Shell Scripting in Python: A Beginner's Guide!

What is a Shell in the OS?

A shell script is just a plain text file that contains instructions/commands for the operating system to follow. These commands are processed, understood, and executed using a shell interface. Automating repetitive processes might be very useful for saving time and reducing human errors.

Shell scripting automates mundane system operations like file backups, resource monitoring, and user account management. System administrators can simplify complicated operations, save time, and improve accuracy by putting a series of instructions into a script.

Understanding Python's Role in Shell Scripting

Several methods exist in Python for carrying out shell commands. A popular approach is to use the subprocess module, which lets you launch new processes, access their return codes, and connect to their input/output/error streams. This module allows you to access the subprocess's input and output streams and perform shell commands at a high level.

Another option is using the 'os.system' method to run shell commands inside Python. Although this approach is straightforward, it requires some capabilities in the' subprocess' module.

Executing Shell Commands with Python Using the Sub-process Module

subprocess.Popen()

subprocess.Popen is a lower-level interface for operating subprocesses.run wraps Popen at a higher level for easier use.

Launch a process and access its input, output, and error streams with Popen. It returns a handle to the running process so that you can wait, check its return code, or kill it.

The run is a simpler method that runs a command and captures its output without requiring a Popen object or stream management. It also lets you provide command options like raising an exception if it fails.

The run is best for running a command and capturing its output. At the same time, Popen is better for controlling the process's input and output streams.

Popen accepts the same parameters as run(), including the command to run and optional arguments like stdin, stdout, stderr, shell, cwd, and env.

Communicate(), poll(), wait(), terminate(), and kill() are Popen class methods for process interaction.

As an example:

import os import result = os.popen('ls -l').read()
print(result)# 
Output: # [Expected ls -l command output]

This example uses os.popen() to run ‘ls—l' and record its output, which can be read and processed.

Unlike subprocess.run(), os.popen() gives more control over command output than os.system(), but it's less versatile and secure.

Also Read: Why Choose Python? Discover Its Core Advantages

subprocess.run()

Python users are advised to use the subprocess.run() function when trying to execute shell commands. This function is a component of the subprocess module and was introduced in Python 3.5. 

Using the subprocess is an easy way to start a subprocess and wait for it to finish.run() method. Optional features include running a specified command with arguments and environment variables and redirecting input and output. Executing a subprocess blocks the run() method until it finishes and produces a CompletedProcess object with the subprocess's return code and output.

Among the arguments that can be passed to the subprocess.run() function are:

• The args command executes together with its parameters, which are given as a string array.
• When set to True, capture_output will record both the standard output and standard error.
• If set to True, the output for stdout and stderr will be in string format; otherwise, it will be in bytes format.
• If the return code is non-zero, the subprocess 'CalledProcessError' is reported, and the boolean value 'check' indicates that the subprocess's return code should be checked.
• The timeout is a number in seconds that determines how long the subprocess must run before being terminated.
• The boolean value "shell" indicates if the command should be executed in a shell. As a result, shell-specific capabilities like variable replacement and wildcard expansion become available when the command is given as a string.

A basic use of subprocess is illustrated below.use run() to run a command in the shell:

import subprocess
result = subprocess.run(['ls', '-l'])
# Output:
# [Expected output from the ls -l command]

This code block imports the subprocess module and runs the ‘ls—l' shell command using run(). The function is supplied with a list of strings, with the command as the first item and its parameters as the others.

The function executes the command, waits, and returns a subprocess. The process is finished. This instance includes the command's return code, output, and errors.

Running shell commands requires error management. The subprocess module handles failures with checks. When the check is True, and the process leaves with a non-zero status, a subprocess. It will throw CalledProcessError.

Using Shell=True

You can perform a shell command as a string instead of a list. The shell=True argument does this. This method is less secure and can expose your program to shell injection attacks, so use it cautiously.

Python's subprocess module runs shell commands well. It offers tremendous control over command execution and output, making it ideal for simple and complex jobs.

Executing Shell Commands with Python Using the OS Module

Using os.system() is the simplistic way to execute a shell command:

• Start by making a new Python file and name it shell_cmd.py or whatever else you choose.
• Then, import the os module into the Python program. This module contains the system function that runs shell commands.
• You can only pass a string to the system() function as a parameter.
• Enter the desired outcome or action by typing it in.

• You should see the corresponding output after running this.py file.
• os.getcwd(): This function returns the path where a process works.
• path: To open a directory, choose its path.
• os.chdir(path): The current working directory has been changed to the path you provided as an argument.

Also Read:
Introduction To Python Basics

Error Handling and Debugging in Python Shell Scripts

When running shell commands from Python, common failures include "Command Not Found" due to missing commands or improper PATH settings and permission issues when Python lacks privileges. These concerns can be fixed by error handling and command installation and permissions.

Command Not Found Errors

Common issues include 'command not found'. Running a command that is outside the PATH generally causes this.

Permission Issues

Common issues include permission errors. Your Python script lacks the permissions needed to run the command.

Running shell commands from Python is powerful yet tricky. For optimal command execution, you must understand these typical issues and how to fix them.

Recommended Read: JSON Python: Read, Write, and Parse JSON Files in Python

Conclusion

Python shell commands have several uses. They're helpful in scripting, automation, and system administration. Python scripts with shell commands can automate tedious operations, manage system resources, and more. Executing Python shell commands can improve scripts. Python developers should know how to execute shell commands because they open up many options. We hope that this guide helps you learn Python shell commands. If you want to master this new skill for your career advancement, consider enrolling in our comprehensive Python Training course.

FAQs

1. How does Python compare to traditional shell scripting languages like Bash?

Unix-like systems benefit from bash scripts for file manipulation, system administration, and task automation. Python, a full-featured programming language, simplifies difficult jobs, data manipulation, and cross-platform applications.

2. Can I execute shell commands directly from Python scripts?

Python modules like os, subprocess, sys, platform, etc. execute shell commands.

3. How can I handle errors and debug Python shell scripts effectively?

Custom exit codes can be supplied to sys.exit() in Python. It is useful for exit code error descriptions. The system terminates with error code 1 if the call argument is not an integer.

4. How can I optimize Python shell scripts for performance?

Dash or ksh is faster and lighter than Bash or zsh, which can improve script performance. Reduce overhead and resource-consuming subprocesses. Instead of pipes or subshells, try built-in commands, process substitution, or command grouping. For large or sophisticated operations, utilize text processing tools like awk, sed, or grep instead of loops. Lastly, arrays or associative arrays can store and access data instead of repeatedly reading or writing files or variables.