Windows exit codes beyond pass/fail: robocopy's bitmask, msiexec's installer codes, and $LASTEXITCODE

What it is

robocopy packs several independent yes/no facts into one exit value; the documented values combine as bits (1 copied, 2 extra, 4 mismatched, 8 failures):

Value	Meaning
0	No files were copied; no failure; no mismatch (destination already matched source)
1	All files were copied successfully
2	Extra files/directories exist in the destination that aren't in the source
3	Files were copied and extra files exist
5	Files were copied and some were mismatched
6	Extra and mismatched files exist, but nothing was copied and nothing failed
7	Files copied, a mismatch was present, and extra files were present
8	Several files did not copy

The documented rule: "Any value equal to or greater than 8 indicates that there was at least one failure during the copy operation" — below 8, the number is informational, not pass/fail.

msiexec returns specific Windows Installer error codes, two of which are successes in disguise:

Code	Name	Meaning
0	ERROR_SUCCESS	Completed normally
1603	ERROR_INSTALL_FAILURE	A fatal error occurred during installation
1618	ERROR_INSTALL_ALREADY_RUNNING	Another install is already in progress
3010	ERROR_SUCCESS_REBOOT_REQUIRED	"A restart is required to complete the install. This message indicates success."
1641	ERROR_SUCCESS_REBOOT_INITIATED	"The installer has initiated a restart. This message indicates success."

$LASTEXITCODE is PowerShell's window into a native program's real code, but its value depends on how the script was invoked: called directly or via &, it is unchanged unless the script calls exit or runs a native command; via pwsh -File, it becomes 1 on an uncaught exception or the exit-specified value, 0 on success; via pwsh -Command, it becomes 1 if the last command set $? false, 0 otherwise — collapsing any more specific native code to 1.

sc.exe uses no small fixed table; it surfaces the underlying Win32 system error code directly, e.g. 1060 (ERROR_SERVICE_DOES_NOT_EXIST, "The specified service does not exist as an installed service") — look codes up in the System Error Codes reference rather than assuming a short enumerated list.

Why it matters

A script that checks robocopy ...; if ($LASTEXITCODE -ne 0) { fail } treats routine "nothing to copy" (0) and "successfully copied" (1) inconsistently, and fails an msiexec run that actually succeeded but needs a reboot (3010/1641).

How to apply

• For robocopy, test if ($LASTEXITCODE -ge 8) { fail }, not -ne 0.
• For msiexec, treat 0, 3010 and 1641 as success (scheduling a reboot for the latter two); treat 1603 and 1618 as real failures.
• Capture $LASTEXITCODE immediately after a native call, before any other command overwrites it, and know which invocation mode (-File vs -Command) collapses its granularity.
• For sc.exe, resolve its numeric result against the system error code reference rather than a hard-coded switch statement.

Pitfalls

• Comparing robocopy's or msiexec's code to a simple 0/non-zero boolean in an automation pipeline.
• Assuming -Command mode preserves a native program's specific code; it is collapsed to 0 or 1 unless the command ends with exit $LASTEXITCODE.