powershellpowershell-7.0

Install-Module not available - not recognized as a name of a cmdlet

I'm trying to take advantage of the PowerShell Gallery which requires Install-Module. For some reason I cannot get this to work on any computer I have access to:

Install-Module: The term 'Install-Module' is not recognized as a name of a cmdlet, function, script file, or executable program.

I tried this on a Windows 8.1, Windows 10 and even installed PowerShell 7.1 on the Windows 10 computer. I see that a lot of people have this issue, but other than installing WMF (which seems to already be included with Win10) I don't see any useful suggestions.

I don't have a lot of experience with PowerShell, and definitely not managing it, so I'm curious if anybody has an idea as to why this is not working.

Edit 1: I seem to have the PowerShellGet module in the following directories:

C:\Program Files\WindowsPowerShell\Modules
C:\Program Files\PowerShell\7\Modules

and the $env:PSModulePath variables contains C:\Program Files\PowerShell\Modules;c:\program files\powershell\7\Modules;C:\Program Files\WindowsPowerShell\Modules;C:\WINDOWS\system32\WindowsPowerShell\v1.0\Modules

Edit 2: This may be related to the __PSLockDownPolicy environment variable, that was set to 4 on all of the affected computers. I was able to figure this out by attempting to manually load the PowerShellGet module, which yielded an error that hinted at this.

After removing that environment variable, it seems to work.

Solution

  • As you've discovered yourself, it was the presence of machine-level environment variable __PSLockDownPolicy that prevented Install-Module from running (see below for general prerequisites).

    This undocumented environment variable, effective on Windows only, is for security reasons only respected if defined at the machine level via the registry (rather than as a per-user or process-specific variable) and can be used to restrict what is allowed in a PowerShell session.

    Values 4 through 7 result in PowerShell operating in constrained language mode, where only a handful of pre-approved .NET types may be used, which as a result prevents the PowerShellGet module that contains the Install-Module from loading - see the conceptual about_Language Modes help topic.

    You can check what language mode is in effect in a given session by executing $ExecutionContext.SessionState.LanguageMode; in constrained mode, the result is ConstrainedLanguage; FullLanguage indicates that no language restrictions are in place.

    Looking at PowerShell's source code, the fact that this variable is checked inside a method named GetDebugLockdownPolicy() suggests that it isn't officially supported.

    For more information, see this archived blog post.

    The simplest way to remove this restriction is to delete the environment variable, by running the following from an elevated PowerShell prompt:

    Remove-ItemProperty 'HKLM:\SYSTEM\CurrentControlSet\Control\Session Manager\Environment' __PSLockdownPolicy

    One mystery remains:

    As you state, importing the PowerShellGet module explicitly, with Import-Module PowerShellGet showed you that the real problem was not that the Install-Module command didn't exist, but that its enclosing module failed to load.

    Doing so revealed that yet another module was the problem, namely the PackageManagement module that PowerShellGet depends on; Import-Module PackageManagement finally reveals: Cannot set property. Property setting is supported only on core types in this language mode, which implies that the (constrained) language mode was the problem.

    The mystery is the error message reported in your question: Install-Module: The term 'Install-Module' is not recognized as a name of a cmdlet, function, script file, or executable program.

    I see a different error message, both in Windows PowerShell 5.1 and PowerShell (Core) 7.1.2, when the constrained language mode is in effect, i.e. when $ExecutionContext.SessionState.LanguageMode reports ConstrainedLanguage:

    Install-Module: The 'Install-Module' command was found in the module 'PowerShellGet', but the module could not be loaded. For more information, run 'Import-Module PowerShellGet'.

    This error message makes it more obvious what the problem is, and would probably have led to a faster diagnosis.

    The following contains general information that is unrelated to the above:

    Installing / troubleshooting of the PowerShellGet module that provides the Install-Module command:

    Install-Module is a (function-based) cmdlet that comes with Windows PowerShell 5.0 and 5.1 as well as all versions of PowerShell (Core) (v6+), as part of the PowerShellGet module.

    If your computers do meet these criteria, the implication is that your PowerShell installation has been corrupted in one or both of the following ways:

    If your computers do not meet these criteria, you have two choices: