Preserve custom shell for PowerShell scripts

[rangampallysonali]

Summary

Fix shell selection logic in ShellOperation._prep_trigger_command when using PowerShell script extensions (.ps1).

Previously, the condition:

if self.shell is None and sys.platform == "win32" or extension == ".ps1":

could evaluate to True whenever extension == ".ps1", even when a custom shell was explicitly provided. This caused Prefect to override the user-specified shell and force "powershell".

The condition has been updated to ensure PowerShell is only selected automatically when no shell has been supplied by the user.

Changes

Changed:

if self.shell is None and sys.platform == "win32" or extension == ".ps1":

to:

if self.shell is None and (
    sys.platform == "win32" or extension == ".ps1"
):

Motivation

This aligns the implementation with the expected behavior described in issue #21808 and preserves user-provided shell values when working with PowerShell scripts.

Testing

Verified that the existing Windows shell override tests continue to pass:

pytest src/integrations/prefect-shell/tests/test_commands_windows.py -k override_shell -v

Results:

• 3 passed
• 0 failed

[rangampallysonali]

Hi maintainers, I opened this PR to address #21808 by preserving a user-provided shell when extension=".ps1" is used.

I verified the relevant Windows shell override tests locally:

pytest src/integrations/prefect-shell/tests/test_commands_windows.py -k override_shell -v

Result: 3 passed.

Please let me know if you'd like me to add a more targeted regression test or adjust the implementation.

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[chatgpt-codex-connector]

Preserve PowerShell exit propagation for custom shells

When a user runs .ps1 commands with a PowerShell-compatible custom shell such as shell="powershell.exe" or shell="pwsh", this branch now skips the auto-PowerShell path, so the later if shell == "powershell" check does not append Exit $LastExitCode. For inputs like ShellOperation(commands=["ping bad-host"], shell="powershell.exe"), the script can terminate normally and Prefect reports success even though the native command failed; Microsoft’s powershell.exe -File docs state successful execution exits 0 unless the script terminates with exit. Please preserve the custom executable name while still treating PowerShell executables as PowerShell for exit-code propagation.

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[zzstoatzz]

thanks for working on this @rangampallysonali ! I dug into this because the precedence fix is definitely needed, but the issue example still seems to fail after this PR

the remaining problem is that ShellOperation preserves shell="powershell.exe -executionpolicy bypass" after this change, but then passes that whole string as the first subprocess.Popen argument. on Windows that becomes a single executable name, so it fails with [WinError 2].

I verified this with a GitHub Actions repro in zzstoatzz/prefect-pack:

• This PR branch fails on Windows: https://github.com/zzstoatzz/prefect-pack/actions/runs/27969751529/job/82772662185
• The same repro passes with a small follow-up change that splits the shell command before appending the temp .ps1 path: https://github.com/zzstoatzz/prefect-pack/actions/runs/27969869936/job/82773055038

The shape of the follow-up fix was:

def _split_shell_command(shell: str) -> list[str]:
    return shlex.split(shell.lower(), posix=sys.platform != "win32") or [shell.lower()]


def _shell_command(shell: str, temp_file_name: str) -> list[str]:
    return [*_split_shell_command(shell), temp_file_name]

and then use _shell_command(shell, temp_file.name) instead of [shell, temp_file.name].

It is also worth recognizing powershell.exe / pwsh as PowerShell executables when deciding whether to append Exit $LastExitCode, otherwise powershell.exe with flags can run but loses the exit-code behavior.

I had opened #22340 while testing, but I am closing it so this contribution can stay here. Feel free to copy anything useful from that branch/body; the important bit is the Windows repro above and adding a regression test for ShellOperation(shell="powershell.exe -executionpolicy bypass", extension=".ps1").