error handling. For example, in a script, if you wanted to check for the existence of a file and then print a custom message if it did not, you could do:

Get-Item -Path john -ErrorAction silentlycontinue;
if( -not $?)
{
    'could not find file.';
     exit
 }`