Running linux commands python

How to Execute Linux Commands in Python

January 13, 2021

Linux is one of the most popular operating systems used by software developers and system administrators. It is open-source, free, customizable, very robust, and adaptable. Making it an ideal choice for servers, virtual machines (VMs), and many other use cases.

Therefore, it is essential for anyone working in the tech industry to know how to work with Linux because it is used almost everywhere. In this tutorial, we are going to look at how we can automate and run Linux commands in Python.

Table of contents

Prerequisites

• Basic understanding of Linux and shell scripting.
• Basic programming skills in Python.

Introduction

Python has a rich set of libraries that allow us to execute shell commands.

A naive approach would be to use the os library:

The os.system() function allows users to execute commands in Python. The program above lists all the files inside a directory. However, we can’t read and parse the output of the command.

In some commands, it is imperative to read the output and analyze it. The subprocess library provides a better, safer, and faster approach for this and allows us to view and parse the output of the commands.

OS	subprocess
os.system function has been deprecated. In other words, this function has been replaced.	The subprocess module serves as a replacement to this and Python officially recommends using subprocess for shell commands.
os.system directly executes shell commands and is susceptible to vulnerabilities.	The subprocess module overcomes these vulnerabilities and is more secure.
The os.system function simply runs the shell command and only returns the status code of that command.	The subprocess module returns an object that can be used to get more information on the output of the command and kill or terminate the command if necessary. This cannot be done in the os module.

Although you can execute commands using the OS module, the subprocess library provides a better and newer approach and is officially recommended. Therefore, we are going to use subprocess in this tutorial. This documentation explores the motivation behind creating this module.

Building an application to ping servers

Let’s use the subprocess library to write a script that pings multiple servers to see whether they are reachable or not. This would be a good use case when you have multiple hosts, servers, or VMs(AWS ec2 instances) and want to check if they are up and running without any problems.

A simple solution is to just ping these servers and see if they respond to the request. However, when you have a considerable amount of machines, it will be extremely tedious and time-consuming to manually ping them. A better approach is to use Python to automate this process.

According to the official documentation, the subprocess module allows you to spawn new processes, connect to their input/output/error pipes, and obtain their return codes.

This module intends to replace several older modules and functions. The subprocess library has a class called Popen() that allows us to execute shell commands and get the output of the command.

Create a Python file and add the following code. We also need to create a file called “servers.txt”, where we can add a list of all the servers we need to ping. The Python script will read from this file and ping each server listed in it.

I have added 4 servers, out of which two exist and the other two do not. Only the servers that exist can be “pinged”.

As you can see in the output, we get the message “name or service not known” for the two servers that did not exist.

In the program above, the ping() function takes a list of servers and returns the output of each running ping command on each server. If a server is unreachable, it displays an output saying “ping: somethingthatdoesntexist: Name or service not known”.

The Popen() is a constructor method of the Popen class and takes in the following arguments:

A list of commands and any additional options these commands might require. For example, the ls command can be used with ‘-l’ option. To execute the ls -l command, the argument list would look like this: [‘ls’, ‘-l’] . The commands are specified as strings. In the example above, we use the ping command with the option -c 1 so that it only sends one packet of data, and the server replies with a single packet . Without this limit, the command would run forever until an external process stops it.

The stdout argument is optional and can be used to set where you want the subprocess to display the output. By default, the output is sent to the terminal. However, if you don’t want to dump a large output onto the terminal, you can use subprocess.PIPE to send the output of one command to the next. This corresponds to the | option in Linux.

The stderr argument is also optional and is used to set where you want the errors to be displayed. By default, it sends the errors to the terminal. Since we need to get a list of servers that cannot be reached, we don’t need to change this. The servers that cannot be reached (error) will be displayed to us on the terminal.

The output of the command is stored in a variable called temp . The communicate() function allows us to read the output and the str function can be used to convert it to a string. Once we get the output, we can parse it to extract only the essential details or just display it as it is. In this example, I am storing the output in a list for future use.

Conclusion

In conclusion, automation is one of the hottest topics in the industry, and almost every company is investing huge amounts of money to automate various manual tasks. In this tutorial, we explored the process of automatically running and analyzing Linux commands on multiple hosts using Python.

An old way of doing this is by using shell scripts. However, using Python gives developers more power and control over the execution and output of the commands. Now that you have understood the basics of executing Linux commands, you can go ahead and experiment with different commands and build more complex and robust applications.

About the author

Adith Bharadwaj is a senior at the National Institute of Engineering (NIE) and a Software Engineer Intern at Cisco — India. Adith has a keen interest in solving challenging problems and is a data science and machine learning enthusiast. When he’s not coding, he loves drawing, working out, and watching great TV shows, movies or anime.

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How to Run Linux Commands With Python on the Raspberry Pi

Python is the language of choice for shell scripting and task automation. It is popular in system administration because it can execute shell commands using only its default libraries. In this tutorial, you will learn how to run Linux shell commands with Python using the os and subprocess modules.

Introduction

Nowadays, automation is a buzzword as hot as a processor running at 100% in a room with no AC. Home management, data extraction, the omniscient talking boxes in the kitchen, and even self-driving cars—they are all products of automation. Well, it makes sense since we’re living in the fourth industrial revolution. Heard of it? It is where humans and machines try to work as one, and it is happening now.

Using a Raspberry Pi even opens more doors in automation. With its GPIO, you can interface the credit-card sized computer into almost anything that throws or receives digital data. And since it is a fully-fledged Linux computer, you can automate your computer tasks as well. What’s more, Python makes it unimaginably easier! There are two ways to run Linux commands with Python: using the os module and using the subprocess module.

Using the OS Module

First is the os module. According to official documentation, the os module provides a portable way of using operating system dependent functionality. Ain’t that convenient? With short python codes, you can already perform standard operating system tasks without having to interact with the Desktop interface. The system method allows you to do exactly this. To use it to run a Linux command, your code should look like below.

Sample Code using system()

This 4-liner checks your current directory, change location to your home directory, and lists all the contents in detail. It’s a pretty straightforward implementation, but there’s a downside. With system() , you are not allowed to store the resulting output as a variable.

Instead, you can use the popen() method, which is still under the os module. It opens a pipe from or to the command line. A pipe connects a command’s output to another command’s input. This makes it accessible within Python. To use popen() to store as a variable, see the example code below.

Sample code using popen()

If you print the stream variable, you will see its return data. This consists of the actual commands executed, the mode, and the address. Furthermore, if you want to get the whole output as one string, change readlines() to read() .

Using the Subprocess Module

The second way to run Linux commands with Python is by using the newer subprocess module. This module allows you to spawn new processes, connect to their input/output/error pipes, and obtain their return codes. It was created to replace both os.system() and os.popen() functions.

The only method that matters in the subprocess is run() . With it, you can do everything we’ve done above and more using different arguments. Use the following codes as reference:

Writing a simple command using subprocess

Using the method like this will execute the command ls in your terminal. Unlike os.system() , it doesn’t work when you add a switch and enter it fully like subprocess.run(‘ls -la’) . This feature allows the method to take care of quoting and escaping problems hence preventing errors with formatting. To execute ls -la , you must pass the command as a list: subprocess.run([‘ls’,’-la’) . Alternatively, you can make the shell argument True to pass the whole thing as a string. Just take note that this can pose a security risk if you’re using an untrusted input source.

Writing a command with switches

Next, to store the command output inside a variable, simply do it just like any other data. The result won’t be what you’re expecting, however. Since the main purpose of ‘run’ is to execute the shell command within python, the result won’t be like the output you see in the terminal. It will be the return data just like in os.open . You can check it using the code below.

Storing the command output to a variable

This sketch dissects the return data of your command using the method’s arguments. Here are some of the frequently used ones:

• args – returns the actual commands executed
• returncode – returns the return code of the output; 0 means no error
• stdout – captured stdout from the child process
• stderr – captured stderr stream from the child process

Since we did not capture the previous code’s output, we will get ‘none’ with both stdout and stderr arguments. To enable the capture output argument, refer to the following code:

If you print x, you will get the list of items in your current directory of type bytes. Convert it to a string by writing x.stdout.decode() . Alternatively, you can pass the argument text=True with the main function. The output should now look exactly the same as what you have in the terminal.

Lastly, we will run Linux commands with Python and save the output you see in the terminal into a text file—a simple task with subprocess. You just need to redirect the stdout stream to your text file using the argument stdout .

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Python Execute Unix / Linux Command Examples

H ow do I execute standard Unix or Linux shell commands using Python? Is there a command to invoke Unix commands using Python programs?

Tutorial details
Difficulty level	Easy
Root privileges	No
Requirements	Python
Est. reading time	N/A

You can execute the command in a subshell using os.system() . This will call the Standard C function system(). This function will return the exit status of the process or command. This method is considered as old and not recommended, but presented here for historical reasons only. The subprocess module is recommended and it provides more powerful facilities for running command and retrieving their results.

os.system example (deprecated)

In this example, execute the date command:

In this example, execute the date command using os.popen() and store its output to the variable called now:

Say hello to subprocess

The os.system has many problems and subprocess is a much better way to executing unix command. The syntax is:

In this example, execute the date command:

You can pass the argument using the following syntax i.e run ls -l /etc/resolv.conf command:

Another example (passing command line args):

In this example, run ping command and display back its output:

The only problem with above code is that output, err = p.communicate() will block next statement till ping is completed i.e. you will not get real time output from the ping command. So you can use the following code to get real time output:

Related media

A quick video demo of above python code:

References:

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Comments on this entry are closed.

307 ms?
That’s a long interval 😉

Hi, please more small and usefull examples with python like it! more code snippets!

A very comprehensive explanation, being useful to beginners to python.

where to find the command of linux

these commands are very helpfull us….please give more example like this.

What exactly does Shell=True does?
please tell the exact usage of the shell argumet.

First off, enjoy all your posts and youtube videos. Recently viewed your tutorial on installing freebsd. So thank you for sharing your knowledge.

I have a query regarding launching an external bash script file (.sh) in freebsd.
For linux I used:
os.system(‘sh ‘ + filepath)
For Mac:
os.system(‘open ‘ + filepath)
And for windows:
os.startfile(filepath)

I am unable to get any of these to work for freebsd. I know startfile is only for windows, however was wondering if there was an equivalent for freebsd without using subprocess. Or if not possible at all how to use subprocess to call a external script.

Also, in freebsd, what would be the equivalent of say:
sudo chown -R user:user file.bundle
as both sudo and chown are not installed by default.

Any help would be appreciated.

What if I want to create a variable in Python, then pass that variable to a bash command line?
Something like this:
….
celsius = sensor.read_temperature()
import subprocess
subprocess.call([“myscript.sh”, “-v”, “-t $celsius”])

Is that possible?

Of course you can. In python’s new formatting it would look like this:
subprocess.call([«myscript.sh», «-v», «-t <>«.format(celsius)])

I use split so I dont have to write literal arrays. Works most of the time.

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