Knowledge about Linux ¶

Linux is a family of Unix-like operating systems, often called distros, such as Debian, Ubuntu, Fedora, or Arch Linux... They were designed to be free, open-source, light, secure, and performant.

➡️ Each distro has its pro and cons according to what specific usage they were designed for (ex: gaming, security, performance...).

🐦 Common distros: Ubuntu, Debian, Arch Linux, Red Hat, CentOS, Kali Linux, Parrot Linux, Linux Mint, Fedora...

🎠 Random distros: Gentoo, QubesOS.

Linux in its simplest form is a command-line interface (CLI), commonly referred to as terminal/shell (or console/command prompt).

Inside the CLI, we can run commands interpreted by a shell 🐚. There are multiple shell languages (sh, csh, tcsh, ksh, bash), however most Linux distributions are using bash (bourne again shell) 🔥.

🏠 Customize your prompt: Bash Prompt Generator/powerline.

👉 Note that shell, the CLI, and a terminal are not the same thing, although they are closely related, hence the common misuse.

👉 You can use CTRL+ALT+F1 to swap to "console mode" (from F1, up to F6), while you can use ALT+F7 to swap back to the "GUI mode".

A command has a name, which may be followed by arguments, including some usually called flag/switches (options). The difference between both is that a flag starts with a - while operands do not.

Below are some examples with the command ls:

$ ls

$ ls toto.txt
$ ls "toto.txt"

$ ls -l -a
$ ls -la

$ ls -la toto/

🌍 Browse the manual (man) to learn more about some command

$ man ls

🤘 Use tab key to autocomplete commands/paths

$ ls /<TAB>
# will display every path starting with "/"

A few takeaways:

• 🛣️ you are writing commands right after the $
• 👉 press ENTER to execute a command
• 💥 press CTRL-C to cancel/kill a command
• ✅ press CTRL-D to end the input
• 🧓 press CTRL-R to search in the command history
• 🏎️ press CTRL-E to move the cursor to the end
• 🧼 press CTRL-U to remove everything before the cursor
• 🛣️ press ALT+B/CTRL+arrow to move between words
• 🚀 Usually, flags can be merged (ex: -l -a is the same as -la).
• 🗺️ Most commands have an option "help": -h, -help, or --help

A path 🛣️ is a suite of one or more folders 📂 that may lead to a regular file 📄. They are separated by a separator which is: / (slash).

$ pwd
/usr/home/toto/Documents/

➡️ The root folder (similar to C: on Windows) contains every other folder. It's the leading / of every path.

➡️ Each user has a home folder with their documents...

🦄 See also: Linux Filesystem and Files.

Absolute and relative paths ¶

A path starting by the root is called absolute path. Otherwise, they are called relative path. There are 3 shortcuts used in relative paths:

• . (dot): will be replaced with the absolute path to the working directory. See the pwd command.
• .. (dot dot): will be replaced with the parent folder of .
• ~ (tilde): path to user home, same as $HOME

$ pwd
/home/example
$ ./toto.txt
$ /home/example/toto.txt # same

➡️ root is its own parent (/../ is the same as /).

🧪 Following slashes are merged (/// becomes /).

Permissions are assigned on a file/folder, and are applied according to the target 🎯 of the permissions (ownership)

• u: user, applied to the user (owner)
• g: group, applied to the main group of the user (ex: students_2022)
• o: others, applied to everyone else

There are 3 well-known levels of permissions 🔒

• r (4): can read
• w (2): can write (=can edit+save, can create)
• x (1): can execute a script, can move through a folder

Giving us something like: u=r+w, g=r, o=r. Instead of letters, we usually use numbers, as it's shorter. We could write u=6, g=4, o=4 which can be shortened again to 644.

groups ¶

Each user is part of a group called after their name (e.g. the user toto is in the group toto). Use /etc/group to see members of each group.

Sudo ¶

There is a super-user 🦸, usually called root, that has absolute control over the machine 👑. They can delegate their privileges to users called sudoers. The command to execute something "as root" is:

# execute "cat [...]" as administrator
$ sudo cat /etc/shadow

➡️ See man sudo_root, and sudo/wheel groups.

See permissions using ls ¶

• The first character is the type of the file
• The 3 following letters rw- are the permissions of u: read+write.
• The 3 following letters r-- are the permissions of g: read.
• The 3 following letters r-- are the permissions of o: read.
• Ignore the 1
• The following string listro is the name of the user u
• The following string listro is the name of the group g

Environment variables (Variables d’environnement) are global variables, mostly used by commands/applications to access information about the system, save configurations...

• HOME: path to the current user home
• USER: username of the current user
• LANG: language of the current user
• SHELL: path to the shell
• PWD: path to the current folder
• RANDOM: return a random value
• DISPLAY: identify display

And, there is PATH. This variable is used to store a list of folders. When you write a command on Linux, then the shell will look for the command's file in the PATH, starting from the first folder inside.

➡️ On Linux, folders in the PATH are separated with :.

Print all environment variables

$ env
$ printenv

Print the value of one environment variable

$ echo $PATH
$ printenv PATH
$ env | grep "^PATH="

Set an environment variable

$ export VAR_NAME=value

Add /home/toto/bin to the PATH

$ export PATH=/home/toto/bin:$PATH

A glob-pattern is an expression using wildcards (motifs), that when evaluated by the shell, will be replaced with a list of files.

For instance, *.h will be replaced with every file -- and directory --, ending with .h. They are mostly used on commands taking too many filenames, in which you don't want to manually write all of them 😎.

If you write the glob-pattern a?c, then it could be abc... But if you write a\?c, or a[?]c, then it will only match a?c. This is called "escaping".

👉 Glob-patterns are pretty similar but different to Regexes.

Everything defined in [] is called a charset. If you want every character between 'a', and 'z', then you could write the charset [a-z]. There are pre-defined charsets if needed

• [[:digit:]] which is [0-9]
• [[:upper:]] which is [A-Z]
• [[:lower:]] which is [a-z]
• [[:space:]] which is [ \n\t]
• [[:alnum:]] which is [a-zA-Z0-9._]

We can run scheduled tasks on Linux using cron jobs. They are an alternative to System Timers.

$ crontab -l # list current user jobs
$ sudo crontab -l -u xxx # list user 'xxx' jobs

To add a new scheduled task, use -e

$ crontab -e

➡️ See also: /etc/cron*, /var/spool/cron/crontabs/*.

The format of the file is schedule /path/to/your/script. Schedule is made of 5 numbers whitespace-separated.

• minute (0-59)
• hour (0-23)
• day (month, 1-31)
• month (1-12)
• day (week, 0-6)

You can generate a schedule using crontab.guru or crontab-generator.

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