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

1. What is the difference in display? What are the “the” and “a” options used for?
2. 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

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

Edit / modify a file

1. In your home directory, create a text file using the vim editor via the command vim test.txt.
2. Press the i key to switch to vim insertion mode, you can then type text (e.g. your first and last name).
3. Then press ESC to exit insertion mode.
4. 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)
5. Check with less that the file contains what you wrote.
6. How big is this file?

Search for files in a directory

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

{% include snippets/000_find_examples.sh %}

1. What is the meaning of the character * here?
2. What is the purpose of the -name option?
3. What is the difference between ls -R and find?
4. 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.

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

sort
a
d
c

then type CTRL+D.

1. 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.

1. View the lines in file.sorted that contain the 1 character (look at the grep command).
2. Count the number of lines in this file
3. How do I count the number of lines that contain 1 in file.sorted?
4. 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.

1. With a text editor¹, create / open your .bashrc file and write alias ll='ls -l' in it.
2. 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.
3. Reload .bashrc in the terminal
4. You can enrich your .bashrc file to customize your terminal with for example:

{% include snippets/bashrc.sh %}

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):

1. 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

1. 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

1. Based on the https://www.linuxtricks.fr/wiki/droits-sous-linux-utilisateurs-groupes-permissions page, change the read/write/execute permissions on your files.
2. 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

1. Write the following C program

{% include snippets/000_minimal.c %}

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