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>Advanced Bash-Scripting Guide: An in-depth exploration of the art of shell scripting</TH
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>Chapter 4. Introduction to Variables and Parameters</TD
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><H1
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><A
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>4.3. Bash Variables Are Untyped</H1
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><P
>Unlike many other programming languages, Bash does not segregate
	its variables by <SPAN
CLASS="QUOTE"
>"type."</SPAN
> Essentially, <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>Bash
	variables are character strings</I
></SPAN
>, but, depending on
	context, Bash permits arithmetic operations and comparisons on
	variables. The determining factor is whether the value of a
	variable contains only digits.</P
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>Example 4-4. Integer or string?</B
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>   1&nbsp;#!/bin/bash
   2&nbsp;# int-or-string.sh
   3&nbsp;
   4&nbsp;a=2334                   # Integer.
   5&nbsp;let "a += 1"
   6&nbsp;echo "a = $a "           # a = 2335
   7&nbsp;echo                     # Integer, still.
   8&nbsp;
   9&nbsp;
  10&nbsp;b=${a/23/BB}             # Substitute "BB" for "23".
  11&nbsp;                         # This transforms $b into a string.
  12&nbsp;echo "b = $b"            # b = BB35
  13&nbsp;declare -i b             # Declaring it an integer doesn't help.
  14&nbsp;echo "b = $b"            # b = BB35
  15&nbsp;
  16&nbsp;let "b += 1"             # BB35 + 1
  17&nbsp;echo "b = $b"            # b = 1
  18&nbsp;echo                     # Bash sets the "integer value" of a string to 0.
  19&nbsp;
  20&nbsp;c=BB34
  21&nbsp;echo "c = $c"            # c = BB34
  22&nbsp;d=${c/BB/23}             # Substitute "23" for "BB".
  23&nbsp;                         # This makes $d an integer.
  24&nbsp;echo "d = $d"            # d = 2334
  25&nbsp;let "d += 1"             # 2334 + 1
  26&nbsp;echo "d = $d"            # d = 2335
  27&nbsp;echo
  28&nbsp;
  29&nbsp;
  30&nbsp;# What about null variables?
  31&nbsp;e=''                     # ... Or e="" ... Or e=
  32&nbsp;echo "e = $e"            # e =
  33&nbsp;let "e += 1"             # Arithmetic operations allowed on a null variable?
  34&nbsp;echo "e = $e"            # e = 1
  35&nbsp;echo                     # Null variable transformed into an integer.
  36&nbsp;
  37&nbsp;# What about undeclared variables?
  38&nbsp;echo "f = $f"            # f =
  39&nbsp;let "f += 1"             # Arithmetic operations allowed?
  40&nbsp;echo "f = $f"            # f = 1
  41&nbsp;echo                     # Undeclared variable transformed into an integer.
  42&nbsp;#
  43&nbsp;# However ...
  44&nbsp;let "f /= $undecl_var"   # Divide by zero?
  45&nbsp;#   let: f /= : syntax error: operand expected (error token is " ")
  46&nbsp;# Syntax error! Variable $undecl_var is not set to zero here!
  47&nbsp;#
  48&nbsp;# But still ...
  49&nbsp;let "f /= 0"
  50&nbsp;#   let: f /= 0: division by 0 (error token is "0")
  51&nbsp;# Expected behavior.
  52&nbsp;
  53&nbsp;
  54&nbsp;#  Bash (usually) sets the "integer value" of null to zero
  55&nbsp;#+ when performing an arithmetic operation.
  56&nbsp;#  But, don't try this at home, folks!
  57&nbsp;#  It's undocumented and probably non-portable behavior.
  58&nbsp;
  59&nbsp;
  60&nbsp;# Conclusion: Variables in Bash are untyped,
  61&nbsp;#+ with all attendant consequences.
  62&nbsp;
  63&nbsp;exit $?</PRE
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><P
>Untyped variables are both a blessing and a curse. They permit
	more flexibility in scripting and make it easier to grind out
	lines of code (and give you enough rope to hang yourself!).
	However, they likewise permit subtle errors to creep in
	and encourage sloppy programming habits.</P
><P
>To lighten the burden of keeping track of variable
        types in a script, Bash <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>does</I
></SPAN
> permit
	<A
HREF="declareref.html"
>declaring</A
> variables.</P
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