New Harmonies

PowerShell for Linux-Based Commands

PowerShell inherently comes with extensive cmdlets that can handle most administration tasks that occur on Windows. When you use PowerShell on Unix-style operating systems, you have to call many command-line tools manually, which explains why I chose ifconfig (the Unix equivalent of the command-line IP configuration utility ipconfig) as the next test candidate. Microsoft provides a turnkey program example with the file ifconfig.Crescendo.json, which you analyze in the first step:

Import-CommandConfiguration .\ifconfig.Crescendo.json

The output is the command table shown in Figure 2. Note that Crescendo is running on Windows 10, an operating system that knows nothing about ifconfig. The output containing information about the control file shows various metadata because the Crescendo specification not only lets you specify the commands to be executed and parameters required by them, but Microsoft lets you store information, which PowerShell then includes in the help system.

Figure 2: Tools to be addressed by Crescendo do not need to be installed on the machine responsible for transpilation.

Open the ifconfig.Crescendo.json file in an editor of your choice (Listing 3). Both Visual Studio and the basic Visual Studio Code let you edit PowerShell files with syntax highlighting: I do not recommend editing PowerShell scripts with Notepad.

01 {
02     "$schema": "../src/Microsoft.PowerShell.Crescendo.Schema.json",
03    "Commands": [
04      {
05        "Verb": "Invoke",
06        "Noun": "ifconfig",
07        "Description": "This is a description of the proxy",
08        "OriginalName": "ifconfig",
09        ""Aliases": [
10           "Get-NetworkConfiguration"
11        ],
12        "Usage": {
13           "Synopsis": "Run invoke-ifconfig"
14        },
15        "Parameters": [
16        {
17              "Name": "Interface",
18              "OriginalName": "",
19              "Description": "This is the description for a parameter",
20              "ParameterType": "string",
21             "DefaultValue": ""
22        }
23      ]
24    }
25   ]
26 }

Again, the OriginalName parameter takes the name of the binary you want PowerShell to activate. In the world of Crescendo, you do not generally describe Unix commands by a fixed path, but by the name you need to enter at the command line. However, there is nothing wrong with specifying the full path of the binary in "sensitive" situations.

Next is the metadata. In addition to the Aliases block, which supplies additional strings intended for activating the command, a Usage block accommodates help text.

Parameters

In many cases, the commands packaged by Crescendo take parameters that allow the user to customize the program's behavior. PowerShell cmdlets are also parameterizable, so Microsoft has created a tool in the Crescendo environment for passing in parameters. Specifically, the Commands block expects another field that describes the parameters. It is important to note that you do not need to declare all the parameters supported by the binary to be called in your Crescendo wrapper. It is sufficient to expose only those settings that you want to make addressable by the caller. In the case of the ifconfig wrapper, the only useful parameter according to the developer is the name of the network interface to be analyzed ("Name": "Interface").

Like the Commands block, the Parameters block has an OriginalName field. It now determines the "prefix" under which the binary file to be called expects the information to be delivered. You can use Description to store information that shows the user more details about the role of the parameter. The ParameterType line defines the supported data type, and DefaultValue is the default value to be passed in. You can use the data types area to add all the variable types supported by PowerShell; besides string, which is used here, integer is a possible data type.

The parameters array lets you specify optional additional attributes. For example, if you pass in

"Mandatory": true

the caller of the wrapper is required to assign a value to this parameter:

"Parameters": [
   {
..."Name": "Id",
..."OriginalName": "",
..."Mandatory": true,
..."ValueFromPipelineByPropertyName": true
.}
],

Another neat example of Crescendo parameters can be found in the wrapper around the Unix tar utility that, first, defines a prefix for the parameter and, second, specifies that the parameter should act as a bit switch:

{
      "Name" : "Detail",
      "OriginalName" : "-l",
      "ParameterType" : "switch"
},

Parameters configured with a "ParameterType" : "switch" type are not supplied with a value within the wrapper call. The user of the wrapper can only omit or use the parameter: If it is found in the call to the cmdlet generated by Crescendo, it will also show up in the call to the binary.

Once again, the next thing to do in the ifconfig example is to transpile the wrapper to create PowerShell code. Note that the commands are still running on Windows:

Export-CrescendoModule -ModuleName ".\ifconfig1.psm1" -ConfigurationFile ".\ifconfig. Crescendo.json"

PowerShell proves to be flexible in terms of script execution. Theoretically, nothing prevents you from adding your module to the Windows 10 workstation cmdlets cache (and executing it there):

Import-Module -Name .\ifconfig1.psm1
Invoke-ifconfig

However, because of the lack of an ifconfig binary, the program fails to execute and outputs an error.

Running Crescendo Wrappers on Ubuntu

Ubuntu 20.04 LTS comes with a version of ifconfig. Unfortunately, Canonical does not equip the Linux distribution with a PowerShell interpreter out of the box, so you will need to install it:

sudo apt-get update
sudo apt-get install -y wget apt-transport-https software-properties-common
wget -q https://packages.microsoft.com/config/ubuntu/16.04/packages-microsoft-prod.deb
sudo dpkg -i packages-microsoft-prod.deb
sudo apt-get update
sudo apt-get install -y powershell

Assuming everything works, you can launch PowerShell by typing pwsh. The next step is to transfer the files from the Windows workstation to the Unix machine and install them:

Import-Module -Name './ifconfig1.psm1' -Force

Now ifconfig can be called from PowerShell with Invoke-ifconfig (Figure 3). Thanks to the parameter declaration, you can also specify which network interface you want to run the command against (e.g., Invoke-ifconfig eth0).

Figure 3: The wrapper generated by Crescendo works fine on Ubuntu.