Bash commands in python script

How to Execute Linux Commands in Python

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

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:

import os cmd = 'ls -l' os.system(cmd) 

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.

Code

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”.

import subprocess  def ping(servers):   # The command you want to execute  cmd = 'ping'   # send one packet of data to the host  # this is specified by '-c 1' in the argument list  outputlist = []  # Iterate over all the servers in the list and ping each server  for server in servers:  temp = subprocess.Popen([cmd, '-c 1', server], stdout = subprocess.PIPE)  # get the output as a string  output = str(temp.communicate())  # store the output in the list  outputlist.append(output)  return outputlist  if __name__ == '__main__':   # Get the list of servers from the text file  servers = list(open('servers.txt'))  # Iterate over all the servers that we read from the text file  # and remove all the extra lines. This is just a preprocessing step  # to make sure there aren't any unnecessary lines.  for i in range(len(servers)):  servers[i] = servers[i].strip('\n')  outputlist = ping(servers)   # Uncomment the following lines to print the output of successful servers  # print(outputlist) 

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.

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How To Run Bash Commands In Python

There are different ways to run bash commands in Python. Lets start with os.system command.

How to use os.system to run Bash Command

Once we have imported the os. We can use os.system and pass it bash command. Lets try ls -ld /home command

The command is executed. We can’t capture the output with os.system

How to use subprocess.check_output to run Bash Commands

To see the output of executed command. There is another way. We need to import Python package subprocess.

import subprocess subprocess.check_output('ls -ld /home',shell=True, universal_newlines=True): 'drwxr-xr-x 14 root root 4096 Nov 28 16:12 /home\n' 

How to use subprocess.run to run Bash Commands

To capture the output in a variable use the run method.

subprocess.run(['ls','-ld','/home'],check=True, stdout=subprocess.PIPE, universal_newlines=True): CompletedProcess(args=['ls', '-ld', '/home'], returncode=0, stdout='drwxr-xr-x 14 root root 4096 Nov 28 16:12 /home\n')

We can also capture the output in a variable

output = subprocess.run(['ls','-ld','/home'],check=True, stdout=subprocess.PIPE, universal_newlines=True)

output can be printed using following command.

output.stdout 'drwxr-xr-x 14 root root 4096 Nov 28 16:12 /home\n' 

We can also printout the errors using following command.

check=True options throw an error if underlying option throws an error. Lets try again.

subprocess.run(['ls','-ld','/home1'],check=True, stdout=subprocess.PIPE, universal_newlines=True)

CalledProcessError: Command ‘[‘ls’, ‘-ld’, ‘/home1′]’ returned non-zero exit status 2.

Lets try without check=True option

subprocess.run(['ls','-ld','/home1'], stdout=subprocess.PIPE, universal_newlines=True) CompletedProcess(args=['ls', '-ld', '/home1'], returncode=2, stdout='')

Run Bash commands using Python Popen

Popen is used for complex commands, such as pipe commands in bash. Lets try a simple pipe command first.

p = subprocess.Popen(['ls','-ld','/home'],stderr=subprocess.PIPE, universal_newlines=True,stdout=subprocess.PIPE) out,err = p.communicate() print(out,err) drwxr-xr-x 14 root root 4096 Nov 28 16:12 /home

p is a Python process. We can query it anytime. Also we can close it.

Lets try accessing p again.

We got the following error.

ValueError: Invalid file object: _io.TextIOWrapper name=46 encoding='UTF-8'> 

How to pipe commands together using Python Popen

Lets calculate number of files in the home directory.

p1 = subprocess.Popen(['ls','-lrt','/home'],stderr=subprocess.PIPE, universal_newlines=True,stdout=subprocess.PIPE) p2 = subprocess.Popen(["wc", "-l"], stdin=p1.stdout, stdout=subprocess.PIPE)

Note, how we have to query p1.stdout to find the output.

Now lets look at the output.

output = p2.communicate() print(output) (b'23\n', None)

Wrap Up!

I have discussed above 5 ways to run Bash commands in Python. There is another utility called pysh which can be used to run Bash commands.

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