Undefined values: Difference between revisions
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=={{header|PowerShell}}== |
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The <code>Get-Variable</code> cmdlet gets the Windows PowerShell variables in the current console. Variables can be filtered by using the <code>-Name</code> parameter. |
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If a variable doesn't exist an error is returned. Using the <code>-ErrorAction SilentlyContinue</code> parameter suppresses the error message and returns <code>$false</code>. |
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<lang PowerShell> |
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if (Get-Variable -Name noSuchVariable -ErrorAction SilentlyContinue) |
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{ |
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$true |
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} |
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else |
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{ |
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$false |
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} |
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</lang> |
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{{Out}} |
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<pre> |
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False |
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</pre> |
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PowerShell represents things like the file system, registry, functions, variables, etc. with '''providers''' known as PS drives. |
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One of those PS drives is Variable. <code>Variable:</code> contains all of the variables that are currently stored in memory. |
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<lang PowerShell> |
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Get-PSProvider |
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</lang> |
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{{Out}} |
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<pre> |
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Name Capabilities Drives |
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---- ------------ ------ |
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Registry ShouldProcess, Transactions {HKLM, HKCU} |
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Alias ShouldProcess {Alias} |
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Environment ShouldProcess {Env} |
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FileSystem Filter, ShouldProcess, Credentials {C, D, E, Q} |
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Function ShouldProcess {Function} |
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Variable ShouldProcess {Variable} |
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Certificate ShouldProcess {Cert} |
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WSMan Credentials {WSMan} |
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</pre> |
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To access the Variable PS drive you'd use the same syntax as you would with the file system by specifying <code>Variable:\</code>. |
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<lang PowerShell> |
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Test-Path Variable:\noSuchVariable |
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</lang> |
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{{Out}} |
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<pre> |
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False |
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</pre> |
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If a variable doesn't exist, it technically has a value of <code>$null</code>. <code>$null</code> is an automatic variable that represents "does not exist." |
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<lang PowerShell> |
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$noSuchVariable -eq $null |
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</lang> |
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{{Out}} |
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<pre> |
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True |
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</pre> |
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=={{header|Prolog}}== |
=={{header|Prolog}}== |