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>Advanced Bash-Scripting Guide: An in-depth exploration of the art of shell scripting</TH
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><H1
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><A
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></A
>24.2. Local Variables</H1
><DIV
CLASS="VARIABLELIST"
><P
><B
><A
NAME="LOCALREF1"
></A
>What makes a variable
        <I
CLASS="FIRSTTERM"
>local</I
>?</B
></P
><DL
><DT
>local variables</DT
><DD
><P
>A variable declared as <I
CLASS="FIRSTTERM"
>local</I
>
	      is one that is visible only within the <A
HREF="special-chars.html#CODEBLOCKREF"
>block of code</A
> in which it
	      appears. It has local <A
HREF="subshells.html#SCOPEREF"
>scope</A
>.
	      In a function, a <I
CLASS="FIRSTTERM"
>local variable</I
> has
	      meaning only within that function block.

	      <A
NAME="AEN18568"
HREF="#FTN.AEN18568"
>[1]</A
>
      </P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="EX62"
></A
><P
><B
>Example 24-12. Local variable visibility</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# ex62.sh: Global and local variables inside a function.
   3&nbsp;
   4&nbsp;func ()
   5&nbsp;{
   6&nbsp;  local loc_var=23       # Declared as local variable.
   7&nbsp;  echo                   # Uses the 'local' builtin.
   8&nbsp;  echo "\"loc_var\" in function = $loc_var"
   9&nbsp;  global_var=999         # Not declared as local.
  10&nbsp;                         # Therefore, defaults to global. 
  11&nbsp;  echo "\"global_var\" in function = $global_var"
  12&nbsp;}  
  13&nbsp;
  14&nbsp;func
  15&nbsp;
  16&nbsp;# Now, to see if local variable "loc_var" exists outside the function.
  17&nbsp;
  18&nbsp;echo
  19&nbsp;echo "\"loc_var\" outside function = $loc_var"
  20&nbsp;                                      # $loc_var outside function = 
  21&nbsp;                                      # No, $loc_var not visible globally.
  22&nbsp;echo "\"global_var\" outside function = $global_var"
  23&nbsp;                                      # $global_var outside function = 999
  24&nbsp;                                      # $global_var is visible globally.
  25&nbsp;echo				      
  26&nbsp;
  27&nbsp;exit 0
  28&nbsp;#  In contrast to C, a Bash variable declared inside a function
  29&nbsp;#+ is local ONLY if declared as such.</PRE
></TD
></TR
></TABLE
><HR></DIV
><DIV
CLASS="CAUTION"
><TABLE
CLASS="CAUTION"
WIDTH="90%"
BORDER="0"
><TR
><TD
WIDTH="25"
ALIGN="CENTER"
VALIGN="TOP"
><IMG
SRC="common/caution.png"
HSPACE="5"
ALT="Caution"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
>Before a function is called, <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>all</I
></SPAN
>
	      variables declared within the function are invisible outside
	      the body of the function, not just those explicitly declared
	      as <I
CLASS="FIRSTTERM"
>local</I
>.
	        <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;
   3&nbsp;func ()
   4&nbsp;{
   5&nbsp;global_var=37    #  Visible only within the function block
   6&nbsp;                 #+ before the function has been called. 
   7&nbsp;}                #  END OF FUNCTION
   8&nbsp;
   9&nbsp;echo "global_var = $global_var"  # global_var =
  10&nbsp;                                 #  Function "func" has not yet been called,
  11&nbsp;                                 #+ so $global_var is not visible here.
  12&nbsp;
  13&nbsp;func
  14&nbsp;echo "global_var = $global_var"  # global_var = 37
  15&nbsp;                                 # Has been set by function call.</PRE
></TD
></TR
></TABLE
>
            </P
></TD
></TR
></TABLE
></DIV
><DIV
CLASS="NOTE"
><TABLE
CLASS="NOTE"
WIDTH="90%"
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><TD
WIDTH="25"
ALIGN="CENTER"
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><IMG
SRC="common/note.png"
HSPACE="5"
ALT="Note"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
><A
NAME="EXITVALANOMALY01"
></A
></P
><P
>As Evgeniy Ivanov points out, when declaring and
              setting a local variable in a single command, apparently the
              order of operations is to <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>first set the variable,
              and only afterwards restrict it to local scope</I
></SPAN
>.
              This is reflected in the <A
HREF="exit-status.html#EXITSTATUSREF"
>return value</A
>.</P
><P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;
   3&nbsp;echo "==OUTSIDE Function (global)=="
   4&nbsp;t=$(exit 1)
   5&nbsp;echo $?      # 1
   6&nbsp;             # As expected.
   7&nbsp;echo
   8&nbsp;
   9&nbsp;function0 ()
  10&nbsp;{
  11&nbsp;
  12&nbsp;echo "==INSIDE Function=="
  13&nbsp;echo "Global"
  14&nbsp;t0=$(exit 1)
  15&nbsp;echo $?      # 1
  16&nbsp;             # As expected.
  17&nbsp;
  18&nbsp;echo
  19&nbsp;echo "Local declared &#38; assigned in same command."
  20&nbsp;local t1=$(exit 1)
  21&nbsp;echo $?      # 0
  22&nbsp;             # Unexpected!
  23&nbsp;#  Apparently, the variable assignment takes place before
  24&nbsp;#+ the local declaration.
  25&nbsp;#+ The return value is for the latter.
  26&nbsp;
  27&nbsp;echo
  28&nbsp;echo "Local declared, then assigned (separate commands)."
  29&nbsp;local t2
  30&nbsp;t2=$(exit 1)
  31&nbsp;echo $?      # 1
  32&nbsp;             # As expected.
  33&nbsp;
  34&nbsp;}
  35&nbsp;
  36&nbsp;function0</PRE
></TD
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></DD
></DL
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="LOCVARRECUR"
></A
>24.2.1. Local variables and recursion.</H2
><P
><A
NAME="RECURSIONREF0"
></A
></P
><TABLE
CLASS="SIDEBAR"
BORDER="1"
CELLPADDING="5"
><TR
><TD
><DIV
CLASS="SIDEBAR"
><A
NAME="AEN18598"
></A
><P
><A
NAME="RECURSIONREF"
></A
></P
><P
><I
CLASS="FIRSTTERM"
>Recursion</I
> is an interesting
		and sometimes useful form of
		<I
CLASS="FIRSTTERM"
>self-reference</I
>.	<A
HREF="biblio.html#MAYERREF"
>Herbert Mayer</A
> defines it
		as <SPAN
CLASS="QUOTE"
>". . . expressing an algorithm by using a
		simpler version of that same algorithm . . ."</SPAN
></P
><P
>Consider a definition defined in terms of itself,
		  <A
NAME="AEN18607"
HREF="#FTN.AEN18607"
>[2]</A
>
		an expression implicit in its own expression,
		  <A
NAME="AEN18610"
HREF="#FTN.AEN18610"
>[3]</A
>
		<SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>a snake swallowing its own
		tail</I
></SPAN
>,
		  <A
NAME="AEN18614"
HREF="#FTN.AEN18614"
>[4]</A
>
		or . . . a function that calls itself.
		  <A
NAME="AEN18617"
HREF="#FTN.AEN18617"
>[5]</A
>
		</P
><P
><A
NAME="RECURSIONDEMO0"
></A
></P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="RECURSIONDEMO"
></A
><P
><B
>Example 24-13. Demonstration of a simple recursive function</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# recursion-demo.sh
   3&nbsp;# Demonstration of recursion.
   4&nbsp;
   5&nbsp;RECURSIONS=9   # How many times to recurse.
   6&nbsp;r_count=0      # Must be global. Why?
   7&nbsp;
   8&nbsp;recurse ()
   9&nbsp;{
  10&nbsp;  var="$1"
  11&nbsp;
  12&nbsp;  while [ "$var" -ge 0 ]
  13&nbsp;  do
  14&nbsp;    echo "Recursion count = "$r_count"  +-+  \$var = "$var""
  15&nbsp;    (( var-- )); (( r_count++ ))
  16&nbsp;    recurse "$var"  #  Function calls itself (recurses)
  17&nbsp;  done              #+ until what condition is met?
  18&nbsp;}
  19&nbsp;
  20&nbsp;recurse $RECURSIONS
  21&nbsp;
  22&nbsp;exit $?</PRE
></TD
></TR
></TABLE
><HR></DIV
><P
><A
NAME="RECURSIONDEMO02"
></A
></P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="RECURSIONDEMO2"
></A
><P
><B
>Example 24-14. Another simple demonstration</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# recursion-def.sh
   3&nbsp;# A script that defines "recursion" in a rather graphic way.
   4&nbsp;
   5&nbsp;RECURSIONS=10
   6&nbsp;r_count=0
   7&nbsp;sp=" "
   8&nbsp;
   9&nbsp;define_recursion ()
  10&nbsp;{
  11&nbsp;  ((r_count++))
  12&nbsp;  sp="$sp"" "
  13&nbsp;  echo -n "$sp"
  14&nbsp;  echo "\"The act of recurring ... \""   # Per 1913 Webster's dictionary.
  15&nbsp;
  16&nbsp;  while [ $r_count -le $RECURSIONS ]
  17&nbsp;  do
  18&nbsp;    define_recursion
  19&nbsp;  done
  20&nbsp;}
  21&nbsp;
  22&nbsp;echo
  23&nbsp;echo "Recursion: "
  24&nbsp;define_recursion
  25&nbsp;echo
  26&nbsp;
  27&nbsp;exit $?</PRE
></TD
></TR
></TABLE
><HR></DIV
></DIV
></TD
></TR
></TABLE
><P
>Local variables are a useful tool for writing recursive
	      code, but this practice generally involves a great deal of
	      computational overhead and is definitely
	      <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>not</I
></SPAN
> recommended in a shell script.

	         <A
NAME="AEN18632"
HREF="#FTN.AEN18632"
>[6]</A
>
	      
	      </P
><P
><A
NAME="FACTORIALREF"
></A
></P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="EX63"
></A
><P
><B
>Example 24-15. Recursion, using a local variable</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;
   3&nbsp;#               factorial
   4&nbsp;#               ---------
   5&nbsp;
   6&nbsp;
   7&nbsp;# Does bash permit recursion?
   8&nbsp;# Well, yes, but...
   9&nbsp;# It's so slow that you gotta have rocks in your head to try it.
  10&nbsp;
  11&nbsp;
  12&nbsp;MAX_ARG=5
  13&nbsp;E_WRONG_ARGS=85
  14&nbsp;E_RANGE_ERR=86
  15&nbsp;
  16&nbsp;
  17&nbsp;if [ -z "$1" ]
  18&nbsp;then
  19&nbsp;  echo "Usage: `basename $0` number"
  20&nbsp;  exit $E_WRONG_ARGS
  21&nbsp;fi
  22&nbsp;
  23&nbsp;if [ "$1" -gt $MAX_ARG ]
  24&nbsp;then
  25&nbsp;  echo "Out of range ($MAX_ARG is maximum)."
  26&nbsp;  #  Let's get real now.
  27&nbsp;  #  If you want greater range than this,
  28&nbsp;  #+ rewrite it in a Real Programming Language.
  29&nbsp;  exit $E_RANGE_ERR
  30&nbsp;fi  
  31&nbsp;
  32&nbsp;fact ()
  33&nbsp;{
  34&nbsp;  local number=$1
  35&nbsp;  #  Variable "number" must be declared as local,
  36&nbsp;  #+ otherwise this doesn't work.
  37&nbsp;  if [ "$number" -eq 0 ]
  38&nbsp;  then
  39&nbsp;    factorial=1    # Factorial of 0 = 1.
  40&nbsp;  else
  41&nbsp;    let "decrnum = number - 1"
  42&nbsp;    fact $decrnum  # Recursive function call (the function calls itself).
  43&nbsp;    let "factorial = $number * $?"
  44&nbsp;  fi
  45&nbsp;
  46&nbsp;  return $factorial
  47&nbsp;}
  48&nbsp;
  49&nbsp;fact $1
  50&nbsp;echo "Factorial of $1 is $?."
  51&nbsp;
  52&nbsp;exit 0</PRE
></TD
></TR
></TABLE
><HR></DIV
><P
>Also see <A
HREF="contributed-scripts.html#PRIMES"
>Example A-15</A
> for an example of
	      recursion in a script. Be aware that recursion is
	      resource-intensive and executes slowly, and is therefore
	      generally not appropriate in a script.</P
></DIV
></DIV
><H3
CLASS="FOOTNOTES"
>Notes</H3
><TABLE
BORDER="0"
CLASS="FOOTNOTES"
WIDTH="100%"
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="5%"
><A
NAME="FTN.AEN18568"
HREF="localvar.html#AEN18568"
>[1]</A
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="95%"
><P
>However, as Thomas Braunberger points out, a local
                variable declared in a function <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>is also visible
                to functions called by the parent
                function.</I
></SPAN
></P
><P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;
   3&nbsp;function1 ()
   4&nbsp;{
   5&nbsp;  local func1var=20
   6&nbsp;
   7&nbsp;  echo "Within function1, \$func1var = $func1var."
   8&nbsp;
   9&nbsp;  function2
  10&nbsp;}
  11&nbsp;
  12&nbsp;function2 ()
  13&nbsp;{
  14&nbsp;  echo "Within function2, \$func1var = $func1var."
  15&nbsp;}
  16&nbsp;
  17&nbsp;function1
  18&nbsp;
  19&nbsp;exit 0
  20&nbsp;
  21&nbsp;
  22&nbsp;# Output of the script:
  23&nbsp;
  24&nbsp;# Within function1, $func1var = 20.
  25&nbsp;# Within function2, $func1var = 20.</PRE
></TD
></TR
></TABLE
></P
><P
>This is documented in the Bash manual:</P
><P
><SPAN
CLASS="QUOTE"
>"Local can only be used within a function;
                it makes the variable name have a visible scope
                restricted to that function <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>and its
			children</I
></SPAN
>."</SPAN
> [emphasis added]
                <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>The ABS Guide author considers this behavior
                to be a bug.</I
></SPAN
></P
></TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="5%"
><A
NAME="FTN.AEN18607"
HREF="localvar.html#AEN18607"
>[2]</A
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="95%"
><P
>Otherwise known as
		  <I
CLASS="FIRSTTERM"
>redundancy</I
>.</P
></TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="5%"
><A
NAME="FTN.AEN18610"
HREF="localvar.html#AEN18610"
>[3]</A
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="95%"
><P
>Otherwise known as
		  <I
CLASS="FIRSTTERM"
>tautology</I
>.</P
></TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="5%"
><A
NAME="FTN.AEN18614"
HREF="localvar.html#AEN18614"
>[4]</A
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="95%"
><P
>Otherwise known as a
		  <I
CLASS="FIRSTTERM"
>metaphor</I
>.</P
></TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="5%"
><A
NAME="FTN.AEN18617"
HREF="localvar.html#AEN18617"
>[5]</A
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="95%"
><P
>Otherwise known as a
		  <I
CLASS="FIRSTTERM"
>recursive function</I
>.</P
></TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="5%"
><A
NAME="FTN.AEN18632"
HREF="localvar.html#AEN18632"
>[6]</A
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="95%"
><P
>Too many levels of recursion may crash a
		   script with a segfault.
		   <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;
   3&nbsp;#  Warning: Running this script could possibly lock up your system!
   4&nbsp;#  If you're lucky, it will segfault before using up all available memory.
   5&nbsp;
   6&nbsp;recursive_function ()		   
   7&nbsp;{
   8&nbsp;echo "$1"     # Makes the function do something, and hastens the segfault.
   9&nbsp;(( $1 &#60; $2 )) &#38;&#38; recursive_function $(( $1 + 1 )) $2;
  10&nbsp;#  As long as 1st parameter is less than 2nd,
  11&nbsp;#+ increment 1st and recurse.
  12&nbsp;}
  13&nbsp;
  14&nbsp;recursive_function 1 50000  # Recurse 50,000 levels!
  15&nbsp;#  Most likely segfaults (depending on stack size, set by ulimit -m).
  16&nbsp;
  17&nbsp;#  Recursion this deep might cause even a C program to segfault,
  18&nbsp;#+ by using up all the memory allotted to the stack.
  19&nbsp;
  20&nbsp;
  21&nbsp;echo "This will probably not print."
  22&nbsp;exit 0  # This script will not exit normally.
  23&nbsp;
  24&nbsp;#  Thanks, Stphane Chazelas.</PRE
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