# Linux

This document collects knowledge, tips and tricks on all things Linux, that don't get their own document.

<!--toc:start-->
- [Linux](#linux)
  - [Setup and Configuration](#setup-and-configuration)
    - [Locales and Keyboard Layouts](#locales-and-keyboard-layouts)
      - [configure layout 'US intl. with deadkeys'](#configure-layout-us-intl-with-deadkeys)
  - [Linux commands](#linux-commands)
    - [combine commands with *xargs*](#combine-commands-with-xargs)
    - [pipe output into clipboard](#pipe-output-into-clipboard)
  - [permissions](#permissions)
    - [octal permissions](#octal-permissions)
  - [Terminal](#terminal)
    - [ANSI escape sequences](#ansi-escape-sequences)
      - [Text Style](#text-style)
      - [Color codes](#color-codes)
        - [8-16 Colors](#8-16-colors)
        - [256 Colors](#256-colors)
        - [RGB Colors](#rgb-colors)
<!--toc:end-->


## Setup and Configuration


### Locales and Keyboard Layouts

Use `localectl` to display and configure locales and keyboard layout:

```sh
# show current locales
localectl status
```


#### configure layout 'US intl. with deadkeys'

To configure `US intl. with deadkeys`, set the following values:

```sh
X11 Layout:  us
X11 Model:   default
X11 Variant: intl
```

Use `localectl` to show the current settings.


## Linux commands

- [Linux commands](#linux-commands)
  - [combine commands with *xargs*](#combine-commands-with-xargs)


### combine commands with *xargs*

Some commands (e.g. `chmod`) can't handle input via pipe and require the input as arguments.
`xargs` takes the standard input as list of arguments seperated by spaces or newlines and executes the given command with the input parsed to a list of arguments.

Usage:
```bash
find . -type f | xargs chmod 644
```


### pipe output into clipboard

> requires `xclip`

One can save command output to the clipboard by piping it to `xclip`:

```sh
# usage
command | xclip -selection clipboard

# working example with 'echo'
echo "hello world" | xclip -selection clipboard
```

> NOTE: the argument `-selection clipboard` is required


## permissions

Linux permissions are ordered as follows:
- owner
- group
- others


### octal permissions

In octal representation the actions (read, write, execute) have the values:
- read = 4
- write = 2
- execute = 1


## Terminal

- [ANSI escape sequences](#ansi-escape-sequences)


### ANSI escape sequences

Ansi escape sequences are used to format output in a terminal.
They **are prefixed with an escape character**, which can be written in a few ways:

- Ctrl-Key `^[`
- Octal `\033` (\0 for octal)
- Hexadecimal `\x1B` (\\x for hexadecimal)
- Unicode `\u001b`

The sequences is started with a `[` followed by the command and arguments.
Arguments are separated with `;`.
**Sequences are terminated with `m`.** 

``` bash
# Example:
\x1b[1;31m  # Set style to bold, red foreground.
```

For the following tables we define `ESC = \x1B`


#### Buffer Control

| Sequence | Description  |
| :------- | :----------- |
| `ESCc`   | clear buffer |


#### Cursor Control

The following table lists codes to control the cursor:

| Sequence               | Description                                            |
| :--------------------- | :----------------------------------------------------- |
| `ESC[H`                | moves cursor to home position (0, 0)                   |
| `ESC[<line>;<column>H` | moves cursor to `<line>`, `<column>`                   |
| `ESC[<#>A`             | moves cursor up # lines                                |
| `ESC[<#>B`             | moves cursor down # lines                              |
| `ESC[<#>C`             | moves cursor right by # columns                        |
| `ESC[<#>D`             | moves cursor left by # columns                         |
| `ESC[<#>E`             | moves cursor to beginning of next line, # lines down   |
| `ESC[<#>F`             | moves cursor to beginning of previous line, # lines up |
| `ESC[<#>G`             | moves cursor to column #                               |
| `ESC[6n`               | request cursor position (reports as `ESC[#;#R`)        |
| `ESC M`                | moves cursor one line up, scrolling if needed          |
| `ESC 7`                | save cursor position (DEC)                             |
| `ESC 8`                | restores the cursor to the last saved position (DEC)   |
| `ESC[s`                | save cursor position (SCO)                             |
| `ESC[u`                | restores the cursor to the last saved position (SCO)   |


#### Text Style

| ESC Code Sequence | Reset Sequence | Description                                                |
| :---------------- | :------------- | :--------------------------------------------------------- |
| `ESC[1;34;{...}m` |                | Set graphics modes for cell, separated by semicolon (`;`). |
| `ESC[0m`          |                | reset all modes (styles and colors)                        |
| `ESC[1m`          | `ESC[22m`      | set bold mode.                                             |
| `ESC[2m`          | `ESC[22m`      | set dim/faint mode.                                        |
| `ESC[3m`          | `ESC[23m`      | set italic mode.                                           |
| `ESC[4m`          | `ESC[24m`      | set underline mode.                                        |
| `ESC[5m`          | `ESC[25m`      | set blinking mode                                          |
| `ESC[7m`          | `ESC[27m`      | set inverse/reverse mode                                   |
| `ESC[8m`          | `ESC[28m`      | set hidden/invisible mode                                  |
| `ESC[9m`          | `ESC[29m`      | set strikethrough mode.                                    |

> **Note:** Some terminals may not support some of the graphic mode sequences listed above.

> **Note:** Both dim and bold modes are reset with the `ESC[22m` sequence.
> The `ESC[21m` sequence is a non-specified sequence for double underline mode and only works in some terminals and is reset with `ESC[24m`.


#### Color codes

Most terminals support 8 and 16 colors, as well as 256 (8-bit) colors.
These colors are set by the user, but have commonly defined meanings.


##### 8-16 Colors

| Color Name | Foreground Color Code | Background Color Code |
| :--------- | :-------------------- | :-------------------- |
| Black      | `30`                  | `40`                  |
| Red        | `31`                  | `41`                  |
| Green      | `32`                  | `42`                  |
| Yellow     | `33`                  | `43`                  |
| Blue       | `34`                  | `44`                  |
| Magenta    | `35`                  | `45`                  |
| Cyan       | `36`                  | `46`                  |
| White      | `37`                  | `47`                  |
| Default    | `39`                  | `49`                  |
| Reset      | `0`                   | `0`                   |

> **Note:** the _Reset_ color is the reset code that resets _all_ colors and text effects, Use _Default_ color to reset colors only.

Most terminals, apart from the basic set of 8 colors, also support the "bright" or "bold" colors.
These have their own set of codes, mirroring the normal colors, but with an additional `;1` in their codes:

```sh
# Set style to bold, red foreground.
\x1b[1;31mHello
# Set style to dimmed white foreground with red background.
\x1b[2;37;41mWorld
```

Terminals that support the [aixterm specification](https://sites.ualberta.ca/dept/chemeng/AIX-43/share/man/info/C/a_doc_lib/cmds/aixcmds1/aixterm.htm) provides bright versions of the ISO colors, without the need to use the bold modifier:

| Color Name     | Foreground Color Code | Background Color Code |
| :------------- | :-------------------- | :-------------------- |
| Bright Black   | `90`                  | `100`                 |
| Bright Red     | `91`                  | `101`                 |
| Bright Green   | `92`                  | `102`                 |
| Bright Yellow  | `93`                  | `103`                 |
| Bright Blue    | `94`                  | `104`                 |
| Bright Magenta | `95`                  | `105`                 |
| Bright Cyan    | `96`                  | `106`                 |
| Bright White   | `97`                  | `107`                 |


##### 256 Colors

The following escape codes tells the terminal to use the given color ID:

| ESC Code Sequence | Description           |
| :---------------- | :-------------------- |
| `ESC[38;5;{ID}m` | Set foreground color. |
| `ESC[48;5;{ID}m` | Set background color. |

Where `{ID}` should be replaced with the color index from 0 to 255 of the following color table:

![256 Color table](https://user-images.githubusercontent.com/995050/47952855-ecb12480-df75-11e8-89d4-ac26c50e80b9.png)

The table starts with the original 16 colors (0-15).
The proceeding 216 colors (16-231) or formed by a 3bpc RGB value offset by 16, packed into a single value.
The final 24 colors (232-255) are grayscale starting from a shade slighly lighter than black, ranging up to shade slightly darker than white.
Some emulators interpret these steps as linear increments (`256 / 24`) on all three channels, although some emulators may explicitly define these values.


##### RGB Colors

More modern terminals supports [Truecolor](https://en.wikipedia.org/wiki/Color_depth#True_color_.2824-bit.29) (24-bit RGB), which allows you to set foreground and background colors using RGB.
These escape sequences are usually not well documented.

| ESC Code Sequence       | Description                  |
| :---------------------- | :--------------------------- |
| `ESC[38;2;{r};{g};{b}m` | Set foreground color as RGB. |
| `ESC[48;2;{r};{g};{b}m` | Set background color as RGB. |

> Note that `38` and `48` corresponds to the 16 color sequence and is interpreted by the terminal to set the foreground and background color respectively.
> Where as `;2` and `;5` sets the color format.