collection/linux.md

Linux

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

• Linux
  • Setup and Configuration
    • Locales and Keyboard Layouts
      • configure layout 'US intl. with deadkeys'
  • Linux commands
    • combine commands with xargs
    • pipe output into clipboard
  • permissions
    • octal permissions
  • Terminal
    • ANSI escape sequences
      • Text Style
      • Color codes
        • 8-16 Colors
        • 256 Colors
        • RGB Colors

Setup and Configuration

Locales and Keyboard Layouts

Use localectl to display and configure locales and keyboard layout:

# show current locales
localectl status

configure layout 'US intl. with deadkeys'

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

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

Use localectl to show the current settings.

Linux commands

• Linux commands
  • 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:

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:

# 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 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.

# 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:

# 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 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:

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 (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.