How do I pass a local variable to a remote `Invoke-Command`?

In PowerShell 4 (3+ actually) the easiest way is to use the Using scope modifier:

Invoke-Command -ComputerName winserver -ScriptBlock { 
    Get-FileHash E:\test\$Using:dest.zip -Algorithm SHA1 
}

For a solution that works with all versions:

Invoke-Command -ComputerName winserver -ScriptBlock {
    param($myDest)

    Get-FileHash E:\test\$myDest.zip -Algorithm SHA1 
} -ArgumentList $dest

To complement briantist's helpful answer:

The script block passed to Invoke-Command is (as intended) executed on the remote machine, using the remote machine's variables by default.

Thus, in order to use a local variable's value,^[1] extra steps are needed (to put it differently: inside a script block executed remotely, you cannot just refer to local variables as you normally would, such as with $dest):

• PS v3+ offers the using: scope modifier for direct use of a local variable's value inside the script block - see briantist's first command.

  • Note that using: only works when Invoke-Command actually targets a remote machine, such as with the -ComputerName parameter.

• The only option that also works in earlier versions is to pass the local variable as a parameter to the script block. - see briantist's second command.

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^{[1] Note that you fundamentally cannot pass a variable itself, i.e. you cannot assign to a local variable in a remote script block.}