TP #1 : First Steps with Linux

Principles

Different categories of files

The different categories of files are:

Normal files: text: mail, program sources, scripts, configuration…; executables: programs in binary code
Directory files (we talk about directory): they are container files that contain references to other files true framework of the tree structure, they allow to organize files by categories.
Special files: located in /dev, these are access points prepared by the system to the devices. Mounting will perform a mapping of these special files to their directories.
Link files: These are files that contain only a reference (pointer) to another file. This makes it possible to use the same file under several names without having to duplicate it on the disk.

Application

The command ls allows you to see a part of the tree structure. For example, on my personal account, the command ls -l (-l gives details about the contents of the tree) gives the following result.

drwx------ 2 jdequidt imaEns  4096 déc.   8  2011 Desktop
drwxr-x--- 2 jdequidt imaEns  4096 sept. 28  2010 Documents
drwxr-x--- 2 jdequidt imaEns  4096 déc.  18  2018 Downloads
lrwxrwxrwx 1 jdequidt imaEns    29 sept.  9 14:09 mixed.csv -> ima2a/mixed_all_unordered.csv
drwxr-x--- 2 jdequidt imaEns  4096 sept. 28  2010 Music
drwxr-x--- 2 jdequidt imaEns  4096 sept. 28  2010 Pictures
-rwxr-xr-x 1 jdequidt imaEns 33024 sept.  9 14:05 prog1
drwxr-x--x 2 jdequidt imaEns  4096 sept.  9 14:08 Public
drwxr-x--x 2 jdequidt imaEns  4096 juin   6 09:56 public_html
-rw-r--r-- 1 jdequidt imaEns   949 sept.  9 14:05 ReadMe.md
drwxr-xr-x 6 jdequidt imaEns  4096 août  27 09:51 tmp
drwxr-x--- 2 jdequidt imaEns  4096 sept. 28  2010 Videos

From the following link https://www.garron.me/en/go2linux/ls-file-permissions.html, identify the:

directories
the text files
the executable file
the symbolic link and to which real file it points
the size in bytes of the file ReadMe.md.
the directory that contains the most subdirectories

References

To have a very synthetic view of the main commands, I advise you to consult the following links: https://dev.to/awwsmm/101-bash-commands-and-tips-for-beginners-to-experts-30je (very well structured) or https://bookmarks.ecyseo.net/?EAWvDw

Preamble

Once you are logged in, launch a terminal (Terminal, Konsole…)
Launch a program (for example xeyes) in interactive mode xeyes & and in non-interactive mode xeyes. What is the difference? Type the command CTRL+C to kill the xeyes process.

When you launch the terminal, in which directory are you located?
Type (successively) the following commands:

cd
pwd
cd /tmp
pwd
cd ~
pwd
cd /tmp
cd /usr/local
pwd
cd -
pwd

Look at the result. Which directory does the ~ (tilde) match? You can always go to your main / home directory, your home (in system jargon) by simply doing cd or cd ~. Similarly cd - allows you to go to the directory where you were previously.
Create the se3 directory in your main directory :

mkdir ~/se3
cd ~/se3

or equivalent with :

cd
mkdir se3
cd se3

Display the contents of this directory with the command ls.
Create an empty file (in the se3 directory) named empty_file.txt with the touch empty_file.txt command.
Display the contents of your se3 directory
It can also be used in this way:

cd /tmp
ls ~/se3

Type cd to return to your home.
What is the content of your home?
Create a tmp directory. What is now the content of your home?
Delete the tmp directory with the rm command.
Display the contents of your home.
Try deleting tmp again, what happens?
Be the following command sequence:

cd /tmp
ls
ls -a
ls -l
ls -l -a
ls -la (equivalent to the previous one)

What is the difference in display? What are the “the” and “a” options used for?
Delete the se3 directory with its contents via:

cd ~
rm -rf se3

See the help of rm (via man rm) and tell why this command is dangerous.

Search in text

By always being in the /etc/dictionaries-common directory
What does the command wc <filename> do? What do the - and -c options do?
What does the grep command do with e.g. the grep house words or grep house words commands?

Edit / modify a file

In your home directory, create a text file using the vim editor via the command vim test.txt.
Press the i key to switch to vim insertion mode, you can then type text (e.g. your first and last name).
Then press ESC to exit insertion mode.
Then press :w to save the file and then :q to exit vim (it is possible to combine the 2 commands into one by typing :wq)
Check with less that the file contains what you wrote.
How big is this file?

Search for files in a directory

Consult the help of the search command with man find.
Then type:

find /etc
find /etc -name "*.d"
find /etc -name "*.cfg" -ls
ls -R /etc

What is the meaning of the character * here?
What is the purpose of the -name option?
What is the difference between ls -R and find?
Try the two commands find /etc -exec wc '{}' + and find /etc -name "*.cfg" -exec wc '{}' +. What does the -exec option do?

Standard Input (stdin)

We remind you that the standard input corresponds to what is typed on the keyboard.

What does the sort command do? Look specifically at what happens when sort has no file as a parameter.
Try it:

sort
a
d
c

then type CTRL+D.

Similarly, try to sort the numbers 2, 11, 1. Explain why 11 is considered to be smaller than 2. Use the appropriate option to get a numeric sort.

Standard Output

What does the echo command do?
Do the following (notice that the second call to echo overwrites the contents of file):

echo test
echo test > file
cat file
echo toto
echo toto > file
cat file

With >> we add at the end without overwriting the file

echo test > file2
cat file2
echo toto >> file2
cat file2

Using sort, generate an .sorted file containing the numbers 2, 1, 11 sorted in ascending order. The result is displayed in the terminal?
Type the following commands and explain what happens:

sort < file.sorted
sort < file.sorted > file.sorted2
cat .sorted2 file

Why are the file.sorted and file2.sorted files different? You can use the diff command to see the differences.

The `pipe`

The pipe allows to connect the result of a command to the input of a new command.

View the lines in file.sorted that contain the 1 character (look at the grep command).
Count the number of lines in this file
How do I count the number of lines that contain 1 in file.sorted?
Try grep 1 file.sorted | wc to check that this command answers the previous question and that it does not generate temporary files.

The `.bashrc` file

This file contains commands that are executed each time you open a terminal. This file usually contains shortcuts or options for the terminal.

With a text editor¹, create / open your .bashrc file and write alias ll='ls -l' in it.
Exit the editor and type ll. What happens? Open a new terminal (without closing the first one) and type ll. What happens? In the first terminal, type source .bashrc and check that the command ll now works.
Reload .bashrc in the terminal
You can enrich your .bashrc file to customize your terminal with for example:

export CLICOLOR=1
# Usdeful for Database module
export PGHOST=serveur-etu.polytech-lille.fr

# Automatic completion
if [ -f /etc/bash_completion ]; then
    source /etc/bash_completion
fi

# Useful commands
alias ll='ls -l --color=auto'
alias la='ls -A'
alias cp='cp -iv'
alias mv='mv -iv'
alias less='less -FSRXc'

export GREP_OPTIONS='--color=auto'

# Prompt modification
function colored_prompt()
{
    local CYAN="\[\033[0;36m\]"
    local GRAY="\[\033[0;38m\]"
    local RED="\[\033[1;31m\]"

    export PS1='\n\[\e[37;46m\] \h \[\e[0m\]\[\e[37;41m\] \u \[\e[0m\]\[\e[37;44m\] \W \[\e[0m\] '
}

colored_prompt

If you wish to have more information on the possibilities of customization, you can consult the following link https://www.howtogeek.com/307701/how-to-customize-and-colorize-your-bash-prompt/

Very useful commands for these 3 years

You will find below some commands that will be very useful for this year (and used very often in TP control for example):

View / copy public files from another account. If you know the login of another person of Polytech Lille, you can consult the authorized files of this account via:

ls ~your_login

Connect to the school’s machines/servers from the outside:

ssh votre_login@portier.polytech-lille.fr -p 2222

at Polytech Lille, you can connect to another machine with the following command

ssh your_login@machine_name

such as for example:

ssh your_login@gayant01

Useful shortcuts

The terminal uses a specific software library called Readline to read the commands you type. It provides several shortcuts to improve your productivity. These shortcuts could be used either using emacs syntax or vi syntax. By default, the emacs syntax is enabled. Below are the most useful shortcuts (the complete list is detailed in the previous link).

Tab: auto-complete commands and files
Up Arrow_ or Down Arrow: allow to navigate through your command history (previous command or next command)
CTRL + r or CTRL + s: backward or forward search in your command history
CTRL + a / CTRL + f: moves the cursor to the beginning / end of the line
ALT + b / ALT + f: moves the cursor backward / forward one word
CTRL + l: clears the screen
CTRL + u: clears the line and copies it to the clipboard
CTRL + k: clears the line after the cursor and copies it to the clipboard
CTRL + w: remove one word backwards from cursor position

Back to file permissions

Based on the https://www.linuxtricks.fr/wiki/droits-sous-linux-utilisateurs-groupes-permissions page, change the read/write/execute permissions on your files.
Create a text file in your Public directory and make it accessible and readable (but not editable) by all people from the group ima3.

Your first `C` program

Write the following C program

#include <stdio.h>

int main()
{
    printf("Hello World !\n");
    return 0;
}

Compile your program with the command clang myfile.c or gcc myfile.c.
Run your program with ./a.out.

nano or gedit for instance… we will use vim later ↩