System and Python Basis - Exercises

1. Exercise: Search for a User's Groups

Create a script returning the names (print) of all groups associated with the user.

Always process in an incremental way:

  • Decompose the module to develop
  • Implement it step by step
  • Test the solution at each step

Initialize your project:

  • Make a new directory : python-data-science for instance.
  • Edit a new Python file: search-group.py with gedit for instance.
  • Test a hello-world script python3 search-group.py

Load ressources

  • Open a file and read-it line by line
  • Apply it to /etc/passwd then /etc/group

I do not know how to open a file ? search the web... w3schools

Search for a specific word (user name) in a lines

  • Detect for instald user in lines
  • Print only lines with instald inside

There is a lot of string methods in Python (for instance, still in W3School) and one of them seems very interesting: find().

Another one split() allows for decomposing a long string regarding a separator character.

Reduce to user's Groups

  • Select only the primary group name in /etc/passwd file for the user bob.
  • Select each group name in /etc/groups, each time bob is present.

2. Exercise: Script as a shell command

The goal is to transform search-group.py code into a shell command, executable like anyother.

Get user name as a command argument

For that, use the sys.argv from the Python package sys. This variable provides the list of the command line arguments.

In the Shell

We aim to call our program by masking the need for the Python3 interpreter. Like that: 

search-user bob
  • Version 1 : By using an alias (typically alias search-user='python3 search-user.py')
  • Version 2 : By using a shebang, to identify the interpreter inside the Python script, and by setting the script executable chmod +x.

Anywhere

The script is only accessible from the current directory. Resolve this problem

  • Version 1 : Modify the alias definition with a global path to the script.
  • Version 2 : Add the current directory to the PATH variable (export PATH=$PATH:~/path/to/)

Anytime

The alais or exports are not available in a new shell. To make them automatically defined, you may add the appropriate lines to your local .bashrc file. This file allows the users to tune their bash shell at start time.

Exercise: Increased Command

  • split() is restrictive much for extracting some element from a string ? Take a look at Regular Expression in Python w3school
  • Handle bugs (no user name in arguments, ...)

  • Add options to your command: --help or -h for help, -i for printing group ids, -t to test if the user is in a specific group, ...

  • Make a new directory : python-data-science for instance.

  • Edit a new Python file: search-group.py with gedit for instance.
  • Test a hello-world script python3 search-group.py