Using the Command Line Interface / Shell on PythonAnywhere

A lot of the assignments in this course require you to use the Bash Shell (Linux command line interface) on PythonAnywhere. In this example:

17:33 ~ $ workon django42
(django42) 17:33 ~ $ python --version
Python 3.9.5
(django42) 17:36 ~ $ python -m django --version
4.0.7

The Linux shell commands in th above example are:

• workon django42
• python --version
• python -m django --version

Each time you start a new shell, you need to type workon django42. If you leave and come back to a shell that is still running, if you see the '(django42)' in your prompt - you do not have to re-run the workon command. It just needs to be done once per shell.

Some Linux Shell Commands

Here are a few Linux commands that we use a lot:

• ls - List the files and directories in the current working directory.

• pwd - Print the current working directory. Shows the current folder name and all its parent folders.

• ls -l - Prints a list of files and folders in the current directory with extra detail.

• cd - Change directory. Which you just type cd with no parameter it takes to your home directory - the directory that you are placed in when you log in.

• cd ~/django_projects - Go into the folder named django_projects directly below your home folder. The ~ on shell commands is a short cut for "the path to my home folder".

• rm sqlite3.db - Remove a file - in this case the file sqlite3.db in the current working directory.

• grep -r autos_create * - Search files in the current folder and below for files that contain the string autos_create and show the file where the search string is found and the line of the file that contains the string.

• clear - Clear the screen

• up-arrow - You can scroll back and forth through previous commands using the up and down arrows on your keyboard. You can press return to re-execute a previous command once you scroll to it.

Other Interfaces

You are not always typing commands to the to the Linux Shell in the command line. Usually the Linux shell prompt ends in dollar sign $. You need to look at the prompt and recognize which program that is asking you for commands.

Here are some other command line environments, what their prompts look like, and (most importantly) how to exit them. You will learn to use them as the course progresses. Make sure to bookmark this page and come back when you get stuck.

Interactive Python

In this example, you run Python interactively so you can type python code to experiment with code fragments:

(django42) 03:37 ~ $ python
Python 3.9.5 (default, May 27 2021, 19:45:35) 
[GCC 9.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> print('Hello world')
Hello world
>>> for i in range(2) : 
...     print(i)
... 
0
1
>>> quit()
(django42) 03:38 ~ $ 

You start in a shell prompt $ and run Python and get the "three chevron" prompt >>> and you can type Python commands until you type quit().

Sometimes you enter a statement like for that required multiple indented python statements until you de-indent and enter a blank line. When python is prompting for more statements it changes the prompt from three chevrons to ....

SQLite Interface

Sometimes you run the SQLite command line interface. Once you start the interface from the shell prompt, your prompt becomes sqlite> until you exit with .quit.

At this prompt you type SQL (database) statements or SQLite commands.

(django42) 03:45 ~/django_projects/mysite (master)$ sqlite3 db.sqlite3 
SQLite version 3.31.1 2020-01-27 19:55:54
Enter ".help" for usage hints.
sqlite> .tables
auth_user_groups            polls2_choice             
auth_user_user_permissions  polls2_question           
sqlite> select * from polls2_choice;
1|42|1213|1
2|Something else|90|1
sqlite> select * 
   ...> from polls2_choice
   ...> ;
1|42|1213|1
2|Something else|90|1
sqlite> .quit
(django42) 03:46 ~/django_projects/mysite (master)$

In the above example .tables is a SQLite command and select * from polls2_choice; is an SQL command. SQL commands terminate with a semicolon and can span multiple lines. So if you type part of a SQL statement without a semicolon, SQLite changes the prompt to ...> to allow you to enter more SQL until you enter a ; at which time the prompt changes back to sqlite>.

You exit the SQLite interface and get back to the Linux Shell with .quit.

The Django Shell

Later in the course, you will be using the Django Shell to interact directly with your database through your Django models. You use python manager.py shell in your project folder to start the Django Shell.

The Django Shell is just a Python Shell but all of your application code has been loaded. So you can execute Django statements as well as python statements.

django42) 03:54 ~/django_projects/mysite (master)$ python manage.py shell                                                                   
Python 3.9.5 (default, May 27 2021, 19:45:35) 
[GCC 9.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
(InteractiveConsole)
>>> print('Hello world')
Hello world
>>> from polls.models import Question
>>> q = Question(question_text='Hello world')
>>> quit()
(django42) 03:55 ~/django_projects/mysite (master)$ 

In the above you are importing the Question model from the polls/models.py file and then you are creating a Question object with some initial text and assigning it to the variable q.

You exit the Django Shell using the same quit() command as you use to exit the Python Shell. Remember that the Django Shell is just a Python Shell with your Django project pre-loaded. That is why you need to run the command in the folder where the manage.py file is housed.