PowerShell code to add/remove, install/uninstall and enable/deactivate feature at Web, Site Collection, Web Application and Farm scope

function Deactivate-SPFeature
{
    param ($FeatureID, $SiteUrl, $WebApplicationUrl)
    $Feature = Get-SPFeature -Identity $FeatureID -ErrorAction SilentlyContinue
    $IsActiveFeature
    If ($Feature -eq $null)
    {
        Write-Warning "The specified feature ($FeatureID) was not found."
        return
    }
    If ($Feature.Scope -eq [Microsoft.SharePoint.SPFeatureScope]::Farm)
    {
        $IsActiveFeature = Get-SPFeature -Identity $FeatureID -Farm -ErrorAction SilentlyContinue
        If ($IsActiveFeature -eq $null)
        {
            Write-Warning "The specified feature ($FeatureID) is not activated on the Farm."
            return;
        }
        Write-Host "Deactivating Feature $FeatureID."
        Disable-SPFeature -Identity $FeatureID  -confirm:$false
        Write-Host "Feature  $FeatureID deactivated sucessfully"  -foregroundcolor Green
        return;
    }
    ElseIf ($Feature.Scope -eq [Microsoft.SharePoint.SPFeatureScope]::WebApplication)
    {
        $IsActiveFeature = Get-SPFeature -Identity $FeatureID -WebApplication $WebApplicationUrl -ErrorAction SilentlyContinue
        If ($IsActiveFeature -eq $null)
        {
            Write-Warning "The specified feature ($FeatureID) is not activated in the Web application ($WebApplicationUrl)."
            return;
        }
        Write-Host "Deactivating Feature $FeatureID."
        Disable-SPFeature -Identity $FeatureID -Url $WebApplicationUrl -confirm:$false
        Write-Host "Feature  $FeatureID deactivated sucessfully"  -foregroundcolor Green
        return;
    }
    ElseIf ($Feature.Scope -eq [Microsoft.SharePoint.SPFeatureScope]::Site)
    {
        $IsActiveFeature = Get-SPFeature -Identity $FeatureID -Site $SiteUrl -ErrorAction SilentlyContinue
    }
    ElseIf ($Feature.Scope -eq [Microsoft.SharePoint.SPFeatureScope]::Web)
    {
        $IsActiveFeature = Get-SPFeature -Identity $FeatureID -Web $SiteUrl -ErrorAction SilentlyContinue
    }
    If ($IsActiveFeature -eq $null)
    {
        Write-Warning "The feature $FeatureID is not activated on the site $SiteUrl"
        return;
    }

    Write-Host "Deactivating Feature $FeatureID."
    Disable-SPFeature -Identity $FeatureID  -Url $SiteUrl -confirm:$false
    Write-Host "Feature  $FeatureID deactivated sucessfully"  -foregroundcolor Green
}

function EnableOrSkip-SPFeature
{
    param ($FeatureID, $SiteUrl)
    $Feature = Get-SPFeature -Identity $FeatureID -ErrorAction SilentlyContinue
    If ($Feature -eq $null)
    {
        Write-Warning "The specified feature ($FeatureID) was not found."
        return
    }
    If ($Feature.Scope -eq [Microsoft.SharePoint.SPFeatureScope]::Farm)
    {
        $Feature = Get-SPFeature -Identity $FeatureID -Farm -ErrorAction SilentlyContinue
        if ($Feature -ne $null)
        {
            Write-Host "Feature  $FeatureID is already activated on the Farm"  -foregroundcolor Green
            return;
        }
        Write-Host "Activating feature $FeatureID"
        Enable-SPFeature -Identity $FeatureID -Confirm:$false 
        Write-Host "Feature $FeatureID activated successfully" -foregroundcolor Green
        return;
    }
    ElseIf ($Feature.Scope -eq [Microsoft.SharePoint.SPFeatureScope]::WebApplication)
    {
        $Feature = Get-SPFeature -Identity $FeatureID -WebApplication $SiteUrl -ErrorAction SilentlyContinue
    }
    ElseIf ($Feature.Scope -eq [Microsoft.SharePoint.SPFeatureScope]::Site)
    {
        $Feature = Get-SPFeature -Identity $FeatureID -Site $SiteUrl -ErrorAction SilentlyContinue
    }
    ElseIf ($Feature.Scope -eq [Microsoft.SharePoint.SPFeatureScope]::Web)
    {
        $Feature = Get-SPFeature -Identity $FeatureID -Web $SiteUrl -ErrorAction SilentlyContinue
    } 
    If ($Feature -ne $null)
    {
        Write-Host "Feature  $FeatureID is already activated on the site $SiteUrl" -foregroundcolor Green
        return;
    }
    Write-Host "Activating feature $FeatureID"
    Enable-SPFeature -Identity $FeatureID -Url $SiteUrl  -Confirm:$false 
    Write-Host "Feature $FeatureID activated successfully" -foregroundcolor Green
}

function AddAndDeployGlobally-SPSolution
{
    param ( $SolutionPackageName, $SolutionPath)
    # Check if solution exists.
    $Solution = Get-SPSolution -Identity $SolutionPackageName -ErrorAction SilentlyContinue
    if ($Solution -ne $null)
    {
        RetractAndRemove-SPSolution $SolutionPackageName
    }
    # Add solution
    Write-Host "Adding solution $SolutionPackageName."
    Add-SPSolution -LiteralPath $SolutionPath
    Write-Host "$SolutionPackageName added successfully."  -foregroundcolor Green
    # Deploy solution
    $Solution = Get-SPSolution | ? {($_.Name -eq $SolutionPackageName) -and ($_.Deployed -eq $false)}
    if($Solution -ne $null)
    {
        # Install solution on Farm
        Write-Host "Deploying solution to Farm."
        if($Solution.ContainsWebApplicationResource)   
        {
            Install-SPSolution -Identity $SolutionPackageName -WebApplication $WebApplication -       GACDeployment -Confirm:$false
        }
        else
        {
            Install-SPSolution -Identity $SolutionPackageName -GACDeployment -Confirm:$false
        }
        Restart-SPAdminV4
        while ($Solution.Deployed -eq $false)
        {
            Write-Host -NoNewLine .
            Start-Sleep 2
        }
        Write-Host "$SolutionPackageName deployed successfully to Farm." -foregroundcolor Green   
    }
}

function RetractAndRemove-SPSolution
{
    param ( $SolutionPackageName )
    # Check if solution exists.
    $Solution = Get-SPSolution | ? {($_.Name -eq $SolutionPackageName) -and ($_.Deployed -eq $true)}  -ErrorAction SilentlyContinue
    if ($Solution -ne $null)   
    {   
        Write-Host "Rectracting solution: $SolutionPackageName" 
        if($Solution.ContainsWebApplicationResource)   
        {   
            Uninstall-SPSolution $SolutionPackageName -AllWebApplications -Confirm:$false   
        }   
        else   
        {   
            Uninstall-SPSolution $SolutionPackageName -Confirm:$false   
        }
        Restart-SPAdminV4
        while ($Solution.JobExists)
        {
            Write-Host -NoNewLine .
            Start-Sleep 2
        }
        Write-Host "$SolutionPackageName retracted successfully."  -foregroundcolor Green
        if ($(Get-SPSolution | ? {$_.Name -eq $SolutionPackageName}).Deployed -eq $false)
        {
            Write-Host "Removing solution: $SolutionPackageName" 
            Remove-SPSolution $SolutionPackageName -Confirm:$false
            Write-Host "$SolutionPackageName removed successfully."  -foregroundcolor Green
        }
    } 
}

function Restart-SPAdminV4
{
    Stop-Service SPAdminV4
    Start-SPAdminJob 
    Start-Service SPAdminV4
}

Source: http://sharepointnadeem.blogspot.com/2011/08/sharepoint-2010-powershell-deployment.html