To list all Environment variables from all App Services and Function Apps, you can make use of below sample PowerShell script:

Connect-AzAccount

$rGs = Get-AzResourceGroup
$allSettings = @()

foreach ($rg in $rGs) {
    $webApps = Get-AzWebApp -ResourceGroupName $rg.ResourceGroupName

    foreach ($webApp in $webApps) {
        $appSettings = (Get-AzWebApp -ResourceGroupName $rg.ResourceGroupName -Name $webApp.Name).SiteConfig.AppSettings
        foreach ($setting in $appSettings) {
            $allSettings += [pscustomobject]@{
                ServiceName     = $webApp.Name
                ResourceGroup   = $rg.ResourceGroupName
                SettingName     = $setting.Name
                SettingValue    = $setting.Value
                SettingType     = "AppSetting"
            }
        }

        $connectionStrings = (Get-AzWebApp -ResourceGroupName $rg.ResourceGroupName -Name $webApp.Name).SiteConfig.ConnectionStrings
        foreach ($conn in $connectionStrings) {
            $allSettings += [pscustomobject]@{
                ServiceName     = $webApp.Name
                ResourceGroup   = $rg.ResourceGroupName
                SettingName     = $conn.Name
                SettingValue    = $conn.ConnectionString
                SettingType     = "ConnectionString"
            }
        }
    }
}

$allSettings | Export-Csv -Path "$HOME/AllEnvironmentVariables.csv" -NoTypeInformation #exported to csv file
$allSettings # print values to terminal

Response:

AllEnvironmentVariables.csv:

You can use the following Azure CLI commands to list configuration settings of App Services and Function Apps.

App Services:

• az webapp config show: Get the details of a web app’s configuration
• az webapp config appsettings list: Get the details of a web app’s settings

Function Apps:

• az functionapp config show: Get the details of an existing function app’s configuration
• az functionapp config appsettings list: Show settings for a function app

If you need to filter variables by type (App Settings, Azure SQL Database connection strings, custom connection strings, etc) take a look into the variable prefixes.