# Linux 

## Absolute Path vs Relative Path in Linux/Unix
One of this blog follower asked us that whatâs the difference between absolute and relative path?

To understand this we have to know what is a path in Linux.
###  What is a path?

A path is a unique location to a file or a folder in a file system of an OS. A path to a file is a combination of / and alpha-numeric characters.
What is an absolute path?

An absolute path is defined as the specifying the location of a file or directory from the root directory(/). In other words we can say absolute path is a complete path from start of actual filesystem from / directory.
###  Some examples of absolute path:
```bash
/var/ftp/pub
/etc/samba.smb.conf
/boot/grub/grub.conf
```
If you see all these paths started from / directory which is a root directory for every Linux/Unix machines.
###  What is the relative path?

Relative path is defined as path related to the present working directory(pwd). Suppose I am located in /var/log and I want to change directory to /var/log/kernel. I can use relative path concept to change directory to kernel

changing directory to /var/log/kernel by using relative path concept.

> $ pwd/var/logcd kernel
 
Note: If you observe there is no / before kernel which indicates itâs a relative directory to present working directory.

Changing directory to /var/log/kernel using absolute path concept.

> $ cd /var/log/kernel
 
Note: We can use an absolute path from any location where as if you want to use relative path we should be present in a directory where we are going to specify relative to that present working directory.
 
Examples of relative path and absolute path for the same operation.

# Basic Linux Command

## help
- Man: an interface to the on-line reference manuals 
> $ man ls

## Wildcard
* Show all picture (multiple char)
> $ ls pic*  

? show only picture between 50 and 59 (only one char)

> $ ls pic5?.jpg  

[] show only if char in [p-P]

> $ ls [pP]ic*

## Files and directory

- ls: list directory contents
> $ ls

> args =  -l -a -r -t -h --help


- Pwd: output the current working directory
> $ pwd

- Cd: Change directory
> $ cd    

back to previous folder
> $ cd -   


- Mkdir: Make directory
> $ mkdir test1 test2 test3

> $ mkdir -p lol/lol/lol     

-p to create parent directory if needed

- Rmdir: Remove directory

> rmdir filename

> rm -rf filename

- Mv: Move directory (can be used to rename a file)
> mv file /opt/movedfile

- Cp: Copy file or directory
> $ cp file /opt/newcopiedfile 

- Touch: change file timestamps but it can also create files   
> $ touch nomdefichier.md  

> $ touch pic{00..99}.jpeg # does not work
  
- Which: Searching the PATH for executable files matching the names of the arguments
> $ which ls

- File: file — determine file type
> $ file myfile

## file viewer
- More: file perusal filter for crt viewing

> $ more filename 

- Less: opposite of more but Less is more ;)

> $ less filename

- Cat: concatenate files and print on the standard output

> $ cat filename

## Users

adduser, addgroup - add a user or group to the system
> sudo adduser steve

```bash
r4v3n@d3bi4n:~/Workspace/test$ sudo adduser steve
[sudo] password for r4v3n: 
Sorry, try again.
[sudo] password for r4v3n: 
Adding user `steve' ...
Adding new group `steve' (1002) ...
Adding new user `steve' (1002) with group `steve' ...
Creating home directory `/home/steve' ...
Copying files from `/etc/skel' ...
New password: 
Retype new password: 
passwd: password updated successfully
Changing the user information for steve
Enter the new value, or press ENTER for the default
	Full Name []: 
	Room Number []: 
	Work Phone []: 
	Home Phone []: 
	Other []: 
Is the information correct? [Y/n] y

```

user skeleton: skeleton files used for new user configuration 
> $ ls /etc/skel/

```bash
─r4v3n at d3bi4n in /etc/skel
╰─» ls -a                                                                        0 (0.002s) < 05:21:24
./  ../  .bash_logout  .bashrc  .profile

```

- Change user
> $ su - marie

## Permissions

- Chmod: change file mode bits

A  combination of the letters **ugoa** controls which users' access to the file will be changed: 
- the user who owns it (u), 
- other users in the file's group (g), 
- other users not in the file's group  (o),
- or  all  users  (a).  

If none of these are given, the effect is as if (a) were given, but bits that are set in the umask are not affected.

![file permissions](./img/file_permission.png)

```bash
marie@d3bi4n:~$ ls -l
total 4
-rw-r--r-- 1 marie marie 12 Apr  7 05:44 test
```

> $ chmod o-r mysecret

> -rw-r--**-**-- 1 marie marie 12 Apr  7 05:44 test

> $ chmod o+**rw** myscret 

> -rw-r--**rw**- 1 marie marie 12 Apr  7 05:44 test


Number alternatives: 
### Groups

Groups: print the groups a user is in
> $ groups

Adds user marie into steve group
> $ adduser marie steve

```bash
r4v3n@d3bi4n:~/Workspace/test$ sudo adduser marie steve
Adding user `marie' to group `steve' ...
Adding user marie to group steve
Done.

marie@d3bi4n:~$ groups
marie steve
```

## Sysadmin tools 

- Htop: Interactive processes viewer
> $ htop

## text editor
- Nano: Nano's ANOther editor, an enhanced free Pico clone
(simple text editor for noobies) 

> $ nano 

> $ nano filename

- VIM: vim - Vi IMproved, a programmer's text editor (PGM)

> $ vim

> $ vim filename 

# APT
> $ apt install
> $ apt remove
> $ apt autoremove
> $ apt update


## Display & Destop Manager

* Architecture:
> BIOS -> GRUB -> Display Manager -> Desktop Environement

* Install Desktop Environement (GUI)

> $ tasksel
> $ apt install gnome
> $ apt remove gnome

* Reconfigurer le display manager
> $ sudo dpkg-reconfigure gdm3

* Installer le display manager

> $ sudo apt install lightdm

> $ sudo apt install gdm3

* remove Desktop environement

> $ sudo apt remove lightdm 

# Services 
- HTOP
- dhclient > get ip
- gnome networkmanager 
- wpa_supplicant > encryption @ wifi