# Linux
- The "operating system" of the UPPMAX and most of the other clusters is **Linux**.

```{questions}
- What is Linux?
- How to use the command line?

```

```{objectives}
- We'll briefly get an overview of Linux
  - How the command line works
  - Some text editors
  - Things to be aware of
```

## What is Linux?

![Content](./img/pingvin.png)

- Daily speaking: The Linux Operating system is a UNIX like and UNIX compatible Operating system.
- Linux is a "Kernel" on which many different programs can run.
- The shell (bash, sh, ksh, csh, tcsh and many more) is one such program.
    
![Content](./img/images.jfif)

 

- Actually, for it to be an OS, it is supplied with GNU software and other additions giving us the name **GNU/Linux**.
  - [Linux naming controversy]((https://en.wikipedia.org/wiki/GNU/Linux_naming_controversy)

 

![Content](./img/gnu.png)

 

- Linux has a multiuser platform at its base which means permissions and security comes easy.

 
### Linux comes in different distributions, dialects or, say, flavours.
- UPPMAX runs CentOS and RedHat

 

![Content](./img/flavours.png)


 
## Using the command line

### Command line with bash (Bourne Again Shell)
- A Unix shell and command language.
- Often default shell

![Content](./img/shell.jpg)

- The command-line interface: the bash prompt $
- bash can be seen as a program that finds and runs other programs
- bash is scripting language that is referred to as a shell
  - (because it sits around the kernel making it easy to interact with)

 ![Content](./img/unix_architecture.jpg)

 
### The prompt

[info]$ <span style="color:blue">program</span> word1 word2 word3 […]

- [info] is configurable, and usually tells you who you are, on what system, and where in the file system.
  
  - Example: 

    ```bash=
      [bjornc@rackham3 linux_tutorial]$
    ```

  - For changing info (only for advanced users!)  Does not matter for this course!:
    - <https://www.cyberciti.biz/tips/howto-linux-unix-bash-shell-setup-prompt.html>
  - The <span style="color:blue">program</span> to run is the first word
  - All words are separated by spaces

![folders](./img/folders.png)
 
### Example bash command

<section>
    
```{image} ./img/mv_inbox.png
:alt: mv inbox
:width: 300px
:align: left
``` 

<br/><br/>

```{image} ./img/program_flags.png
:alt: program flags
:width: 300px
:align: left
```
    
<br/><br/><br/><br/><br/><br/><br/><br/><br/><br/><br/><br/><br/><br/><br/><br/><br/><br/>
   

</section>

 

- Terminal screen shows
    
</p>

![Content](./img/screen.png)               

 
### Tab Completion
<section>

```{image} ./img/tab.png
:alt: tab
:width: 200px
:align: left
```

<br/><br/><br/><br/><br/>
</section>

- Whenever you’re writing a path or filename on the bash prompt, you can strike the ‘tab’ key to
ask Bash to complete what you’re writing.

- Get in the habit of this — it will save you many hours!

 
## Editing files with file/text editors

![Content](./img/edit.png)


### gedit 
- graphical user interface — GUI, needs X-server

- Also graphical editor within MobaXterm

### nano 
- keyboard shortcuts shown on-screen)
- Cheatsheet: [http://staffwww.fullcoll.edu/sedwards/nano/UsefulNanoKeyCommands.html](http://staffwww.fullcoll.edu/sedwards/nano/UsefulNanoKeyCommands.html)
  - ^ = Ctrl, M = meta key)
    - Windows M = Alt
    - On Mac: in the Terminal.app go to Preferences -> Settings -> Keyboard and turn on "Use option as meta key": then M = Alt


```{solution} Not to try today if you haven't used before!!!

- If you start one of these editors you may have difficulties to exit!

**vim**
- fast and powerful, once you learn it
- on UPPMAX started with command ``vi``

  - 1. Insert mode (type like normal text editor. Press ``i`` for insert mode)
  - 2. Command mode (give commands to the editor to get things done . Press `<ESC>` for command mode)
  - Cheat sheet: [https://coderwall.com/p/adv71w/basic-vim-commands-for-getting-started](https://coderwall.com/p/adv71w/basic-vim-commands-for-getting-started)
 
**gvim**
- ``vi``m with a GUI, lots of features very Fast

**emacs**
- fast and powerful, once you learn it
  - Cheat sheet: [https://www.gnu.org/software/emacs/refcards/pdf/refcard.pdf](https://www.gnu.org/software/emacs/refcards/pdf/refcard.pdf)
    - (C = `<Ctrl>`)
- also With GUI
   - ``emacs –nw``
     - keeps you in terminal window.
```

## The graphical editors
- When starting the graphical versions of an editor, add ``&`` to be able to use the command line while program is open.
  - Ex: `gedit &`
  - If not, you can `<Ctrl>+z` and type `bg` to send program to background.

```{discussion} Try out later!
   **Try them out and pick one favorite editor!**
```

## Typical sources of error

![Content](./img/cross.png)

```{Warning}
- Capitalization matters in file names and program names
- Spaces matter.
  - Always have a space after the program name.
  - Don’t add spaces within file names.
- Check that you are in the right place in the file system.
- File permissions. Check that the right read, write and execute permission are set. See next session.
```


 
## Caution!!

![Content](./img/caution.png)


```{Warning}

- There is no undo for:
  - copy (`cp`),
  - move (`mv`), and
  - remove (`rm`).
- **Beware of overwriting files and deleting the wrong ones.**
```
 
```{Note}
- **Tip: make "`rm`" ask if you really want to erase:**
  - Within a session: Type in the command prompt

        alias rm='rm -i'

  - Override asking with 

        rm –f <>

  - Edit file `.bashrc` in `home` directory by adding the alias line for this to start everytime.
- This will also work for ``mv`` and ``cp``!
```

```{Note}
- If you do destroy your data, email UPPMAX support, we may be able to help.
```

 ```{keypoints}
- Linux Operating system is a UNIX-like and UNIX compatible Operating system.
- Typical command:
    $ program word1 word2 word3 […]
- Example of file editors
    - terminal
        - nano
        - vim
        - emacs
    - graphical:
      - gedit
- Tips
    - use Tab completion
    - capitalization and spaces matters
    - no undo:s for copying, moving and removing
      - Solution: ``alias rm='rm -i' ``
```