Terminal codes (ANSI/VT100) introduction¶

Terminal (control) codes are used to issue specific commands to your terminal. This can be related to switching colors or positioning the cursor, i.e. anything that can't be done by the application itself.

How it technically works¶

A terminal control code is a special sequence of characters that is printed (like any other text). If the terminal understands the code, it won't display the character-sequence, but will perform some action. You can print the codes with a simple echo command.

Note: I see codes referenced as "Bash colors" sometimes (several "Bash tutorials" etc...): That's a completely incorrect definition.

The tput command¶

Because there's a large number of different terminal control languages, usually a system has an intermediate communication layer. The real codes are looked up in a database for the currently detected terminal type and you give standardized requests to an API or (from the shell) to a command.

One of these commands is tput. Tput accepts a set of acronyms called capability names and any parameters, if appropriate, then looks up the correct escape sequences for the detected terminal in the terminfo database and prints the correct codes (the terminal hopefully understands).

The codes¶

In this list I'll focus on ANSI/VT100 control codes for the most common actions - take it as quick reference. The documentation of your terminal or the terminfo database is always the preferred source when something is unclear! Also the tput acronyms are usually the ones dedicated for ANSI escapes!

I listed only the most relevant codes, of course, any ANSI terminal understands many more! But let's keep the discussion centered on common shell scripting ;-)

If I couldn't find a matching ANSI escape, you'll see a :?: as the code. Feel free to mail me or fix it.

The ANSI codes always start with the ESC character. (ASCII 0x1B or octal 033) This isn't part of the list, but you should avoid using the ANSI codes directly - use the tput command!

All codes that can be used with tput can be found in terminfo(5). (on OpenBSD at least) See OpenBSD's terminfo(5) under the Capabilities section. The cap-name is the code to use with tput. A description of each code is also provided.

General useful ASCII codes¶

The Ctrl-Key representation is simply associating the non-printable characters from ASCII code 1 with the printable (letter) characters from ASCII code 65 ("A"). ASCII code 1 would be ^A (Ctrl-A), while ASCII code 7 (BEL) would be ^G (Ctrl-G). This is a common representation (and input method) and historically comes from one of the VT series of terminals.

Cursor handling¶

Erasing text¶

General text attributes¶

Foreground coloring¶

Background coloring¶

Misc codes¶

Save/restore screen¶

Used capabilities: smcup, rmcup

You've undoubtedly already encountered programs that restore the terminal contents after they do their work (like vim). This can be done by the following commands:

# save, clear screen
tput smcup
clear

# example "application" follows...
read -n1 -p "Press any key to continue..."
# example "application" ends here

# restore
tput rmcup

These features require that certain capabilities exist in your termcap/terminfo. While xterm and most of its clones (rxvt, urxvt, etc) will support the instructions, your operating system may not include references to them in its default xterm profile. (FreeBSD, in particular, falls into this category.) If tput smcup appears to do nothing for you, and you don't want to modify your system termcap/terminfo data, and you KNOW that you are using a compatible xterm application, the following may work for you:

echo -e '\033[?47h' # save screen
echo -e '\033[?47l' # restore screen

Certain software uses these codes (via their termcap capabilities) as well. You may have seen the screen save/restore in less, vim, top, screen and others. Some of these applications may also provide configuration options to disable this behaviour. For example, less has a -X option for this, which can also be set in an environment variable:

export LESS=X
less /path/to/file

Similarly, vim can be configured not to "restore" the screen by adding the following to your ~/.vimrc:

set t_ti= t_te=

Additional colors¶

Some terminal emulators support additional colors. The most common extension used by xterm-compatible terminals supports 256 colors. These can be generated by tput with seta{f,b} [0-255] when the TERM value has a -256color suffix. Some terminals also support full 24-bit colors, and any X11 color code can be written directly into a special escape sequence. (More infos) Only a few programs make use of anything beyond 256 colors, and tput doesn't know about them. Colors beyond 16 usually only apply to modern terminal emulators running in graphical environments.

The Virtual Terminal implemented in the Linux kernel supports only 16 colors, and the usual default terminfo entry for TERM=linux defines only 8. There is sometimes an alternate "linux-16color" that you can switch to, to get the other 8 colors.

Bash examples¶

Hardcoded colors¶

printf '%b\n' 'It is \033[31mnot\033[39m intelligent to use \033[32mhardcoded ANSI\033[39m codes!'

Colors using tput¶

Directly inside the echo:

echo "TPUT is a $(tput setaf 2)nice$(tput setaf 9) and $(tput setaf 5)user friendly$(tput setaf 9) terminal capability database."

With preset variables:

COL_NORM="$(tput setaf 9)"
COL_RED="$(tput setaf 1)"
COL_GREEN="$(tput setaf 2)"
echo "It's ${COL_RED}red${COL_NORM} and ${COL_GREEN}green${COL_NORM} - have you seen?"

Misc¶

HOME function

home() {
  # yes, actually not much shorter ;-)
  tput home
}

Silly but nice effect¶

#!/bin/bash

DATA[0]="     _/  _/    _/                            _/    "
DATA[1]="  _/_/_/_/_/  _/_/_/      _/_/_/    _/_/_/  _/_/_/ "
DATA[2]="   _/  _/    _/    _/  _/    _/  _/_/      _/    _/"
DATA[3]="_/_/_/_/_/  _/    _/  _/    _/      _/_/  _/    _/ "
DATA[4]=" _/  _/    _/_/_/      _/_/_/  _/_/_/    _/    _/  "

# virtual coordinate system is X*Y ${#DATA} * 5

REAL_OFFSET_X=0
REAL_OFFSET_Y=0

draw_char() {
  V_COORD_X=$1
  V_COORD_Y=$2

  tput cup $((REAL_OFFSET_Y + V_COORD_Y)) $((REAL_OFFSET_X + V_COORD_X))

  printf %c ${DATA[V_COORD_Y]:V_COORD_X:1}
}


trap 'exit 1' INT TERM
trap 'tput setaf 9; tput cvvis; clear' EXIT

tput civis
clear

while :; do

for ((c=1; c <= 7; c++)); do
  tput setaf $c
  for ((x=0; x<${#DATA[0]}; x++)); do
    for ((y=0; y<=4; y++)); do
      draw_char $x $y
    done
  done
done

done

Mandelbrot set¶

This is a slightly modified version of Charles Cooke's colorful Mandelbrot plot scripts (original w/ screenshot) -- ungolfed, optimized a bit, and without hard-coded terminal escapes. The colorBox function is memoized to collect tput output only when required and output a new escape only when a color change is needed. This limits the number of tput calls to at most 16, and reduces raw output by more than half. The doBash function uses integer arithmetic, but is still ksh93-compatible (run as e.g. bash ./mandelbrot to use it). The ksh93-only floating-point doKsh is almost 10x faster than doBash (thus the ksh shebang by default), but uses only features that don't make the Bash parser crash.

#!/usr/bin/env ksh

# Charles Cooke's 16-color Mandelbrot
# http://earth.gkhs.net/ccooke/shell.html
# Combined Bash/ksh93 flavors by Dan Douglas (ormaaj)

function doBash {
    typeset P Q X Y a b c i v x y
    for ((P=10**8,Q=P/100,X=320*Q/cols,Y=210*Q/lines,y=-105*Q,v=-220*Q,x=v;y<105*Q;x=v,y+=Y)); do
        for ((;x<P;a=b=i=c=0,x+=X)); do
            for ((;a**2+b**2<4*P**2&&i++<99;a=((c=a)**2-b**2)/P+x,b=2*c*b/P+y)); do :
            done
            colorBox $((i<99?i%16:0))
        done
        echo
    done
}

function doKsh {
    integer i
    float a b c x=2.2 y=-1.05 X=3.2/cols Y=2.1/lines
    while
        for ((a=b=i=0;(c=a)**2+b**2<=2&&i++<99&&(a=a**2-b**2+x,b=2*c*b+y);)); do :
        done
        . colorBox $((i<99?i%16:0))
        if ((x<1?!(x+=X):(y+=Y,x=-2.2))); then
            print
            ((y<1.05))
        fi
        do :
    done
}

function colorBox {
    (($1==lastclr)) || printf %s "${colrs[lastclr=$1]:=$(tput setaf "$1")}"
    printf '\u2588'
}

unset -v lastclr
((cols=$(tput cols)-1, lines=$(tput lines)))
typeset -a colrs
trap 'tput sgr0; echo' EXIT
${KSH_VERSION+. doKsh} ${BASH_VERSION+doBash}

A much more sophisticated version by Roland Mainz can be found here

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