Ver el código de «Basic Administration»

In this practice we will learn how to use the command interpreter (also known as ''[https://www.traficantes.net/sites/default/files/pdfs/En%20el%20principio%20fue...-TdS.pdf command line]'') and to learn basic notions of Linux system administration.

= Step 0: Brief introduction to the Linux system =

== Basic structure of the file system ==

In a Linux system, all folders and files in the file system start at the root folder which is represented by the /.

 /
 ├── bin
 ├── usr
 │ ├─── local
 │ ├─── bin
 │ └── ..
 ├── dev
 │ ├─── sda
 │ ├─── sda1
 │ └── ..
 ├─── home
 │ ├─── practice
 │ │ ├─── topic1.pdf
 │ │ ├─── bulletin1.pdf
 │ │ └── ..
 │ ├─── professor
 │ └── ..
 └─── etc
     ├─── firefox
     ├─── libvirt
     ├── ..
     └── ..

As you can see, the file system follows a tree structure.

== Basic notions ==

General command format:

 command [-options] [arguments]

In Linux, use of lowercase and uppercase in file names is meaningful.

== Folders . and .. ==

Every folder on a Linux system has two pseudofolders, the '''.''' and the '''..''':

* The pseudofolder '''.''' refers to the parent folder that contains this folder.
* The pseudofolder '''..''' refers to the current folder, it is therefore a self-reference.

In the case of the root folder, the pseudofolder '''.''' and '''..''' refer to the root folder itself, it is therefore an exception.

== Current working directory and the command cd ==

Each command interpreter has a ''current working directory'' ('''c'''urrent '''w'''orking '''d'''irectory). The current working folder can be modified with the command '''cd''' ('''c'''hange '''d'''irectory, in English).

For example:

<syntaxhighlight lang="bash">
$ cd ..
</syntaxhighlight>

It would place us in the parent folder, using a relative path.

If we retype from /home " cd .. " takes us to '''/$''' 

To place us in the root, we can use an absolute path:

<syntaxhighlight lang="bash">
$ cd /
</syntaxhighlight>

In case we want to get back to the user folder, it would be enough to invoke '''cd''' without further or '''cd ~ '''

<syntaxhighlight lang="bash">
$ cd
</syntaxhighlight>

'''VERY IMPORTANT''': '''cd..''' is not the same as '''cd ..''', the space after the cd is required.

== Absolute paths, relative paths ==

Absolute or relative paths are used to refer to a file or folder.

Absolute paths always start with /, so they take the root folder (/) as a reference point. For example, an absolute path to the '''test''' folder that is stored in the user's '''ubuntu''' folder is '/home/ubuntu/test/''.

The relative path takes the current working folder as reference. To find out the current working folder we have the command '''pwd'''.

<syntaxhighlight lang="bash">
$ pwd
/home/ubuntu
</syntaxhighlight>

Based on the current working folder, using the pseudofolder '''.''', relative paths can be constructed. For example, to reference a file '''file.txt''' in the temporary folder, we can use the relative path ''../../tmp/file.txt'''.

Although it is probably more convenient to use an absolute path in this case, which would be '''/tmp/file.txt'''.

== Command interpreter ==

The shell is a textual application launcher that uses the keyboard as an input device. Using the keyboard, you type the name of the program you want to launch and when you press the ''enter'' key, the shell executes the program. By default, the command interpreter used by Ubuntu is '''bash'''.

== man: see the man page of a command ==

The '''man''' command will be useful for viewing man pages.

Its basic structure is:

<syntaxhighlight lang="bash">
man <command>
</syntaxhighlight>

Although it is also used with the option ''-a'' of All, which also displays entries beginning with ''.'':

<syntaxhighlight lang="bash">
man -a <command>
</syntaxhighlight>

This program takes as input the name of the command you want to query its man page, for example:

<syntaxhighlight lang="bash">
man ls
</syntaxhighlight>

It gives us man page information for the ''ls'' command.

To exit the manual page, press the ''q'' key (the first letter of the word ''quit'', in English).

quit = exit

Usually, programs offer help options, such as ''--help''.

<syntaxhighlight lang="bash">
man --help
</syntaxhighlight>

The compact version can also be used when specifying options:

<syntaxhighlight lang="bash">
man -h
</syntaxhighlight>

In general, all commands usually offer a helper option.

= Step 1: Basic operations with the file system =

== ls ==

Lists the files and folders. If nothing is specified, displays the files and folders contained in the current working folder.

<syntaxhighlight lang="bash">
ls
</syntaxhighlight>

If a path is specified, displays the folders and files contained in that path, for example:

<syntaxhighlight lang="bash">
$ ls /
bin boot data dev etc home initrd.img initrd.img.old lib lib64 lost+found media mnt opt proc root run sbin snap srv sys tmp usr var vmlinuz vmlinuz.old
</syntaxhighlight>

When using ''/'' as the absolute path, it displays the contents of the root folder.

The most commonly used options of this program are usually:

* -a: shows '''hidden''' files and folders. In Linux, any file or folder whose name starts with . is considered ''hidden''. This also includes the pseudofolders . and . in the list.

* -l: displays in list and provides data such as last modified date, owner, group, size in bytes and name:

<syntaxhighlight lang="bash">
$ ls -a

. ..  .bash_history .bash_logout .bashrc .bashrc .profile .ssh
</syntaxhighlight>

Both options can be combined:

<syntaxhighlight lang="bash">
$ ls -la /

total 92
drwxr-xr-x 24 root root root 4096 Aug 28 09:50 .
drwxr-xr-x 24 root root root 4096 Aug 28 09:50 ...
drwxr-xr-xr-x 2 root root root 4096 Aug 22 12:37 bin
drwxr-xr-x 4 root root root 4096 Aug 22 12:37 boot
drwx------ 3 root root 4096 Aug 28 09:50 data
drwxr-xr-x 18 root root root 3780 Aug 28 09:46 dev
[...]

</syntaxhighlight>

Other useful options are:

* -s : shows the size in blocks of each file.

* -t : brings the day and time of modification

* -R : lists also the subfolders 

* --color : shows the content colored


The ''permissions'' are used on the basis of three capabilities:

* Read, which is coded with ''r'' (read).
* Writing, which is coded with ''w'' (write). (write).
* Execution, which is coded with '''x''' (execute). (execute).

The first column shows the permissions that encode:

* Type of entry: ''d'' indicates '''directory''' (''folder''').
* Permissions of the owner, first third of rwx
* Group permissions, second string of rwx
* Permissions for the rest of the world, third string of rwx.

Next, the owner and group are displayed, followed by the last modification date and the name of the file or folder.

The help in ls only works with ''' ls --help ''' not with ls -h

== mkdir ==

Creates a new directory.

<syntaxhighlight lang="bash">
$ mkdir test
</syntaxhighlight>

Creates a test folder in the current directory.

Let's check that it has been created properly:

<syntaxhighlight lang="bash">
$ ls
test
</syntaxhighlight>

With the ''-p'' option, it allows to create successive nested folders at once, for example:

<syntaxhighlight lang="bash">
$ mkdir -p a/b/c
</syntaxhighlight>

Which creates the following folder structure.

 a
 └── b
     └── c

== touch ==

Creates a new empty file (no contents) with the name we specify.

<syntaxhighlight lang="bash">
$ touch file.txt
</syntaxhighlight>

We check that the file has been created correctly.

<syntaxhighlight lang="bash">
$ ls
file.txt
</syntaxhighlight>

== cp ==

Copies a file or directory.

<syntaxhighlight lang="bash">
$ cp file.txt file_copy.txt
</syntaxhighlight>

To copy a folder and its contents we have to use the -r (recursive) option.

<syntaxhighlight lang="bash">
$ cp -r test test_copy
</syntaxhighlight>

* We check the result with the ''ls'' command.

* If we are in the destination folder, the structure would be as follows:

<syntaxhighlight lang="bash">
$ cp <source path name> <file name>
</syntaxhighlight>

== mv ==

Move file or folder.

<syntaxhighlight lang="bash">
mv file_copy.txt file2.txt
</syntaxhighlight>

It is also used for renaming.

<syntaxhighlight lang="bash">
mv test_copy test_copy2
</syntaxhighlight>

== rm ==

* Deletes an empty file or folder:
<syntaxhighlight lang="bash">
rm <name>
</syntaxhighlight>

* To delete a non-empty folder and its contents:
<syntaxhighlight lang="bash">
rm -r <name>
</syntaxhighlight>

''IMPORTANT'': Never do ''rm -r *''

* EXERCISES:

# Create the following structure within the /tmp directory (use the mkdir, touch and cp commands):

 /tmp
 ├── folder1.
 ├── folder2
 │ ├─── folder1.txt
 │ └└── file2.md
 └── folder3.
     ├── file1.txt
     └─── file2.md

# Based on the previous exercise, change the structure to the following (use mv and rm commands):

 /tmp
 └── folder.
     ├─── file1.txt
     └─── file2.md

= Step 2: Screen print and output redirection =

== echo ==

It is used to print a line of text on the screen or environment variables. It also allows us to write to files.

<syntaxhighlight lang="bash">
echo <option> <text> 
</syntaxhighlight>

<syntaxhighlight lang="bash">
echo "Hello world"
</syntaxhighlight>

There is difference between quotes \" and single quotes \'.

A few options:

* -n, to tell echo not to add a line break after the message

An environment variable is an edited variable with some value, by default there are some created, such as the HOME variable, which contains the user's directory:

<syntaxhighlight lang="bash">
echo $HOME
</syntaxhighlight>

We will take advantage and create a variable ourselves:

<syntaxhighlight lang="bash">
ME="my name"
</syntaxhighlight>

We check:

<syntaxhighlight lang="bash">
echo $ME
</syntaxhighlight>

== Output redirection ==

> and >> is used to redirect an output. For example, using the command echo,
we can send a text inside a file:

<syntaxhighlight lang="bash">
echo "This is a test" > file.txt
</syntaxhighlight>

The difference between > and >> is that > overwrites whatever is in the file and
adds the content, and >> does not overwrite, it just adds the new content.

Let's do the test:

<syntaxhighlight lang="bash">
echo "This is a test" > file.txt
echo "This is a test2" > file.txt
echo "This is a test" >> file2.txt
echo "This is a test2" >> file2.txt
</syntaxhighlight>

In any case, if the file does not exist, it is created.

= Step 3: Advanced file and folder handling =

== cat ==

* Displays the contents of a file:

<syntaxhighlight lang="bash">
cat file.txt
cat file1 file2
</syntaxhighlight>

* Creates a file:

<syntaxhighlight lang="bash">
cat > filename
</syntaxhighlight>

SAVE FILE: Press ENTER and then CTRL + D

* Add more lines to the file: 

<syntaxhighlight lang="bash">
cat >> filename
</syntaxhighlight>

* Concatenate files:

<syntaxhighlight lang="bash">
cat file1 file2 > file3
</syntaxhighlight>

== grep ==

Used to locate matches of a certain pattern in files and displays the match, if found, on screen.

<syntaxhighlight lang="bash">
grep <option> <pattern> <files to search for>.
</syntaxhighlight>
For example, to search for the pattern ''test'' in the file ''file.txt''.

<syntaxhighlight lang="bash">
$ grep -n test file.txt
1:This is a test
</syntaxhighlight>

* Main command options:

-c : Writes the number of lines found.

-i : Is case insensitive.

-l : Displays the names of the files containing the searched characters.

-n : Each line is preceded by its number in the file.

-s : Messages indicating that a file cannot be opened are not displayed.

-v : Displays only lines that do not meet the condition.

== wc ==

Used to count the number of words, characters, lines or bytes contained in a file.

The most commonly used options are:

* -l: displays the number of lines contained in the file.
* w: shows the number of words.
-m: shows the number of characters
-c: shows the number of bytes.

Let's try and count everything in the file file2.txt:

<syntaxhighlight lang="bash">
wc -l file2.txt
wc -w file2.txt
wc -m file2.txt
wc -c file2.txt
</syntaxhighlight>

We can count more than one file at a time:

<syntaxhighlight lang="bash">
wc -l file.txt file2.txt
</syntaxhighlight>

It will show us the count of each file and the total.

== find ==

To search for files we can use the ''find'' command.

For example, to search for the file ''python3'' in the whole directory tree (from the root) we can use the command:

<syntaxhighlight lang="bash">
find / -name python3
</syntaxhighlight>

We can restrict the search to files with ''-type f''

<syntaxhighlight lang="bash">
find / -type f -name python3
</syntaxhighlight>

or for directories with ''-type d''

== Pipe (|) ==

The pipeline will serve us to connect several commands at the same time, let's see
some examples, which is always better understood:

<syntaxhighlight lang="bash">
cat file2.txt | grep test
grep -nr test file2.txt | wc -l
</syntaxhighlight>

In the first example, we display the contents of file2.txt, and this output is passed to the grep command.
We pass it to the grep command, which searches for lines containing test.

In the second example, we look for all lines containing the word test, and this output is passed to the w
output is passed to the wc command, which counts the lines in the output of the grep command.
grep command.

== Redirections ==

Let's look at a few concepts before we continue:

* Standard input: represents the data that an application needs to run, such as a data file or information entered from the terminal and is represented in the terminal as type 0.
* Standard output: it is the way that the applications use to show you information, there we can see the progress or simply the messages that the application wants to give you at a certain moment and it is represented in the terminal as type 1.
* Standard error: it is the way in which the programs inform you about the problems that can be found at the moment of the execution and it is represented in the terminal as type 2.

Redirections are used to move information from one type to another.

We have already seen previously the use of > and >>, and also the |, let's see some more:

* command < file: Takes the file input.
* command 2> file: Sends the error output of command to file
* command 2>&1: Send the error output to the standard output
* command &> file: Send the standard and error output to file; equivalent to command > file 2>&1

= Step 4: Users and groups =

== whoami ==

Used to get the current user.

<syntaxhighlight lang="bash">
whoami
</syntaxhighlight>

Most likely we will get ubuntu.


== who ==

This is to find out who is currently on the machine logged in.

<syntaxhighlight lang="bash">
who
</syntaxhighlight>

It's probably just us:

<syntaxhighlight lang="bash">
ubuntu pts/0 2019-08-28 13:51 (192.168.122.1)
</syntaxhighlight>


== su and sudo ==

su (substitute user): used to change user. For example:

<syntaxhighlight lang="bash">
su root
</syntaxhighlight>

Here you will be prompted for the password of the user you want to change to.

sudo (super user do): not all distributions integrate it, because it's less secure than su.
secure than su, this command allows us to execute a command as another user,
by default, if we do not give user, it will do it with the root user. If sudo
password, it asks for the user's password, since the user is the one who has permissions to use sudo or not.
has permissions to use sudo or not.

<syntaxhighlight lang="bash">
sudo ls -a /root
</syntaxhighlight>

If we had for example a user 'practica' and we wanted to run a
command on his behalf:

<syntaxhighlight lang="bash">
sudo -u practica ls
</syntaxhighlight>

As root I can add users so they can sudo like this:

<syntaxhighlight lang="bash">
adduser sudo user
</syntaxhighlight>

This makes the user ''user'' able to do sudo.


== adduser and useradd ==

Both commands are used to create a new user, but they have their differences.

useradd is a system command, and adduser is a script written in perl that uses the useradd command.
uses the useradd command. The difference at first glance is that adduser will
create the /home/user directory and useradd needs the -m option to do the same thing.
option to do the same. You can use whichever one you want, although useradd is more recommended,
as it will work the same on all distributions.

Let's add a user practica with its directory in /home (note that we need super user permissions to do this).
we need super user permissions to perform this task):

<syntaxhighlight lang="bash">
sudo useradd -m practice
</syntaxhighlight>

Let's check that the home directory is created and that the user practica
is working:

<syntaxhighlight lang="bash">
ls /home # we will see the practica directory.
sudo -u practica whoami # it will tell us that we are the user practica
</syntaxhighlight>


== passwd ==

It is used to modify the password of a user, for example, we are going to modify the password of the user we have previously created.
the password of the user we have previously created:

<syntaxhighlight lang="bash">
sudo passwd practice
</syntaxhighlight>

It will ask us twice for the password, and we will have it updated. Let's go
now try the su command to change the user:

<syntaxhighlight lang="bash">
su practice
whoami
</syntaxhighlight>

We will see that we have changed the user, to close this user, we type
exit' or press 'CTLR + D'.


== deluser and userdel ==

Both are used to delete a user, similar to adduser and useradd, userdel is the command and deluser is the script.
command and deluser is the script.

Let's delete the user we have created, including deleting his /home.
(-r option):

<syntaxhighlight lang="bash">
userdel -r practice
</syntaxhighlight>

Let's check that we no longer have user or /home:

<syntaxhighlight lang="bash">
ls /home
sudo -u practice whoami
</syntaxhighlight>


== addgroup and groupadd ==

Both are used to create a group. Groups are used to group users
and that they have the same permissions. For example, when we install some
tools like docker, it creates the docker group automatically so that we can easily add a user to the group.
so that we can easily add a user to the group, and this one has the permissions to work with that tool.
permissions to work with that tool.

Let's create a new group, let's call it guest:

<syntaxhighlight lang="bash">
sudo groupadd guest
</syntaxhighlight>


== usermod and groups ==

usermod is used to assign a group to a user.

groups is used to view the groups a user belongs to.

Let's add a practica user, we will add him to the guest group and we will see the groups he belongs to
we will see the groups to which he belongs:

<syntaxhighlight lang="bash">
sudo useradd -m practica
sudo usermod -G guest practica
groups practica
</syntaxhighlight>

We will see that the user practica belongs to the guest group.


== delgroup and groupdel ==

Both are used to delete a group. Let's delete the group we created
previously:

<syntaxhighlight lang="bash">
sudo groupdel guest
</syntaxhighlight>

We can notice now that the user practica does not belong to the guest group:

<syntaxhighlight lang="bash">
groups practica
</syntaxhighlight>


= Step 5: Permissions =

== Nomenclature rwx and numeric ==

If we do an ls -l, the first string that appears is the permissions of the file (-rwxrw).
file (-rwxrw-r--), we can divide it in 4 elements:

- (rwx) (rw-) (r--)
| | | |
type owner group others

* type: the type of the file, it can be d (directory), l (symbolic link), - (normal file)
* owner: owner permissions
* group: group permissions
* others: permissions for other users

Meaning of permissions:

* r: read permissions
* w: write permissions
* x: execute permissions

Finally, there is an abbreviated mode for these permissions which is used with
numbers.

rwx
001 in binary is 1 in decimal. Execute permissions
010 in binary is 2 in decimal. Write permissions
100 in binary is 4 in decimal. Read permissions

The sum of the permissions, gives us the numerical value. Let's see an example:
rwx rw- r--
111 110 100
 7 6 4

== chmod ==

Allows you to change the access permissions to a file or directory. We can give
permissions in two different ways.


=== character mode ===

Let's first see some details:

User classes:

* owner (u)
* group (g)
* others (o)
* all (a)

Modifier:

* add (+)
* delete (-)
* overwrite (=)

To give permissions we would have to select the class of user the modifier
and the permissions, let's see an example:

<syntaxhighlight lang="bash">
chmod u+x file # add execute permissions to the user
chmod go-w file # remove write permissions to the group and other users
chmod u=rwx,go=r file # give the user all permissions, and the group and others only read permissions
</syntaxhighlight>


=== Octal mode ===

With octacl mode, it is as if we always use modify overwrite,
let's take some examples:

<syntaxhighlight lang="bash">
chmod 744 file # To the user we give all permissions, and to the group and others we only give read permissions.
chmod 777 file # Everyone has permissions for everything
chmod 600 file # only the owner has read and write permissions
</syntaxhighlight>


=== Examples ===

Let's see how permissions work with some examples. We are going to create
a test file and we are going to remove the write permissions to our user, and we are going to give it
user, and we are going to give write permissions to others:

<syntaxhighlight lang="bash">
touch tests
ls -l tests # this way we will see that by default the permissions are rw-rw-r--
chmod u-w,o+w tests
ls -l tests # now the permissions should be -w-rw-rw-
</syntaxhighlight>

Once the permissions are given, let's see that with our user we can read but not
write:

<syntaxhighlight lang="bash">
cat tests
echo "testing" > tests
exit
</syntaxhighlight>

And now let's see that the user tests, can read and write:

<syntaxhighlight lang="bash">
your practice
cat tests
echo "testing" > tests
</syntaxhighlight>


== chown ==

Allows you to change the owner of a file or directory.

Let's for example now pass the file tests to the user practica:

<syntaxhighlight lang="bash">
sudo chown practice tests
ls -l tests
</syntaxhighlight>

We will see that the current user is now tests, although we see that the ls shows
the following:

<syntaxhighlight lang="bash">
-r--rw-rw- 1 practice ubuntu 9 Aug 29 09:34 tests
</syntaxhighlight>

There is one thing to note, and that is that files and directories are assigned a user and a group by default.
a user and a group, by default the same user is used as group
when we create a new file or directory. If we want that when we change
the owner, we can also change the group of the file, we can do it in the following way
following way:

<syntaxhighlight lang="bash">
sudo chown practice:practice tests
</syntaxhighlight>

Once this is done, let's now test that the permissions are still working
as before, now with the ubuntu user, we should be able to read and write to the file tests.
the file tests, and with the user practica, only read, this is so because
now the owner is practica:

<syntaxhighlight lang="bash">
# tests ubuntu user
cat tests
echo test >> tests
# tests user practica
your practice
cat tests
echo "testing" > tests
exit
</syntaxhighlight>


= Step 6: Processes =


== ps ==

Used to see what processes are running on the system, let's see an example:

<syntaxhighlight lang="bash">
ps aux # with the aux option we will show all the processes in the system.
</syntaxhighlight>

Each line is a process, and each process displays its PID, user, amount of
memory and cpu used, command and other details.

<syntaxhighlight lang="bash">
man kill & # adding the & to the end of a command causes it to run in the background, so it will stay open.
</syntaxhighlight>

The output of this command will show us the PID of the process we just executed, let's check it.
run, let's check it:

<syntaxhighlight lang="bash">
ps aux | grep "man kill"
</syntaxhighlight>

We will see that the PID matches.

== kill ==

This command will be used to kill a process. kill is used followed by a signal to send the process followed by one or more PIDs.
signal to send to the process followed by one or more PIDs. to see the available signals, we can list them: == == == This command is used to kill a process.
to see the available signals, we can list them:

<syntaxhighlight lang="bash">
kill -l
</syntaxhighlight>

The most commonly used are SIGTERM and SIGKILL, the first one tries to end the process in an unabrupt way, the second one
the first one tries to terminate the process in a less abrupt way, and the second one is usually used when the process
does not heed this first signal. We are now going to terminate the process we created
before, if we don't remember the PID, let's look it up again:

<syntaxhighlight lang="bash">
ps aux | grep "man kill"
</syntaxhighlight>

We send signal to terminate the process and check that it has finished:

<syntaxhighlight lang="bash">
kill -SIGTERM 8470
ps aux | grep "man kill"
</syntaxhighlight>

We can also notice that in the signal listing, there are some numbers, we can
use those numbers instead of the words, for example:

<syntaxhighlight lang="bash">
man kill &
kill -15 PID_PREVIOUS_COMAND
</syntaxhighlight>

We check that the process has finished correctly:

<syntaxhighlight lang="bash">
ps aux | grep "man kill"
</syntaxhighlight>


= Step 7: CPU, RAM and Disk status =

== top ==

This command will help us to see the list of processes and the status of CPU and memory.
memory.

Let's try the command and observe the output in detail:

<syntaxhighlight lang="bash">
top
</syntaxhighlight>

To exit, we press q.


== df and du ==

df (disk free) and du (disk usage). Both are utilities for displaying disk usage.
the disks.

With df we will show the space information on each mounted device:

<syntaxhighlight lang="bash">
df
</syntaxhighlight>

These commands that show us the size of the files, almost always have
an option to show it in a more readable format (-h)

<syntaxhighlight lang="bash">
df -h
</syntaxhighlight>

Much better.

Now let's look at the du command, which will show the size of a file or directory and its subdirectories.
directory and its subdirectories, let's not forget the -h:

<syntaxhighlight lang="bash">
du -h /home/ubuntu
du -h /home/ubuntu/tests
</syntaxhighlight>

If we only want to know the total of a folder and we are not interested in its subfolders, we can use
subfolders, we can use the -s option:

<syntaxhighlight lang="bash">
du -sh /home/ubuntu
</syntaxhighlight>


== free ==

Used to view the memory status, as always, -h option:

<syntaxhighlight lang="bash">
free -h
</syntaxhighlight>


== lsblk ==

Shows us the information of all block devices (hard disks,
pendrivers, CD_ROM, SSD, ...).

<syntaxhighlight lang="bash">
lsblk
</syntaxhighlight>

In the following practice this command will be used more thoroughly, as we will
we will be working with devices.


= Step 8: Software package management and repositories =

Linux systems include, in addition to the basic operating system tools, software repositories that can be optionally installed by the system administrator. The manufacturers of Linux distributions include ready-to-install software packages that are integrated with the system.

A package includes the software necessary for a certain application to function properly, as well as its dependency packages.

In Ubuntu, the software package and repository management tool is called '''apt''''.

=== List of software package repositories ===

In Ubuntu the repository list is located in the /etc/apt/sources.list file. We can check which repositories we have added:

<syntaxhighlight lang="bash">
cat /etc/apt/sources.list
</syntaxhighlight>

=== Updating the software package listing ===

To update the package listing we will use the command:

<syntaxhighlight lang="bash">
sudo apt update
</syntaxhighlight>

=== Installing a new package ===

To install a new package, we will use the install command, for example, let's install tree, similar to ls but shows the directory tree:

<syntaxhighlight lang="bash">
sudo apt install tree
</syntaxhighlight>

=== Uninstallation of a package ===

We will use the purge command or the remove command, purge removes everything, and remove keeps the configuration if the package had it. Let's remove the previously installed package:

<syntaxhighlight lang="bash">
sudo apt purge tree
</syntaxhighlight>

= Step 9: Remote administration with ssh =

''ssh'' (Secure SHell) allows you to remotely administer a system from the shell. In order to access the virtual machine via ''[[ssh]]'', the ''openssh-server'' package must be installed.

 sudo apt-get install openssh-server

Once installed, we need to query the IP address of the virtual machine

 ip address

From the hypervisor we can access by ''ssh'' to the virtual machine with the command:

 ssh user@ip

Being ''user'' the user name with which you access the virtual machine and the ''ip'' address shown by the command ''ip address''.

For example, if the selected user is ''ubuntu'' and the IP is 192.168.122.123, then the invocation to ''ssh'' is the following:

 ssh ubuntu@192.168.122.123

In case openssh-server is not installed on the virtual machine, the connection may be rejected: 
 ssh: connect to host 192.168.122.123 port 22: Connection refused

To solve the problem it is necessary to install OpenSSH client on the virtual machine

 sudo apt install openssh-client

And then install OpenSSH server

 sudo apt install openssh-server


= Exercise =

* Create a file 'x.txt' with this content (use echo or cat!)

 hello world
 1234567890
 bye bye
 test test

* create a folder x that has a subfolder y
* move 'x.txt' to subfolder y
* make a copy of 'x.txt' to folder 'x' whose name is 'x.txt.backup'
* make a copy of the entire 'x' folder and name it 'x.backup'
* remove the 'x.backup' folder
* find all folder in your filesystem whose name is 'bin'
* count the number of folders whose name is 'bin' in your filesystem
* check if there is a user 'practica' in /etc/passwd
* check for all files in your home folder with read+write permission for the group and store it in a file called "group_permissions.txt"