Python 3

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Introduction to Python

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The Command Prompt: Moving Around and Looking

the command prompt

The Command Prompt includes powerful commands for working with files and programs.

the command prompt (or Terminal prompt, or command line) is a text-based interface to the operating system
the command prompt was the original interface to your computer, before Windows File Explorer or the Mac Finder GUIs we use today
the command prompt is still used extensively by programmers and IT professionals to run programs and read and write files

opening the command prompt: Windows

In your Windows search, look for one of the following, and open it:

Anaconda Prompt (preferred, if you have it installed)
PowerShell
Command Prompt

You should see something similar to the following:

C:\Users\david>                       < -- command line

After opening this window, note the blinking cursor: this is your computer's operating system, awaiting your next command. (Please note that there may be small differences between your output and this illustration; these can usually be ignored.)

opening the command prompt: Mac or Linux*

in your Spotlight search, look for Terminal, and open it:

You should see something similar to the following:

Last login: Thu Sep  3 13:46:14 on ttys001

Davids-MBP-3:~ david$                 < -- command line

After opening the command prompt program on your computer, note the blinking cursor: this is your computer's operating system awaiting your next command. (Please note that there may be small differences between your output and this illustration; these can usually be ignored.)

Also note that the Apple Macintosh runs "on top of" and is actually powered by the Unix operating system; when we open a Terminal window, we are talking directly to Unix. Linux is another version of Unix, so the commands we explore are the same on either operating system.

the present working directory (pwd)

Your command line session is located at one particular directory on the file tree at any given time.

On Windows, the pwd is automatically displayed at the prompt:

C:\Users\david>

On Mac/Linux, type pwd and hit [Enter]:

Davids-MBP-3:~ david$ pwd
/Users/david

we can move around the filesystem, and will see the present working directory change
your pwd is the reference point for any files we might want to open or other directories to where we might want to travel

listing files in a directory: Windows

dir is the command to list the contents of a directory.

Type dir and hit [Enter]:

C:\Users\david> dir

 Volume Serial Number is 0246-9FF7

 Directory of C:\Users\david

08/29/2020  11:37 AM    <DIR>          .
08/29/2020  11:37 AM    <DIR>          ..
08/29/2020  11:28 AM    <DIR>          Contacts
08/29/2020  12:50 PM    <DIR>          Desktop
... etc ...

The contents of the directory include all files and folders that can be found in it.

listing files in a directory: Mac

ls is the command to list the contents of a directory.

Type ls and hit [Enter]:

Davids-MBP-3:~ david$ ls

Applications          Downloads         Movies
Desktop               Dropbox           Music
Documents             Library           Public
... etc ...

The contents of the directory include all files and folders that can be found in it.

visualizing the directory tree

Starting from the root, each folder may have files and other folders within.

C:\Users
├── david           <--- my pwd when I open my Terminal
│   ├── Desktop
│   │   └── python_data
│   │        ├── 00
│   │        ├── 01
│   │        │    ├─ 1.1.py
│   │        │    ├─ 1.2.py
│   │        │    ├─ 1.3.py
│   │        │    etc.
│   │        ├── 02
│   │        │    ├─ 2.1.py
│   │        │    ├─ 2.2.py
│   │        │    ├─ 2.3.py
             etc.

Most of us are familiar with the file tree, since we see it in one form or another when we browse files on our computers.
When we are moving around the file tree, it is best to imagine where we are in the file system - in which directory, and at what level.
(The above structure shows that I have placed my python_data/ folder in Desktop/ -- you may have placed it elsewhere.)

moving around the directory tree with 'cd'

cd stands for 'change directory'. This command works for both Windows and Mac.

on Mac/Linux:

Davids-MBP-3:~ david$ pwd
/Users/david

Davids-MBP-3:~ david$ cd Desktop

Davids-MBP-3:~ david$ pwd
/Users/david/Desktop

on Windows:

C:\Users\david> cd Desktop
C:\Users\david\Desktop>

moving to the child directory

To visit a directory "below" where we are, we simply name the child dir.

We move "down" the directory tree by using the name of the next directory -- this extends the path:

C:\Users\david> cd Desktop

C:\Users\david\Desktop> cd python_data

C:\Users\david\Desktop\python_data> cd 02

C:\Users\david\Desktop\python_data\02>

We can also travel multiple levels by specifying a longer path:

C:\Users\david> cd Desktop\python_data\02

C:\Users\david\Desktop\python_data\02>

(Please that on Windows we use the backslash separator (\); on Mac it is the forward slash(/).)

moving to the parent directory

The '..' (double dot) indicates the parent directory and can move us one directory "up".

If we'd like to travel up the directory tree, we use the special .. directory value, which signifies the parent directory:

C:\Users\david\Desktop\python_data\02> cd ..

C:\Users\david\Desktop\python_data> cd ..

C:\Users\david\Desktop> cd ..

C:\Users\david>

We can also travel multiple levels with multiple ../'s:

C:\Users\david\Desktop\python_data\02> cd ..\..\..

C:\Users\david>

(Please that on Windows we use the backslash separator (\); on Mac it is the forward slash(/).)

using ls or dir with cd

These two commands together allow us explore our filesystem.

Here is an example journey through some folders, viewing the contents of each folder as we move (this shows Mac/Linux output, but in Windows you may replace ls with dir):

Davids-MBP-3:Desktop david$ pwd
/Users/david/Desktop

Davids-MBP-3:Desktop david$ ls
python_data

Davids-MBP-3:Desktop david$ cd python_data

Davids-MBP-3:python_data david$ pwd
/Users/david/Desktop/python_data

Davids-MBP-3:python_data david$ ls
00
01
02
... etc.

Davids-MBP-3:python_data david$ cd 02

Davids-MBP-3:02 david$ ls
2.1.py       2.2.py       2.3.py       2.4.py       2.5.py
2.6.py       2.7.py       2.8.py       2.9.py       2.10.py

Davids-MBP-3:02 david$ pwd
/Users/david/Desktop/python_data/02

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