Shell Programming in Python

Snake Shell

© Lead Image © Nelli Valova, 123RF.com

© Lead Image © Nelli Valova, 123RF.com

Article from Issue 267/2023
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Create lightweight Raspberry Pi scripts with Xonsh, a Python shell that lets you write scripts in Python with Bash commands mixed in.

For Raspberry Pi users, Xonsh [1] offers many opportunities to write some extremely lightweight scripts, with Python connecting to physical devices and Bash utilities accessing system and file resources.

In this article, we look at two lean Xonsh projects for the Raspberry Pi. The first program connects to a DHT11 temperature and humidity sensor and shows the results in a Bash dialog in just five lines. The second project calls the Bash top utility in a lean eight lines to show the Raspberry Pi idle time and user time on a 16x2 LCD screen.

Getting Started

Xonsh has two requirements: Python 3.8 or greater and a Bash shell. To install and run Xonsh on a Raspberry Pi, Ubuntu, or Debian system, enter:

sudo apt install xonsh
xonsh

When the Xonsh shell opens, the terminal label changes so you can see that you are working in Xonsh rather than a standard Bash shell (Figure 1). When the shell first starts up, two options are presented: xonfig tutorial, which opens a browser help window, and xonfig web, which allows users to tweek the xonsh shell configuration.

Figure 1: The Xonsh terminal.

Within the Xonsh shell, you can enter both Bash and Python statements, such as:

$ # Mix Bash and Python Lines
$ echo "Time is:" $(date +%T) ; # Bash
Time is: 12:59:51
$ print("%d + %d = %d" % (2,3,2+3)) ; # Python
2 + 3 = 5

Xonsh in the Terminal

The Xonsh shell is designed for Python users, so some handy features are available from the command prompt. The first of these features is code highlighting. Xonsh highlights comments, strings, functions, and command statements (Figure 2).

Figure 2: Xonsh highlights Bash and Python statements.

Xonsh also handles Python indentation errors for single-line statements and non-control statements. Figure 3 shows an example of Python code that successfully runs with both overindented and underindented statements.

Figure 3: Xonsh manages Python indentation errors.

The next useful feature is Python help (Figure 4). The Tab key offers a dropdown dialog of Python objects or methods from the typed string. The arrow keys let you select the required help options within the dialog.

Figure 4: Xonsh offers Python help.

Bash Zenity Dialogs with Python

Python has some great graphic libraries such as Tkinter, Qt, and PySimpleGUI. These libraries are excellent for complex GUI applications, but they can be overkill if you only need a simple dialog.

The Zenity utility [2] is a quick way to present a variety of dialogs in just one line of Bash code. Zenity is preloaded on Raspberry Pi OS and most Linux installations, so no added installation is usually required.

The basic syntax for a Zenity information dialog is:

$ zenity --info --title=myTitle --text=myMessage

In Xonsh, Python statements and variables are used directly in a Bash statement as:

@(<Python statement>)

For example, the Bash echo command can print a Python statement:

$ import sys
$ echo "OS: " @(sys.platform)
OS:  linux

These two steps can be combined to show the OS in a Zenity dialog:

$ import sys
$ zenity --info --title=System_OS --text=@(sys.platform)

Once you have these two concepts working (Zenity and Python embedded in Bash), you can start creating some quick and easy Raspberry Pi applications.

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