Python on CLI

In Back-end

Advantages over bash

Libraries: system operations, reading files, listing directories, writing for loops, checking for exit codes…
Autocomplete with IDEs
Robust testing suite
iPython console
Python / Miniconda is available on most systems
Robust error checking with try and catch blocks
Python libraries deal with OS peculiarities under the hood, so it may run everywhere

Installation

Check if you have iPython installed
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which python
which ipython

Install Miniconda with Python 3 and update

bash Miniconda3-latest-Linux-x86_64.sh
conda update conda
conda config --add channels conda-forge
conda update -y --all
conda install -y ipython

❗ Known issues installing
Avoid using the global area: use environments
Clean up everything in case of wrong installation
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conda clean --all
conda update -y --all

Create Environments with Conda

1 2	# Different projects should have their own isolated software environments. conda create -n my-project ipython package1 package2 package2

Python Libraries for System Administration

Handy Packages

os package. You can use it to list directories, check if files exist, check if symlinks exist, make directories, run system commands, get and set environmental variables, and more. It’s great!
subprocess module
shutil
pprint
pytest

Running code

Basic notions

Run script
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python name-of-script.py
Help
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help(os)

Import packages

import os
import subprocess
import shutil
from pprint import pprint

Common file and directory operations

Working directory

# Get your current working directly
# This returns a string
my_cwd = os.getcwd()
print(my_cwd)

List of directory contents

# List the contents of a directory
# This returns a list
dir_list = os.listdir()
for item in dir_list:
  print(item)

Absolute path name of file

1 2	# Get the Absolute Path name of a file (file + current working dir) os.path.abspath('some-file')

File basename

1 2	#Get the basename - returns file os.path.basename('/path/to/file')

Split directory path

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# Split a directory path - platform independent
os.path.split(os.getcwd())
# Out[17]: ('/Users', 'thali')

Check path existence

# Check if a path exists
os.path.exists('/path/on/filesystem')
# Check if a path is a symlink
os.path.islink()

Move files and directories

Copy directory

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# Copy a directory
# cp -rf
shutil.copytree('src', 'dest')

Copy file

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# Copy a file
# cp -rf
shutil.copyfile('file1', 'file2')

Move directory

  # Move a directory
  # mv
  shutil.move('src', 'dest')
  ````

>❗ Not everything is going to be available through python libraries, such as installing system libraries
    ```python
    # Run an arbitrary system command
    command = "echo 'hello'"
    result = subprocess.run(command.split(' '), stdout=subprocess.PIPE, stderr=subprocess.PIPE)
    #Print the stdout and stderr
    print(result.stdout)
    print(result.stderr)

Write to files

Create a new file

# Write to a file (and create it if it doesn't exist)
# echo "hello" > hello.txt
f= open("hello.txt","w+")
f.write("hello!")
f.close()

Append to file

# Append to a file
# echo "hello" >> hello.txt
f = open("hello.txt", "a+")
f.write("hello again!")
f.close()

Tests

Tests mostly work by using a function called assert, which is essentially saying “make sure this is true and if not die loudly”.
Put this function in a file called test_my_code.py and run as pytest test_my_code.py.

def test_system_command():
    """Test the exit code of a system command"""
    command = "echo 'hello'"
    result = subprocess.run(command.split(' '), stdout=subprocess.PIPE)
    assert result.returncode == 0