How to Execute External Commands in Python

Written by Rahul, Updated on March 7, 2020

The Python subprocess module allows to spawn new processes, execute external commands from Python scripts. You can install the latest version of Python using these tutorials. Also, there is number of IDE available for Python. Like Install PyCharm Python IDE on Ubuntu systems.

A Simple Python Example

Let’s create your first program to list all files available in the current directory. You can add any number of command-line parameters with the comm (,) separated.

#!/usr/bin/python3

import subprocess

subprocess.call(["ls", "-l"])

#!/usr/bin/python3

import subprocess

subprocess.call(["ls", "-l"])

Where-

import statement is used to load the subprocess module form the Python standard library
call is the function of subprocess module, is used to execute external commands

Python Print without New Line

Python commands terminate with new line output “\n”, which you can overwrite using end=”” means that the next command output will be on the same line. See the below example.

#!/usr/bin/python3

import subprocess

print("\nToday is ", end="")
subprocess.call(["date","+%D"])

#!/usr/bin/python3

import subprocess

print("\nToday is ", end="")

subprocess.call(["date","+%D"])

Result:

Today is 01/11/18

Python Example with Shell Expansion

The default subprocess.call doesn’t expand shell wildcards or perform command substitution. This can be overridden by passing True value for shell argument. Remember that the use of shell=True could be a security issue for your system by executing the harmful command.

#!/usr/bin/python3

import subprocess

# Executing command without shell expansion
subprocess.call(['echo', 'Welcome $USER'])

# Executing command with shell expansion
subprocess.call('echo Welcome $USER', shell=True)

#!/usr/bin/python3

import subprocess

# Executing command without shell expansion

subprocess.call(['echo', 'Welcome $USER'])

# Executing command with shell expansion

subprocess.call('echo Welcome $USER', shell=True)

Result:

Welcome $USER
Welcome root

You can see in the above output that the first command prints a variable name because it was executed without shell expansion. The second command executed with shell expansion get the value of the USER environment variable. Also, you can see that the entire command is now passed as a string and not as a list of strings.

Other Useful Python Examples

You can make your scripts more readable format and beautiful by storing a long command in a variable and then execute it.

#!/usr/bin/python3

import subprocess

cmd = "grep -Re 'Fatal' /var/log/ > searches.txt"
subprocess.call(cmd, shell=True)

#!/usr/bin/python3

import subprocess

cmd = "grep -Re 'Fatal' /var/log/ > searches.txt"

subprocess.call(cmd, shell=True)

You can also store the output of any command to a variable using subprocess.getoutput. This will also store if any error message generated.

#!/usr/bin/python3

import subprocess

print("The output of 'pwd' command is:", flush=True)

output = subprocess.getoutput('pwd')
print(output)

#!/usr/bin/python3

import subprocess

print("The output of 'pwd' command is:", flush=True)

output = subprocess.getoutput('pwd')

print(output)

Result:

The output of 'pwd' command is:
/root/Python/Scripts

How to Execute External Commands in Python

A Simple Python Example

Python Print without New Line

Python Example with Shell Expansion

Other Useful Python Examples

Rahul

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