Intro

The following is a Bash script found on Rosetta Code. This script implements a basic version of the game of Snake and seems to have been contributed by Mark J. Reed. Let’s see how it works. Here’s a screenshot after a particularly unsuccessful game:



@@@@                                                                  *
                                                                      
                                                                      
                                                                      
                                   GAME OVER                               
                                Time: 27 seconds                           
                                Final length: 4

Bash functions

My Bash is a bit rusty, so here is some relevant documentation:

typeset declares a variable of a certain type. The -i flag means integer, -a is for arrays.
clear, clears the screen.
tput output special operators to terminal, specifically (man terminfo):
- tput cup set cursor position
- tput civis set cursor to invisible
- tput cnorm set cursor back to normal
- tput el clear to end of line
stty set how terminal interacts, specifically:
- stty -echo disable input echo
- stty echo enable input echo
read [-t <t>] [-N <n>] [-s] <var> reads <n> number of characters from input in -s silent mode, on <t> seconds, put result in <var>.
$((RANDOM)) or $RANDOM draws a random integer in the range 0 - 32767.
Anything in double parentheses can do arithmetic and allows for a C-like syntax.

Code

We start by defining a main function and a center function.

file:snake.sh

function main {
  <<main>>
}

<<function-center>>
main "$@"

The center function seems to be used to print messages that are centered on the console.

«function-center»

function center {
  typeset -i width=$1 i
  shift
  typeset message=$(printf "$@")
  tput cuf $(( (width-${#message}) / 2 ))
  printf '%s' "$message"
}

Setup

We’ll see a lot of tput cup "$y" "$x" && printf '0' in here. This is just: put the character 0 on location x, y.

«main»

typeset -i game_over=0
typeset -i height=$(tput lines) width=$(tput cols)

# start out in the middle moving to the right
typeset -i dx dy hx=$(( width/2 )) hy=$(( height/2 ))
typeset -a sx=($hx) sy=($hy)
typeset -a timeout
clear
tput cup "$sy" "$sx" && printf '@'
tput cup $(( height/2+2 )) 0
center $width "Press h, j, k, l to move left, down, up, right"

The fx and fy coordinates tell us where the food is.

«main»

# place first food
typeset -i fx=hx fy=hy
while (( fx == hx && fy == hy )); do
  fx=$(( RANDOM % (width-2)+1 )) fy=$(( RANDOM % (height-2)+1 ))
done
tput cup "$fy" "$fx" && printf '*'

This little bit also makes the script work with Zshell.

«main»

# handle variations between shells
keypress=(-N 1) origin=0
if [[ -n $ZSH_VERSION ]]; then
  keypress=(-k)
  origin=1
fi

Set the terminal to non-echo mode and hide the cursor.

«main»

stty -echo
tput civis

Wait until a key is pressed.

«main»

typeset key
read "${keypress[@]}" -s key

«main»

typeset -i start_time=$(date +%s)

Main loop

We’ll look at several stages in the main loop:

read a key
handle potential key press
update game state and display

The loop ends when game_over is signaled.

«main»

tput cup "$(( height/2+2 ))" 0 && tput el
while (( ! game_over )); do
  <<read-key-stroke>>
  <<handle-input>>
  <<update-state>>
done

We read a potential key stroke with a timeout of say 0.1 seconds, by calling read -n 1 -t 0.1 -s key. After that function is done, and a key has been pressed, the variable $key will contain the corresponding character.

«read-key-stroke»

timeout=(-t $(printf '0.%04d' $(( 2000 / (${#sx[@]}+1) )) ) )
if [[ -z $key ]]; then
  read "${timeout[@]}" "${keypress[@]}" -s key
fi

The game uses hjkl to move around, like vim. If you prefer asdw, this is where to change that. Since I use Dvorak layout, this becomes aoe,.

«handle-input»

case "$key" in
  a) if (( dx !=  1 )); then dx=-1; dy=0; fi;;
  o) if (( dy != -1 )); then dy=1;  dx=0; fi;;
  ,) if (( dy !=  1 )); then dy=-1; dx=0; fi;;
  e) if (( dx != -1 )); then dx=1;  dy=0; fi;;
  q) game_over=1; tput cup 0 0 && print "Final food was at ($fx,$fy)";;
esac
key=

«update-state»

(( hx += dx, hy += dy ))
# if we try to go off screen, game over
if (( hx < 0 || hx >= width || hy < 0 || hy >= height )); then
   game_over=1
else
  # if we run into ourself, game over
  for (( i=0; i<${#sx[@]}; ++i )); do
    if (( hx==sx[i+origin] && hy==sy[i+origin] )); then
      game_over=1
      break
    fi
  done
fi
if (( game_over )); then
   break
fi
# add new spot
sx+=($hx) sy+=($hy)

The next bit also takes care of updating the screen.

«update-state»

if (( hx == fx  && hy == fy )); then
  # if we just ate some food, place some new food out
  ok=0
  while  (( ! ok )); do
    # make sure we don't put it under ourselves
    ok=1
    fx=$(( RANDOM % (width-2)+1 )) fy=$(( RANDOM % (height-2)+1 ))
    for (( i=0; i<${#sx[@]}; ++i )); do
      if (( fx == sx[i+origin] && fy == sy[i+origin] )); then
        ok=0
        break
      fi
    done
  done
  tput cup "$fy" "$fx" && printf '*'
  # and don't remove our tail because we've just grown by 1
else
  # if we didn't just eat food, remove our tail from its previous spot
  tput cup ${sy[origin]} ${sx[origin]} && printf ' '
  sx=( ${sx[@]:1} )
  sy=( ${sy[@]:1} )
fi
# draw our new head
tput cup "$hy" "$hx" && printf  '@'

Post mortem

When it is game over, some messages are printed and we return the terminal to normal behaviour.

«main»

typeset -i end_time=$(date +%s)
tput cup $(( height / 2 -1 )) 0 && center $width 'GAME OVER'
tput cup $(( height / 2 ))  0 &&
    center $width 'Time: %d seconds' $(( end_time - start_time ))
tput cup $(( height / 2 + 1 )) 0 &&
    center $width 'Final length: %d' ${#sx[@]}
echo
stty echo
tput cnorm