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Introduction to Linux

Here we are going to teach you how to interact with a Linux machine. There are two ways that a computer can be controlled; the first is through the graphical user interface (GUI), which is most likely how you navigated to this page, and the second is via the command line. Which instead of clicking and dragging, involves issuing written commands to an interactive terminal that are then executed and the output is displayed to you in text.

On macOS and Linux, open the Terminal App, on Windows open CMD. Opening a terminal should give you a prompt like this 

[Bennu:~/Documents] Osiris%
Where,
- Bennu = hostname
- ~/Documents = current location
- Osiris = username

Files and directories

Everything in Linux is either a file or directory (folder). A file is basically a chunk of data stored on a hard drive with a name to place it in the file system. Directories only contain data on the files and directories that are in them. To give you an idea of this, a basic outline of the Linux file system is given below: 

/
├── bin
├── boot
├── dev
├── etc
├── home
    ├── Osiris
        ├── file.py
        ├── Documents
            ├── cool-stuff.py
        ├── Downloads
        ├── Music

First we need to figure out where we are, to do so, use the pwd (present working directory) command. 

% pwd
/home/Osiris
This is the home directory, this directory has a special key, the ~ and it can be used as a substitute for the above.

To see what is in this directory use the ls command. 

% ls
Documents Downloads Music
To get more information we can add a flag to this command, add the -l (long) flag to see more flags. Use man ls to read the manual. 
% ls -l
total 0
drwx------@  27 Osiris  staff   864 11 May 12:53 Desktop
drwx------@  37 Osiris  staff  1184 12 May 16:55 Documents
drwx------+  12 Osiris  staff   384  6 Oct  2021 Music
We can also look inside a directory by adding an argument to ls. 
% ls -l Documents
total 0
-rw-r--r-- 1 Osiris staff   32 Aug 27 11:34  cool-stuff.py
You can also call files relative to home using the ~ key, e.g. ls ~/Downloads.

Three more special keys are the ., .., and -. The first two allow you to move relative to the current directory so instead of writing ~/current/working/directory/file, you can simply write ./file. Similarly, if the file was in the directory above, i.e. ~/current/working/file2, we can write ../file2. These are useful when you want to execute scripts. Finally, the - key allows us to navigate back to our previous directory. 

% pwd
/home/Osiris/Documents

% cd ../Music

% pwd
/home/Osiris/Music

% cd -

% pwd
/home/Osiris/Documents

Play around with these commands to navigate through your computer. Once you feel comfortable getting around this way, proceed to the next section.

Creating a new directory

This action is the same as the "New Folder" action when using a file browser. 

% mkdir new

% ls
cool-stuff.py new/

Learning more about commands

Linux has two ways that we can learn more about a command within the terminal. The first is using Manual pages, man, these contain all of the information that you will need to effectively use any command. 

% man vim

VIM(1)                            General Commands Manual                           VIM(1)

NAME
       vim - Vi IMproved, a programmer's text editor

SYNOPSIS
       vim [options] [file ..]
       vim [options] -
       vim [options] -t tag
       vim [options] -q [errorfile]

       ex
       view
       gvim gview evim eview
       rvim rview rgvim rgview

DESCRIPTION
       Vim is a text editor that is upwards compatible to Vi.  It can be used to edit all
       kinds of plain text.  It is especially useful for editing programs.

       There are a lot of enhancements above Vi: multi level undo, multi windows and
       buffers, syntax highlighting, command line editing, filename completion, on-line
       help, visual selection, etc..  See ":help vi_diff.txt" for a summary of the
       differences between Vim and Vi.

       While running Vim a lot of help can be obtained from the on-line help system, with
       the ":help" command.  See the ON-LINE HELP section below.
       ...
The other method is to use the --help flag after the command. 
% vim --help

VIM - Vi IMproved 8.2 (2019 Dec 12, compiled Feb 12 2022 04:26:50)

Usage: vim [arguments] [file ..]       edit specified file(s)
   or: vim [arguments] -               read text from stdin
   or: vim [arguments] -t tag          edit file where tag is defined
   or: vim [arguments] -q [errorfile]  edit file with first error

Arguments:
   --           Only file names after this
   -v           Vi mode (like "vi")
   -e           Ex mode (like "ex")
   -E           Improved Ex mode
   -s           Silent (batch) mode (only for "ex")
   -d           Diff mode (like "vimdiff")
   -y           Easy mode (like "evim", modeless)
   -R           Read only mode (like "view")
...

Viewing files

If you want to view the contents of a file there are two commands that you can use to do so. The first is better suited to small files. 

% cat cool-stuff.py

print("Maths is cool!")
The second method calls up an interactive pager, that displays over the entirety of your terminal window. It will also allow you to scroll through and search for text. Use the j and k keys to scroll, and when you are ready to quit press q. 
% less cool-stuff.py

Again, please take the time to explore your computer and view the contents of files on your system before proceeding.

Deleting, copying, and renaming and moving

The command to copy files is cp, to copy a file run the copy command followed by the name of the file that you want to copy and the name that you want the copy to have. This command is handy when you'd like to try something new with your code, but you're worried about breaking it. 

% cp cool-stuff.py cool-stuff.py.bak

In Linux, moving something and renaming it are the same thing. To do this use the mv command, this command takes two arguments: the path of the file to move/rename and the new path/new name. 

% mv cool-stuff.py really-cool-stuff.py

To delete a file we use the rm command.

Danger

The rm command is not like deleting files using a GUI! This command does not move files to the Bin, instead it permanently deletes them, there is (almost) no way to recover them.

% ls
cool-stuff.py cool-stuff.py.bak

% rm cool-stuff.py.bak

% ls
cool-stuff.py

Building code

The code that we write in programming languages is designed to be easy to read for humans. However, computers are unable to understand this, they only understand what is written in binary. So we need to convert our code to binary, to do this we use a compiler. An example of a C compiler is gcc, to compile code using this we write: 

% cat fun.c
#include <stdio.h>

int main() {
        printf("Wow, coding is fun!\n");
        return 0;
}

% gcc fun.c

% ls
a.out fun.c

Copy the code above and try to compile the code yourself.

Permissions and running code

To run code, we simply call it in the command line. 

% ./a.out
Wow, coding is fun!
Now, this may not have ran for you. The reason for this is permissions. Remember early when we ran ls -l, that there was at the beginning of a line something like this: -rw-------? This is telling us several things. This first character will be either a - for a file, or a d for directories - so the above string relates to a file. Following the first character are the remaining characters which have three characters for three groups of people: 1) the owner of the file, 2) users in the group of the file, and 3) everyone else. The three groups of characters for each group of people tells us if they can read (r) the file, write (w) to the file, and execute (x) the file. So in the case above, only the owner of the file can read and write to its contents.

In order to run code, we need to be allowed to execute it. To change our permissions we can use the chmod command. To let anybody execute the file a.out we can run: 

% chmod a+x a.out
% ls -l a.out
-rwx--x--x 1 Osiris  staff  197  2 Mar  2018 a.out

Extra information

If you'd like to learn more about the options for the chmod command, I encourage you to read its manual using man.

As a sidenote, the command to change the ownership of a file is chown and again if you'd like to know more about it, use its man page.

Redirection of output

Now that you know how to run a few basic commands and programs, I think it is a good time to teach you how we can redirect their output. The default behaviour is to display their output to stdout so that we can see it. However, we are also able to redirect the output to a file. To do this we use the > operator followed by the file we want to store the output in. Note, the echo command just outputs what you put into it. 

% echo "Wow! This guide has been super helpful for me :)" > tootrue.txt

% cat tootrue.txt
Wow! This guide has been super helpful for me :)
Another operator is >>, this operator will append the output to a file if it already exists, otherwise it is the same as >. 
% echo "I agree!" >> tootrue.txt

% cat tootrue.txt
Wow! This guide has been super helpful for me :)
I agree!

The last kind of redirect, redirects the output between programs. The operator for this is a pipe, |. 

% ls -l /
total 108
drwxr-xr-x   2 root root  4096 Sep 11 06:37 bin
drwxr-xr-x   4 root root  4096 Sep 19 06:59 boot
drwxr-xr-x  19 root root  4000 Sep  4 15:07 dev
drwxr-xr-x 186 root root 12288 Sep 19 06:56 etc
drwxr-xr-x  43 root root  4096 Sep  4 15:33 home
lrwxrwxrwx   1 root root    33 Sep 19 06:58 initrd.img -> boot/initrd.img-4.15.0-64-generic
lrwxrwxrwx   1 root root    33 Sep 19 06:58 initrd.img.old -> boot/initrd.img-4.15.0-62-generic
drwxr-xr-x  25 root root  4096 Sep  4 16:09 lib
drwxr-xr-x   2 root root  4096 Sep  4 16:09 lib32
drwxr-xr-x   2 root root  4096 Jul 25  2018 lib64
drwx------   2 root root 16384 Sep  4 13:21 lost+found
drwxr-xr-x   2 root root  4096 Jul 25  2018 media
drwxr-xr-x   2 root root  4096 Jul 25  2018 mnt
drwxr-xr-x   6 root root  4096 Sep 18 16:47 opt
dr-xr-xr-x 259 root root     0 Sep  4 14:13 proc
drwx------   5 root root  4096 Sep 10 15:55 root
drwxr-xr-x  41 root root  2420 Sep 19 15:14 run
drwxr-xr-x   2 root root 12288 Sep 19 06:56 sbin
drwxr-xr-x   4 root root  4096 Sep  4 13:28 snap
drwxr-xr-x   2 root root  4096 Jul 25  2018 srv
dr-xr-xr-x  13 root root     0 Sep  4 20:10 sys
drwxrwxrwt  80 root root 12288 Sep 19 15:09 tmp
drwxr-xr-x  12 root root  4096 Sep  4 16:09 usr
drwxr-xr-x  19 root root  4096 Sep  4 15:05 var
lrwxrwxrwx   1 root root    30 Sep 19 06:58 vmlinuz -> boot/vmlinuz-4.15.0-64-generic
lrwxrwxrwx   1 root root    30 Sep 19 06:58 vmlinuz.old -> boot/vmlinuz-4.15.0-62-generic

% ls -l / | grep lib
drwxr-xr-x  25 root root  4096 Sep  4 16:09 lib
drwxr-xr-x   2 root root  4096 Sep  4 16:09 lib32
drwxr-xr-x   2 root root  4096 Jul 25  2018 lib64
So here, we redirected the output of the ls command into the grep command. Where grep restricts the output only to lines that contain a pattern, in this case "lib".

Remote access with SSH

The command ssh is used to access other machines connected across a network, including the internet. To call ssh we write: 

ssh [username]@[host]
Where username is the username of the account on the machine (host) that you'd like to access. This will result in a prompt asking you to enter a password in order to allow the connection.

Depending on the frequency that you'll be accessing the machine, it could quickly become tiresome to keep entering the password. Fortunately, there is a way to avoid this, and that is SSH Keys. This allows you to authenticate by using an ssh key pair, which are a pair of text files with special data. On your own computer, you can generate them using the following. 

% ssh-keygen
Generating public/private rsa key pair.
Enter file in which to save the key (/home/Osiris/.ssh/id_rsa): 
Created directory '/home/Osiris/.ssh'.
Enter passphrase (empty for no passphrase): 
Enter same passphrase again: 
Your identification has been saved in /home/Osiris/.ssh/id_rsa.
Your public key has been saved in /home/Osiris/.ssh/id_rsa.pub.
The key fingerprint is:
SHA256:zJ2SQwYgAS4x9kKXthtPetW+2De1GttGHfUACfYVTIU Osiris@Bennu
The key's randomart image is:
+---[RSA 2048]----+
|++ooo..   o..=++.|
|+oo+   . . ...E .|
|.o...   +   .  .o|
|. .o . * + .   ..|
|    * . S o   . .|
|   o o   +   o . |
|    .   o ..o .  |
|       . o o+o   |
|          .o+.   |
+----[SHA256]-----+
This creates the files ~/.ssh/id_rsa, which is your private key and should never be moved, and ~/.ssh/id_rsa.pub, which is your public key and can be copied to any machine you like. You can put your public key on another computer using this command. 
% ssh-copy-id [username]@[host]
This will place your public key onto the host computer, so that next time when you try to connect ssh will see your public key on the computer and check for the private key on your computer. Which means that you'll no longer need to enter your password when connecting.

Other useful commands

Command Use
rsync Copy files and directories between machines using ssh
ps List processes running
kill Stop process running
head Display top x lines of file, 10 by default
tail Display bottom x lines of file
where List the path to command's location
whichis If multiple versions, list all their paths
apropos Searches man pages by keyword, useful to find new commands

Next step

Once you feel comfortable with the contents of this page, you can continue on to the next page: GitHub - Part 1. If there is anything that you don't feel confident using please return to that section and practice it some more. Remember, if you ever need more information on how a command works, you can read its manual using man {CMD}.