Shells and Consoles: Windows into your Computer¶

We’ll do a lot of work in a console which is a view of your computer you may not yet be familiar with.

A console lets you issue commands that manipulate your computer directly. The consoles we will use are normally applications that you can run from your normal programs menu.

Look for a program with a name like:

Konsole (KDE Linux)
Gnome-Terminal (Gnome Linux)
X-Term (Generic Linux, such as a Raspberry Pi)
Terminal (OS-X)
Windows Subsystem for Linux (WSL Ubuntu on Windows 10)

which, when run will connect you to a program called a shell which is responsible for:

listening to the commands you type
trying to run those commands
letting you see the results of the command

Your shell prompt should look something like this:

example@397026a860f1:~$

Your console may have a blinking square, or a solid white square just after the $ character. You may have a more involved shell prompt that tells you more information, but generally it will include:

your linux username example here
the machine you are running on @397026a860f1 here
your current directory ~ here (which means your home directory)
the $ character, which traditionally is used as the prompt on linux

As you work on the command line, you’ll type commands and hit the Enter key to tell the shell to execute the commands. The shell may print out an error if it doesn’t know the command, or it may spend a long time running the command, or it may just immediately spit out the result of the command.

Exploring the Computer¶

Most computer operating systems use the idea of directories (sometimes called folders) and files. These together are the filesystem.

Some operating systems, such as Linux or OS-X have a single tree of directories and files that starts at the / directory (called the root directory). Others, such as Windows, have a separate tree for every piece of storage (disks, usb keys) plugged into the machine.

Looking Around¶

Your shell (running in your console) keeps track of where you are in the filesystem. We saw above that many shells will display this as part of their prompt.

You can ask the computer where am I:

example@397026a860f1:~$ pwd
/home/example

Which says, if you start at the root directory which is named / go to the sub-directory named home and then to the sub-directory named example which is where your console (shell) normally starts when you open a console.

So why didn’t pwd say ~ when we asked? ~ is a shorthand meaning my home directory, which is something that your administrator can set to a different location.

Let’s ask what is here:

example@397026a860f1:~$ ls -l
total 0

Which tells us there are currently no files or directories in this home directory.

The -l part was what is called a flag that changes how the ls command works. If we had called the ls command without that flag, we would have seen:

example@397026a860f1:~$ ls
example@397026a860f1:~$

The flag here means long listing which includes a count of the number of things the directory listing includes:

example@397026a860f1:~$ ls -l ..
total 4
drwxr-xr-x 2 example example 4096 Apr 30 21:19 example

Here we added what’s called an argument, a thing on which to act. The special name .. means the parent directory of the current directory. So our command here means “give me a long listing of the parent of this directory”.

Moving Around¶

We can change where we are in the shell with the command cd (change directory):

If we want to go up a directory:

example@397026a860f1:~$ pwd
/home/example
example@397026a860f1:~$ cd ..
example@397026a860f1:/home$ pwd
/home
example@397026a860f1:/home$ ls
example
example@397026a860f1:/home$ cd ..
example@397026a860f1:/$ pwd
/
example@397026a860f1:/$ ls
bin  boot  dev  etc  home  lib  lib32  lib64  libx32  media  mnt  opt  proc  root  run  sbin  srv  sys  tmp  usr  var
example@397026a860f1:/$ cd ~
example@397026a860f1:~$

Here we’ve walked up from /home/example all the way to the root directory / of the machine. We can see that this directory seems to have a lot of directories inside of it, one of which is the home directory. We then jump right back to our home directory by doing cd ~.

Moving Things Around¶

You can create your own directories and files in your home directory.

Let’s create a folder:

example@397026a860f1:~$ mkdir school-work
example@397026a860f1:~$ ls
school-work
example@397026a860f1:~$ cd school-work/
example@397026a860f1:~/school-work$

Let’s create a file to move around:

example@397026a860f1:~/school-work$ touch a-special-file.txt example@397026a860f1:~/school-work$ ls a-special-file.txt

Oh, but the file is in the wrong place, we want it in a folder just for this project:

example@397026a860f1:~/school-work$ mkdir this-project example@397026a860f1:~/school-work$ mv a-special-file.txt this-project/ example@397026a860f1:~/school-work$ ls this-project example@397026a860f1:~/school-work$ ls this-project/ a-special-file.txt

After this, we’ve created a structure that looks like this:

/
`-- home
    `-- example
        `-- school-work
            `-- this-project
                `-- a-special-file.txt

Some Hints¶

If you start a program and it doesn’t come back, hitting <ctrl-c> (that is, holding down the ctrl key while you tap the c key) will generally stop a program
If you want to know how to use a program, the program man will generally give you information about how to use a common program
Most programs also can provide their own help if you pass –help to them
The program echo will echo what you type
You can put the output of a command into a file with:
```
$ echo "A cat is cool" > somefile.txt
```
There are thousands of commands available in Linux, either already installed, or installable by pulling in new software
A command is a piece of software, it is a reusable piece of code that can be called from the shell
A flag is an optional argument you may add to a command to change how it behaves, normally it has a - character for short flags or a – for longer flag names.

Now that we can get around in the shell let’s move onto starting python