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>Advanced Bash-Scripting Guide: An in-depth exploration of the art of shell scripting</TH
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><A
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>Chapter 37. Bash, versions 2, 3, and 4</H1
><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="BASHVER2"
></A
>37.1. Bash, version 2</H1
><P
><A
NAME="BASH2REF"
></A
></P
><P
>        The current version of <I
CLASS="FIRSTTERM"
>Bash</I
>, the one
	you have running on your machine, is most likely version 2.xx.yy,
	3.xx.yy, or 4.xx.yy.
	      <TABLE
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><TD
><PRE
CLASS="SCREEN"
> <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>echo $BASH_VERSION</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>3.2.25(1)-release</TT
>
 	      </PRE
></TD
></TR
></TABLE
>
      </P
><P
>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.</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="EX77"
></A
><P
><B
>Example 37-1. String expansion</B
></P
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><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;
   3&nbsp;# String expansion.
   4&nbsp;# Introduced with version 2 of Bash.
   5&nbsp;
   6&nbsp;#  Strings of the form $'xxx'
   7&nbsp;#+ have the standard escaped characters interpreted. 
   8&nbsp;
   9&nbsp;echo $'Ringing bell 3 times \a \a \a'
  10&nbsp;     # May only ring once with certain terminals.
  11&nbsp;     # Or ...
  12&nbsp;     # May not ring at all, depending on terminal settings.
  13&nbsp;echo $'Three form feeds \f \f \f'
  14&nbsp;echo $'10 newlines \n\n\n\n\n\n\n\n\n\n'
  15&nbsp;echo $'\102\141\163\150'
  16&nbsp;     #   B   a   s   h
  17&nbsp;     # Octal equivalent of characters.
  18&nbsp;
  19&nbsp;exit</PRE
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><P
><A
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><DIV
CLASS="EXAMPLE"
><HR><A
NAME="EX78"
></A
><P
><B
>Example 37-2. Indirect variable references - the new way</B
></P
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><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;
   3&nbsp;# Indirect variable referencing.
   4&nbsp;# This has a few of the attributes of references in C++.
   5&nbsp;
   6&nbsp;
   7&nbsp;a=letter_of_alphabet
   8&nbsp;letter_of_alphabet=z
   9&nbsp;
  10&nbsp;echo "a = $a"           # Direct reference.
  11&nbsp;
  12&nbsp;echo "Now a = ${!a}"    # Indirect reference.
  13&nbsp;#  The ${!variable} notation is more intuitive than the old
  14&nbsp;#+ eval var1=\$$var2
  15&nbsp;
  16&nbsp;echo
  17&nbsp;
  18&nbsp;t=table_cell_3
  19&nbsp;table_cell_3=24
  20&nbsp;echo "t = ${!t}"                      # t = 24
  21&nbsp;table_cell_3=387
  22&nbsp;echo "Value of t changed to ${!t}"    # 387
  23&nbsp;# No 'eval' necessary.
  24&nbsp;
  25&nbsp;#  This is useful for referencing members of an array or table,
  26&nbsp;#+ or for simulating a multi-dimensional array.
  27&nbsp;#  An indexing option (analogous to pointer arithmetic)
  28&nbsp;#+ would have been nice. Sigh.
  29&nbsp;
  30&nbsp;exit 0
  31&nbsp;
  32&nbsp;# See also, ind-ref.sh example.</PRE
></TD
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><HR></DIV
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="RESISTOR"
></A
><P
><B
>Example 37-3. Simple database application, using indirect variable
	  referencing</B
></P
><TABLE
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><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# resistor-inventory.sh
   3&nbsp;# Simple database / table-lookup application.
   4&nbsp;
   5&nbsp;# ============================================================== #
   6&nbsp;# Data
   7&nbsp;
   8&nbsp;B1723_value=470                                   # Ohms
   9&nbsp;B1723_powerdissip=.25                             # Watts
  10&nbsp;B1723_colorcode="yellow-violet-brown"             # Color bands
  11&nbsp;B1723_loc=173                                     # Where they are
  12&nbsp;B1723_inventory=78                                # How many
  13&nbsp;
  14&nbsp;B1724_value=1000
  15&nbsp;B1724_powerdissip=.25
  16&nbsp;B1724_colorcode="brown-black-red"
  17&nbsp;B1724_loc=24N
  18&nbsp;B1724_inventory=243
  19&nbsp;
  20&nbsp;B1725_value=10000
  21&nbsp;B1725_powerdissip=.125
  22&nbsp;B1725_colorcode="brown-black-orange"
  23&nbsp;B1725_loc=24N
  24&nbsp;B1725_inventory=89
  25&nbsp;
  26&nbsp;# ============================================================== #
  27&nbsp;
  28&nbsp;
  29&nbsp;echo
  30&nbsp;
  31&nbsp;PS3='Enter catalog number: '
  32&nbsp;
  33&nbsp;echo
  34&nbsp;
  35&nbsp;select catalog_number in "B1723" "B1724" "B1725"
  36&nbsp;do
  37&nbsp;  Inv=${catalog_number}_inventory
  38&nbsp;  Val=${catalog_number}_value
  39&nbsp;  Pdissip=${catalog_number}_powerdissip
  40&nbsp;  Loc=${catalog_number}_loc
  41&nbsp;  Ccode=${catalog_number}_colorcode
  42&nbsp;
  43&nbsp;  echo
  44&nbsp;  echo "Catalog number $catalog_number:"
  45&nbsp;  # Now, retrieve value, using indirect referencing.
  46&nbsp;  echo "There are ${!Inv} of  [${!Val} ohm / ${!Pdissip} watt]\
  47&nbsp;  resistors in stock."  #        ^             ^
  48&nbsp;  # As of Bash 4.2, you can replace "ohm" with \u2126 (using echo -e).
  49&nbsp;  echo "These are located in bin # ${!Loc}."
  50&nbsp;  echo "Their color code is \"${!Ccode}\"."
  51&nbsp;
  52&nbsp;  break
  53&nbsp;done
  54&nbsp;
  55&nbsp;echo; echo
  56&nbsp;
  57&nbsp;# Exercises:
  58&nbsp;# ---------
  59&nbsp;# 1) Rewrite this script to read its data from an external file.
  60&nbsp;# 2) Rewrite this script to use arrays,
  61&nbsp;#+   rather than indirect variable referencing.
  62&nbsp;#    Which method is more straightforward and intuitive?
  63&nbsp;#    Which method is easier to code?
  64&nbsp;
  65&nbsp;
  66&nbsp;# Notes:
  67&nbsp;# -----
  68&nbsp;#  Shell scripts are inappropriate for anything except the most simple
  69&nbsp;#+ database applications, and even then it involves workarounds and kludges.
  70&nbsp;#  Much better is to use a language with native support for data structures,
  71&nbsp;#+ such as C++ or Java (or even Perl).
  72&nbsp;
  73&nbsp;exit 0</PRE
></TD
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><DIV
CLASS="EXAMPLE"
><HR><A
NAME="CARDS"
></A
><P
><B
>Example 37-4. Using arrays and other miscellaneous trickery
	  to deal four random hands from a deck of cards</B
></P
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><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# cards.sh
   3&nbsp;
   4&nbsp;# Deals four random hands from a deck of cards.
   5&nbsp;
   6&nbsp;UNPICKED=0
   7&nbsp;PICKED=1
   8&nbsp;
   9&nbsp;DUPE_CARD=99
  10&nbsp;
  11&nbsp;LOWER_LIMIT=0
  12&nbsp;UPPER_LIMIT=51
  13&nbsp;CARDS_IN_SUIT=13
  14&nbsp;CARDS=52
  15&nbsp;
  16&nbsp;declare -a Deck
  17&nbsp;declare -a Suits
  18&nbsp;declare -a Cards
  19&nbsp;#  It would have been easier to implement and more intuitive
  20&nbsp;#+ with a single, 3-dimensional array.
  21&nbsp;#  Perhaps a future version of Bash will support multidimensional arrays.
  22&nbsp;
  23&nbsp;
  24&nbsp;initialize_Deck ()
  25&nbsp;{
  26&nbsp;i=$LOWER_LIMIT
  27&nbsp;until [ "$i" -gt $UPPER_LIMIT ]
  28&nbsp;do
  29&nbsp;  Deck[i]=$UNPICKED   # Set each card of "Deck" as unpicked.
  30&nbsp;  let "i += 1"
  31&nbsp;done
  32&nbsp;echo
  33&nbsp;}
  34&nbsp;
  35&nbsp;initialize_Suits ()
  36&nbsp;{
  37&nbsp;Suits[0]=C #Clubs
  38&nbsp;Suits[1]=D #Diamonds
  39&nbsp;Suits[2]=H #Hearts
  40&nbsp;Suits[3]=S #Spades
  41&nbsp;}
  42&nbsp;
  43&nbsp;initialize_Cards ()
  44&nbsp;{
  45&nbsp;Cards=(2 3 4 5 6 7 8 9 10 J Q K A)
  46&nbsp;# Alternate method of initializing an array.
  47&nbsp;}
  48&nbsp;
  49&nbsp;pick_a_card ()
  50&nbsp;{
  51&nbsp;card_number=$RANDOM
  52&nbsp;let "card_number %= $CARDS" # Restrict range to 0 - 51, i.e., 52 cards.
  53&nbsp;if [ "${Deck[card_number]}" -eq $UNPICKED ]
  54&nbsp;then
  55&nbsp;  Deck[card_number]=$PICKED
  56&nbsp;  return $card_number
  57&nbsp;else  
  58&nbsp;  return $DUPE_CARD
  59&nbsp;fi
  60&nbsp;}
  61&nbsp;
  62&nbsp;parse_card ()
  63&nbsp;{
  64&nbsp;number=$1
  65&nbsp;let "suit_number = number / CARDS_IN_SUIT"
  66&nbsp;suit=${Suits[suit_number]}
  67&nbsp;echo -n "$suit-"
  68&nbsp;let "card_no = number % CARDS_IN_SUIT"
  69&nbsp;Card=${Cards[card_no]}
  70&nbsp;printf %-4s $Card
  71&nbsp;# Print cards in neat columns.
  72&nbsp;}
  73&nbsp;
  74&nbsp;seed_random ()  # Seed random number generator.
  75&nbsp;{               # What happens if you don't do this?
  76&nbsp;seed=`eval date +%s`
  77&nbsp;let "seed %= 32766"
  78&nbsp;RANDOM=$seed
  79&nbsp;} # Consider other methods of seeding the random number generator.
  80&nbsp;
  81&nbsp;deal_cards ()
  82&nbsp;{
  83&nbsp;echo
  84&nbsp;
  85&nbsp;cards_picked=0
  86&nbsp;while [ "$cards_picked" -le $UPPER_LIMIT ]
  87&nbsp;do
  88&nbsp;  pick_a_card
  89&nbsp;  t=$?
  90&nbsp;
  91&nbsp;  if [ "$t" -ne $DUPE_CARD ]
  92&nbsp;  then
  93&nbsp;    parse_card $t
  94&nbsp;
  95&nbsp;    u=$cards_picked+1
  96&nbsp;    # Change back to 1-based indexing, temporarily. Why?
  97&nbsp;    let "u %= $CARDS_IN_SUIT"
  98&nbsp;    if [ "$u" -eq 0 ]   # Nested if/then condition test.
  99&nbsp;    then
 100&nbsp;     echo
 101&nbsp;     echo
 102&nbsp;    fi                  # Each hand set apart with a blank line.
 103&nbsp;
 104&nbsp;    let "cards_picked += 1"
 105&nbsp;  fi  
 106&nbsp;done  
 107&nbsp;
 108&nbsp;echo
 109&nbsp;
 110&nbsp;return 0
 111&nbsp;}
 112&nbsp;
 113&nbsp;
 114&nbsp;# Structured programming:
 115&nbsp;# Entire program logic modularized in functions.
 116&nbsp;
 117&nbsp;#===============
 118&nbsp;seed_random
 119&nbsp;initialize_Deck
 120&nbsp;initialize_Suits
 121&nbsp;initialize_Cards
 122&nbsp;deal_cards
 123&nbsp;#===============
 124&nbsp;
 125&nbsp;exit
 126&nbsp;
 127&nbsp;
 128&nbsp;
 129&nbsp;# Exercise 1:
 130&nbsp;# Add comments to thoroughly document this script.
 131&nbsp;
 132&nbsp;# Exercise 2:
 133&nbsp;# Add a routine (function) to print out each hand sorted in suits.
 134&nbsp;# You may add other bells and whistles if you like.
 135&nbsp;
 136&nbsp;# Exercise 3:
 137&nbsp;# Simplify and streamline the logic of the script.</PRE
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