How to Execute External Commands in Python

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"])

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 below example.

#!/usr/bin/python3
import subprocess

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

[Output]

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 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)

[Output]

Welcome $USER
Welcome root

You can see in above output that first command prints variable name because it was executed without shell expansion. The second command executed with shell expansion get the value of 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)

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)

[Output]

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