37.1. Bash, version 2

 The current version of Bash, the one you have running on your machine, is most likely version 2.xx.yy, 3.xx.yy, or 4.xx.yy.

bash$ echo $BASH_VERSION
3.2.25(1)-release

The version 2 update of the classic Bash scripting language added array variables, string and parameter expansion, and a better method of indirect variable references, among other features.

Example 37-1. String expansion

#!/bin/bash

# String expansion.
# Introduced with version 2 of Bash.

#  Strings of the form $'xxx'
#+ have the standard escaped characters interpreted.

echo $'Ringing bell 3 times \a \a \a'
     # May only ring once with certain terminals.
     # Or ...
     # May not ring at all, depending on terminal settings.
echo $'Three form feeds \f \f \f'
echo $'10 newlines \n\n\n\n\n\n\n\n\n\n'
echo $'\102\141\163\150'
     #   B   a   s   h
     # Octal equivalent of characters.

exit

Example 37-2. Indirect variable references - the new way

#!/bin/bash

# Indirect variable referencing.
# This has a few of the attributes of references in C++.

a=letter_of_alphabet
letter_of_alphabet=z

echo "a = $a"           # Direct reference.

echo "Now a = ${!a}"    # Indirect reference.
#  The ${!variable} notation is more intuitive than the old
#+ eval var1=\$$var2

echo

t=table_cell_3
table_cell_3=24
echo "t = ${!t}"                      # t = 24
table_cell_3=387
echo "Value of t changed to ${!t}"    # 387
# No 'eval' necessary.

#  This is useful for referencing members of an array or table,
#+ or for simulating a multi-dimensional array.
#  An indexing option (analogous to pointer arithmetic)
#+ would have been nice. Sigh.

exit 0

# See also, ind-ref.sh example.

Example 37-3. Simple database application, using indirect variable referencing

#!/bin/bash
# resistor-inventory.sh
# Simple database / table-lookup application.

# ============================================================== #
# Data

B1723_value=470                                   # Ohms
B1723_powerdissip=.25                             # Watts
B1723_colorcode="yellow-violet-brown"             # Color bands
B1723_loc=173                                     # Where they are
B1723_inventory=78                                # How many

B1724_value=1000
B1724_powerdissip=.25
B1724_colorcode="brown-black-red"
B1724_loc=24N
B1724_inventory=243

B1725_value=10000
B1725_powerdissip=.125
B1725_colorcode="brown-black-orange"
B1725_loc=24N
B1725_inventory=89

# ============================================================== #

echo

PS3='Enter catalog number: '

echo

select catalog_number in "B1723" "B1724" "B1725"
do
  Inv=${catalog_number}_inventory
  Val=${catalog_number}_value
  Pdissip=${catalog_number}_powerdissip
  Loc=${catalog_number}_loc
  Ccode=${catalog_number}_colorcode

  echo
  echo "Catalog number $catalog_number:"
  # Now, retrieve value, using indirect referencing.
  echo "There are ${!Inv} of  [${!Val} ohm / ${!Pdissip} watt]\
  resistors in stock."  #        ^             ^
  # As of Bash 4.2, you can replace "ohm" with \u2126 (using echo -e).
  echo "These are located in bin # ${!Loc}."
  echo "Their color code is \"${!Ccode}\"."

  break
done

echo; echo

# Exercises:
# ---------
# 1) Rewrite this script to read its data from an external file.
# 2) Rewrite this script to use arrays,
#+   rather than indirect variable referencing.
#    Which method is more straightforward and intuitive?
#    Which method is easier to code?

# Notes:
# -----
#  Shell scripts are inappropriate for anything except the most simple
#+ database applications, and even then it involves workarounds and kludges.
#  Much better is to use a language with native support for data structures,
#+ such as C++ or Java (or even Perl).

exit 0

Example 37-4. Using arrays and other miscellaneous trickery to deal four random hands from a deck of cards

#!/bin/bash
# cards.sh

# Deals four random hands from a deck of cards.

UNPICKED=0
PICKED=1

DUPE_CARD=99

LOWER_LIMIT=0
UPPER_LIMIT=51
CARDS_IN_SUIT=13
CARDS=52

declare -a Deck
declare -a Suits
declare -a Cards
#  It would have been easier to implement and more intuitive
#+ with a single, 3-dimensional array.
#  Perhaps a future version of Bash will support multidimensional arrays.

initialize_Deck ()
{
i=$LOWER_LIMIT
until [ "$i" -gt $UPPER_LIMIT ]
do
  Deck[i]=$UNPICKED   # Set each card of "Deck" as unpicked.
  let "i += 1"
done
echo
}

initialize_Suits ()
{
Suits[0]=C #Clubs
Suits[1]=D #Diamonds
Suits[2]=H #Hearts
Suits[3]=S #Spades
}

initialize_Cards ()
{
Cards=(2 3 4 5 6 7 8 9 10 J Q K A)
# Alternate method of initializing an array.
}

pick_a_card ()
{
card_number=$RANDOM
let "card_number %= $CARDS" # Restrict range to 0 - 51, i.e., 52 cards.
if [ "${Deck[card_number]}" -eq $UNPICKED ]
then
  Deck[card_number]=$PICKED
  return $card_number
else  
  return $DUPE_CARD
fi
}

parse_card ()
{
number=$1
let "suit_number = number / CARDS_IN_SUIT"
suit=${Suits[suit_number]}
echo -n "$suit-"
let "card_no = number % CARDS_IN_SUIT"
Card=${Cards[card_no]}
printf %-4s $Card
# Print cards in neat columns.
}

seed_random ()  # Seed random number generator.
{               # What happens if you don't do this?
seed=`eval date +%s`
let "seed %= 32766"
RANDOM=$seed
#  What are some other methods
#+ of seeding the random number generator?
}

deal_cards ()
{
echo

cards_picked=0
while [ "$cards_picked" -le $UPPER_LIMIT ]
do
  pick_a_card
  t=$?

  if [ "$t" -ne $DUPE_CARD ]
  then
    parse_card $t

    u=$cards_picked+1
    # Change back to 1-based indexing (temporarily). Why?
    let "u %= $CARDS_IN_SUIT"
    if [ "$u" -eq 0 ]   # Nested if/then condition test.
    then
     echo
     echo
    fi                  # Each hand set apart with a blank line.

    let "cards_picked += 1"
  fi  
done  

echo

return 0
}

# Structured programming:
# Entire program logic modularized in functions.

#===============
seed_random
initialize_Deck
initialize_Suits
initialize_Cards
deal_cards
#===============

exit


# Exercise 1:
# Add comments to thoroughly document this script.

# Exercise 2:
# Add a routine (function) to print out each hand sorted in suits.
# You may add other bells and whistles if you like.

# Exercise 3:
# Simplify and streamline the logic of the script.