Pester: Cannot mock external commands

[ephracis]

I am trying to test code that calls on external binaries (in my example certreq and openssl.exe). Obviously I want to mock these calls in my tests but when I try, Pester complains:

You did not declare a mock of the certreq Command.

Here's a small reproducable example:

Mock echo { }
echo foo
Assert-MockCalled echo

Running Powershell 3.0 on Windows 7.

[nohwnd]

The echo is an alias of Write-Output so you need to assert-mockcalled Write-Output not echo to make it work.

But you still may mock echo and call echo in the sut.

[ephracis]

Well echo was just an example. But in my case I want to mock certreq and ../vendor/openssl.exe. I get the same error: Assert-MockCalled complains that I never mocked the command.

[dlwyatt]

Mocking external commands is really flaky, unfortunately. My personal preference is to wrap all calls to those commands in PowerShell functions, and then I can easily mock calls to those functions instead. For example:

function Invoke-CertReq
{
    certreq.exe @args
}

[ephracis]

Well, I was afraid of that. Thanks anyway. :)

[ephracis]

Ok, follow up to your suggestion: how do I use this to verify parameters sent to the Invoke-CertReq function?

Mock Invoke-CertReq -ParameterFilter { ??? }

[dlwyatt]

I'd probably do some analysis of the default $args variable inside the ParameterFilter script block. You'd have this same problem trying to mock the external command directly, though. Anything that's not a PowerShell command, has no parameter metadata to make it easier on you.

[kajbonfils]

Regarding the parameterfiltering, I did like this to mock a call to an external program.

function Invoke-Nuget
{
    & "/tools/nuget.exe" $args
}

Invoke-Nuget list foo.bar -prerelease

In my test i do like this:
Assert-MockCalled -CommandName Invoke-Nuget -Times 1 -ParameterFilter {$args[0] -eq "list" -and $args[1] -eq "foo.bar" -and $args[2] -eq "-prerelease"}
This works fine.

[nohwnd]

@kajbonfils Are you showing a solution or asking a question? :)

[kajbonfils]

@nohwnd Sorry, that wasn't clear. It was a solution. I have updated the response.

[it-praktyk]

I assume that the issue can be closed now. Feel free to reopen if you need more assistance.

[bgshacklett]

Can we get some clarification on the final answer? Is wrapping the native executable and testing against the wrapper the recommended method at this point?

[it-praktyk]

Yes, wrapping native commands to functions and mocking functions is recommended way.

[asears]

@nohwnd et al, is there additional guidance on this for Pester 5 and mocking az / databricks / kubectl / jq cli tools and other python, go or exe/com external cli tools?

Examples of being able to pester test python / go cli tools such as these would be useful, esp within powershellcore.

Creating a wrapper for mocking each cli tool's callout seems a bit inefficient and a workaround to accommodate the testing framework.

[asears]

Nice! This seems to work, since the cli tools are nuget/pip installed packages.

Mock 'jq.exe' -MockWith { $true }
Mock 'databricks.exe' -MockWith { $true }

exe extension is optional?

[nohwnd]

@asears Yes, it is possible since forever, but the version without .exe works since https://github.com/pester/Pester/issues/1043

[hansEricG]

Hi!

If I want to mock an external command with a parameter, e.g. this

qwinsta /server localhost

Is this possible without using a wrapper?
(I've managed to make it work for a parameterless call to qwinsta, but once I added the arguments the mock stopped working).
(I'm using Pester 5)

[nohwnd]

how does your mock look like?

[hansEricG]

Thank you for your reply/question. However, I think the problem I experienced with external mocks was related to the issue I reported earlier today: https://github.com/pester/Pester/issues/1696. The problem, which I wasn't able to diagnose at that time, seems to be related to a non-empty scriptblock (as described in #1696).

However, the failure output is a little different, so I prepared a minimal example to trigger it, in case it would prove helpful.

BeforeAll {
    function FunctionThatInvokesQwinsta ($Server='localhost') { 
        qwinsta /server $Server
    }
}

Describe "Mocking external command bug?" {

    It 'works for mock with empty scriptblock' {
        Mock qwinsta { }
        FunctionThatInvokesQwinsta | Should -Invoke qwinsta
    }

    It 'reports CommandNotFoundException when non-empty scriptblock' {
        Mock qwinsta { "Some output" }
        FunctionThatInvokesQwinsta | Should -Invoke qwinsta
    }
}

The output is

[-] Mocking external command bug?.reports CommandNotFoundException when non-empty scriptblock 104ms (103ms|1ms)
 CommandNotFoundException: Could not find Command Some output

Tests completed in 258ms
Tests Passed: 1, Failed: 1, Skipped: 0 NotRun: 0