Check if batch file ran successfully in powershell

Change your batch file as follows:

@echo off
setlocal enabledelayedexpansion

schtasks /query /tn T1

:: Analyze the exit code (error level)
:: and map it onto a custom exit code.
:: Note that schtasks doesn't report *specific* error
:: exit codes: 1 means that *something* went wrong,
:: which we assume to be that the task doesn't exist.
if ERRORLEVEL 1 (
  echo Task not scheduled
  set ec=101
) else ( 
  echo Task already scheduled
  set ec=100
)

:: Getting here implies success.
:: Use the custom exit code to report the specific success status.
:: Note: If this batch file is aborted due a syntax error,
::       the caller will see exit code 255. 
exit /b %ec% 

Then invoke it from your PowerShell script as follows:

# Note: 
#  * No need to call via `cmd /c` in this case - you can invoke batch
#    files directly.
#  * Doing so has the added advantage of PowerShell *itself* reporting
#    a statement-terminating error if the batch file cannot b found,
#    which we handle via a try / catch statement.
try {

  $out = & $ScriptPath 2>&1 | Out-String

  # No need for `echo` (alias for `Write-Output`) - implicit output will do.
  "Output: $out"

  # Check the automatic $LASTEXITCODE variable for the exit code from the
  # most recently executed external program.
  # We infer success if we see one of the two custom exit codes.
  $ok = $LASTEXITCODE -in 100, 101

} catch {
  # This only happens if the batch file doesn't exist.
  $ok = $false
  $LASTEXITCODE = 2 # Simulated exit code: File not found.
}

if ($ok) { # task exists or doesn't exist.
  "Worked"
} else {   # something fundamental went wrong
  "Failed with exit code $LASTEXITCODE"
}

Note:

• While a batch file can have syntax errors - which causes its execution to be aborted and the caller to see exit code 255 - any other error is reported via exit codes (error levels) - and it is up to each command / executable to properly set an exit code, with 0 by convention signaling success, and any nonzero value failure. Not all executables adhere to this convention:

  • Some always report 0, even when error conditions are encountered.
  • Some only ever report two exit codes: 0 for success, and 1 for any error - schtasks.exe falls into this category.
  • Some communicate specific outcomes with specific exit codes, allowing you to detect specific (error conditions).
  • Some, notably robocopy.exe, even use (of necessity) nonzero exit codes to communicate specific success states - this technique is also used in the code above.

• If the only thing you need to know is whether the batch file exited abnormally, due to a syntax error, you can make do without the custom exit code and simply use
  $ok = $LASTEXITCODE -ne 255

• Since your batch file sets a specific exit code explicitly with exit /b <n>, direct invocation from PowerShell (with &, the call operator) works fine, but note that
  cmd.exe /c call <batch-file> - note the call - is generally the most robust way to invoke a batch file - see this answer.