How does echo $? work?

Question

I am writing some PowerShell scripts to do some build automation. I found here that echo $? returns true or false depending on previous statement. I just found that echo is alias for Write-Output. Write-Host $? also works. But I am still not clear how this $? works. Can someone kindly say some words bout this. Searching for echo $? on net did not give me much.

Answer 1

To complement Martin Brandl's helpful answer with more detailed information:

tl;dr

• Automatic variable $? (see Get-Help about_Automatic Variables) contains a Boolean that reflects whether any non-terminating error occurred in the most recent statement.

  • Since $? is set after every statement, you must either check it right after the statement of interest, or save it for later inspection.
  • See below for potentially counter-intuitive behavior.

• Automatic variable $LASTEXITCODE complements this by recording the specific exit code of the most recently executed external command-line utility (such as findstr).

  • $LASTEXITCODE complements $? in that $? reflects only abstract success or failure of the external utility - exit code 0 is mapped to $True, any nonzero exit code to $False - whereas $LASTEXITCODE contains the actual exit code.
  • Since $LASTEXITCODE is only set for external command-line utilities, its value typically remains valid longer than $?, which is set after every statement.

---

There are many subtleties around how $? is set, and what, precisely, its value indicates:

• $? only reflects the occurrence of nonterminating errors, because (the much rarer) terminating errors by default terminate execution of the current command line / script, and to handle them you need to use try / catch (preferred) or trap (see the Get-Help about_Try_Catch_Finally and Get-Help about_Trap).

  • Conversely, you can opt to have non-terminating errors treated as terminating ones, using preference variable $ErrorActionPreference or common cmdlet parameter
    -ErrorAction (alias -EA) - see Get-Help about_Preference_Variables and Get-Help about_CommonParameters.

• Unless explicitly ignored (with the common -ErrorAction Ignore cmdlet parameter), all non-terminating errors (and caught terminating errors) are collected in the automatic $Error collection, in reverse chronological order; that is, element $Error[0] contains the most recent error.

• For commands to which multiple input objects were passed, $? containing $False only tells you that processing of at least one input object failed. In other words: an error could have occurred for any subset of the input objects, including all of them.

  • To determine the exact error count and the offending input objects, you must examine the $Error collection.

• With non-remoting indirect-execution cmdlets to which you pass a target command to execute - such as Invoke-Expression, Start-Process and Start-Job and Invoke-Command without the -ComputerName parameter (which doesn't involve remoting - see below) - $? only reflects whether the target command could be invoked in principle, irrespective of whether that command then reports errors or not.

  • A simple example: Invoke-Expression '1 / 0' sets $? to $True(!), because Invoke-Expression was able to parse and invoke the expression, even though the expression itself then fails.
  • Again, inspecting the $Error collection tells you if and what errors the target command reported.

• With remoting (invariably indirect-execution) cmdlets, notably with Invoke-Command with the -ComputerName parameter (as is typical), but also with implicitly remoting cmdlets, $? does reflect whether the target command reported any errors.

  • A simple example (must be run from an elevated console and assumes that the local machine is already set up for remoting):
    Invoke-Command -ComputerName . { 1 / 0 }, because remoting is involved, indeed sets $? to $False to reflects the failure of target command 1 / 0.
    ^{Note that even though the local computer (.) is targeted, use of -ComputerName invariably uses remoting.}

  • Note that, by design, remoting reports normally terminating errors that occur remotely as non-terminating ones, presumably so that a normally terminating error on one target machine doesn't abort processing on all others.

---

• Examples of commands that DO reflect errors in $?:

  # Invoking a non-existing cmdlet or utility directly.
  NoSuchCmd
  
  # Ditto, via call operator &.
  # Note, however, that using a *script block* with & behaves differently - see below.
  & 'NoSuchCmd'
  
  # Invoking a cmdlet with invalid parameter syntax.
  Get-ChildItem -NoSuchParameter
  
  # Invoking a cmdlet with parameter values that cause a (non-terminating) runtime error.
  Get-ChildItem NoSuchFile
  
  # Invoking an external utility that reports a nonzero exit code. 
  findstr -nosuchoptions
  # The specific exit code is recorded in $LASTEXITCODE, 
  # until the next external utility is called.
  
  # Runtime exceptions
  1 / 0
  
  # A cmdlet that uses remoting:
  # (Must be run from an elevated session, and the local machine must
  # be configured for remoting first - run `winrm quickconfig`).
  # Note that remoting would NOT be involved WITHOUT the -ComputerName parameter, 
  # in which case `$?` would only reflect whether the script block could be
  # _invoked_, irrespective of whether its command(s) then fail or not.
  Invoke-Command -ComputerName . { 1 / 0 }
  
  # A .NET method that throws an exception.
  # Note: Outside of a `try/catch` handler, this is a non-terminating error.
  # Inside a `try/catch` handler, .NET exceptions are treated as terminating
  # and trigger the `catch` block.
  [System.IO.Path]::IsPathRooted('>')
  

• Examples of commands that DO NOT reflect errors in $?:

  <#
    Non-remoting indirect execution cmdlets:
  
    $? reflects only whether the specified command could be 
    *invoked*, irrespective of whether the command itself then failed or not.
  
    In other words: $? is only $False if the specified command could not even be
    executed, such as due to invalid parameter syntax, an ill-formed target
    command, or a missing target executable. 
  
  #>
  
  # Invoking a command stored in a script block.
  & { 1 / 0 }
  
  # Invoking an expression stored in a string.
  Invoke-Expression '1 / 0'
  
  # Starting a background job.
  Start-Job { 1/ 0 }
  
  # The *non-remoting* form of Invoke-Command (WITHOUT -ComputerName).
  Invoke-Command { 1 / 0 }
  

Answer 2

You find a complete Punctuation chart here. The answer (taken from the chart):

Execution status of the last operation ($true or $false); contrast with $LastExitCode that reports the exit code of the last Windows-based program executed.