Writing a Python program to execute external commands

• In python standard library we have a module name subprocess Refer Here
• Lets write a simple program to create, list and remove the file using external linux commands

#!/usr/bin/env python3
import subprocess


def create_check_remove_file(file_name="frompython.txt"):
    """
    This function will create a file called as frompython.txt
    equivalent linux commands
    1. touch frompython.txt
    2. ls
    3. rm frompython.txt
    """
    # subprocess.call(["touch", file_name])
    capture_output(["touch", file_name])
    print(f"created file {file_name}")
    # subprocess.call(["ls"])
    capture_output(["ls"])
    print("listed the files")
    # subprocess.call(["rm", file_name])
    capture_output(["rm", file_name])
    print(f"removed the file {file_name}")
    capture_output(["ls"])
    pass


def capture_output(commands):
    """
    This command will capture the return code of the commands executed
    and the stdout
    :param commands: commands to be executed
    """
    result = subprocess.run(commands, stdout=subprocess.PIPE)
    print(f"return code: {result.returncode}")
    command = " ".join(commands)
    print(f"output from the {command} executed are {result.stdout.decode('utf-8')}")


if __name__ == "__main__":
    create_check_remove_file()

Python Programs to Read Command line arguments

• Lets try to build a python program which

python sum.py 1 2 3 4 5 6 7 8 9 10
55

• There are 3 ways to read command line args

  1. In the sys module we have argv
  2. Using the getopt module which has getopt method (getopt.getopt)

• Argv Example to calculate squares

#!/usr/bin/env python3

import sys


def sum_of_numbers():
    """
    This method will print the sum of arguments passed from command line
    :return: sum of numbers from cmdline
    """
    # naive method
    # result = 0
    # for argument in sys.argv[1:]:
    #     result = result + int(argument)
    # return  result

    # list comprehension
    arguments = [int(argument) for argument in sys.argv[1:]]
    return sum(arguments)


def sum_of_squares():
    """
    This method will print the sum of arguments passed from command line
    :return: sum of squares numbers passed from cmdline
    """
    # list comprehension
    arguments = [int(argument) * int(argument) for argument in sys.argv[1:]]
    return sum(arguments)


if __name__ == "__main__":
    print(sum_of_numbers())
    print(sum_of_squares())

• In Majority of the cases we might have named arguments, but the above command works only for positional args

python calc.py -a 1 2 3 4 5 6 7 8 9 10
55

python calc.py -m 1 2 3
6

• Lets explore getopts Refer Here
• Exercise: Debug this program.

#!/usr/bin/env python3

import getopt
import sys


def parse_args():
    """
    This method will parse arguments
    """
    opts, args = getopt.getopt(sys.argv, "a:m:")
    print(opts)
    print(args)


if __name__ == "__main__":
    parse_args()