Home Page Home Page
 Home | Software Development | System/Network Administration | Corporate Services | Resources | About Us
Monthly Server Management One-time Server Services Other Services
Network Administration Network Monitoring Network Security High Availability Load Balancing Data Backup and Recovery
Linux HOWTOs Linux Guides New RFCs Vulnerability list
Partners Careers Site Map
Bash, version2

34.1. Bash, version2

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

bash$ echo $BASH_VERSION
2.05.b.0(1)-release
	      
The version 2 update of the classic Bash scripting language added array variables, [1] string and parameter expansion, and a better method of indirect variable references, among other features.

Example 34-1. 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.
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'   # Bash
                           # Octal equivalent of characters.

exit 0

Example 34-2. 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 ${!variable} notation is greatly superior to the old "eval var1=\$$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

#  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

Example 34-3. Simple database application, using indirect variable referencing

#!/bin/bash
# resistor-inventory.sh
# Simple database application using indirect variable referencing.

# ============================================================== #
# 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=.25
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:"
  echo "There are ${!Inv} of [${!Val} ohm / ${!Pdissip} watt] resistors in stock."
  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?


# 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 34-4. Using arrays and other miscellaneous trickery to deal four random hands from a deck of cards

#!/bin/bash

# Cards:
# 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 %= $CARDS"
if [ "${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
    # Separate hands.

    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 0



# 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.

Notes

[1]

Chet Ramey promises associative arrays (a Perl feature) in a future Bash release. As of version 3, this has not yet happened.