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>Advanced Bash-Scripting Guide: An in-depth exploration of the art of shell scripting</TH
></TR
><TR
><TD
WIDTH="10%"
ALIGN="left"
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><A
HREF="untyped.html"
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>Prev</A
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><TD
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>Chapter 4. Introduction to Variables and Parameters</TD
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><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="OTHERTYPESV"
></A
>4.4. Special Variable Types</H1
><DIV
CLASS="VARIABLELIST"
><DL
><DT
><TT
CLASS="REPLACEABLE"
><I
>Local variables</I
></TT
></DT
><DD
><P
>Variables <A
HREF="subshells.html#SCOPEREF"
>visible</A
> only within a <A
HREF="special-chars.html#CODEBLOCKREF"
>code block</A
> or function (see
	    also <A
HREF="localvar.html#LOCALREF"
>local variables</A
> in
	    <A
HREF="functions.html#FUNCTIONREF"
>functions</A
>)</P
></DD
><DT
><A
NAME="ENVREF"
></A
><TT
CLASS="REPLACEABLE"
><I
>Environmental variables</I
></TT
></DT
><DD
><P
>Variables that affect the behavior of the shell and
	      user interface</P
><DIV
CLASS="NOTE"
><TABLE
CLASS="NOTE"
WIDTH="90%"
BORDER="0"
><TR
><TD
WIDTH="25"
ALIGN="CENTER"
VALIGN="TOP"
><IMG
SRC="common/note.png"
HSPACE="5"
ALT="Note"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
>In a more general context, each <A
HREF="special-chars.html#PROCESSREF"
>process</A
> has an
		<SPAN
CLASS="QUOTE"
>"environment"</SPAN
>, that is, a group of
		variables that the process may reference. In this sense,
		the shell behaves like any other process.</P
><P
>Every time a shell starts, it creates shell variables that
		correspond to its own environmental variables. Updating
		or adding new environmental variables causes the
		shell to update its environment, and all the shell's
		<I
CLASS="FIRSTTERM"
>child processes</I
> (the commands it
		executes) inherit this environment.</P
></TD
></TR
></TABLE
></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
>The space allotted to the environment is limited.
	        Creating too many environmental variables or ones that use up
		excessive space may cause problems.</P
><P
>	          <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="SCREEN"
> <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>eval "`seq 10000 | sed -e 's/.*/export var&#38;=ZZZZZZZZZZZZZZ/'`"</B
></TT
>
 
 <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>du</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>bash: /usr/bin/du: Argument list too long</TT
>
 	          </PRE
></TD
></TR
></TABLE
>
	      </P
><P
>Note: this <SPAN
CLASS="QUOTE"
>"error"</SPAN
> has been fixed, as of
	        kernel version 2.6.23.</P
><P
>(Thank you, Stphane Chazelas for the clarification,
	        and for providing the above example.)</P
></TD
></TR
></TABLE
></DIV
><P
>If a script sets environmental variables, they need to be
	      <SPAN
CLASS="QUOTE"
>"exported,"</SPAN
> that is, reported to the
	      <I
CLASS="FIRSTTERM"
>environment</I
> local to
	      the script. This is the function of the <A
HREF="internal.html#EXPORTREF"
>export</A
> command.</P
><A
NAME="CHILDREF"
></A
><DIV
CLASS="NOTE"
><TABLE
CLASS="NOTE"
WIDTH="90%"
BORDER="0"
><TR
><TD
WIDTH="25"
ALIGN="CENTER"
VALIGN="TOP"
><IMG
SRC="common/note.png"
HSPACE="5"
ALT="Note"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
>A script can <B
CLASS="COMMAND"
>export</B
> variables only
	      to child <A
HREF="special-chars.html#PROCESSREF"
>processes</A
>,
	      that is, only to commands or processes which that
	      particular script initiates. A script invoked from
	      the command-line <TT
CLASS="REPLACEABLE"
><I
>cannot</I
></TT
>
	      export variables back to the command-line environment.
	      <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
><A
HREF="internal.html#FORKREF"
>Child processes</A
>
	      cannot export variables back to the parent processes that
	      spawned them.</I
></SPAN
></P
><P
><A
NAME="CHILDREF2"
></A
><TT
CLASS="USERINPUT"
><B
>Definition:</B
></TT
>
	      A <I
CLASS="FIRSTTERM"
>child process</I
> is a
	      subprocess launched by another process, its <A
HREF="internal.html#PARENTREF"
>parent</A
>.</P
></TD
></TR
></TABLE
></DIV
></DD
><DT
><A
NAME="POSPARAMREF1"
></A
><TT
CLASS="REPLACEABLE"
><I
>Positional parameters</I
></TT
></DT
><DD
><P
>Arguments passed to the script from the command
	      line
		<A
NAME="AEN2450"
HREF="#FTN.AEN2450"
>[1]</A
>
	      : <TT
CLASS="VARNAME"
>$0</TT
>, <TT
CLASS="VARNAME"
>$1</TT
>,
	      <TT
CLASS="VARNAME"
>$2</TT
>, <TT
CLASS="VARNAME"
>$3</TT
> . . .</P
><P
><A
NAME="SCRNAMEPARAM"
></A
><TT
CLASS="VARNAME"
>$0</TT
> is
	       the name of the script itself,
	      <TT
CLASS="VARNAME"
>$1</TT
> is the first argument,
	      <TT
CLASS="VARNAME"
>$2</TT
> the second, <TT
CLASS="VARNAME"
>$3</TT
>
	      the third, and so forth.

	      <A
NAME="AEN2464"
HREF="#FTN.AEN2464"
>[2]</A
>

	      <A
NAME="BRACKETNOTATION"
></A
>
	      After <TT
CLASS="VARNAME"
>$9</TT
>, the arguments must be enclosed
	      in brackets, for example, <TT
CLASS="VARNAME"
>${10}</TT
>,
	      <TT
CLASS="VARNAME"
>${11}</TT
>, <TT
CLASS="VARNAME"
>${12}</TT
>.</P
><P
>The special variables <A
HREF="variables2.html#APPREF"
>$* and $@</A
>
	      denote <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>all</I
></SPAN
> the positional parameters.</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="EX17"
></A
><P
><B
>Example 4-5. Positional Parameters</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;
   3&nbsp;# Call this script with at least 10 parameters, for example
   4&nbsp;# ./scriptname 1 2 3 4 5 6 7 8 9 10
   5&nbsp;MINPARAMS=10
   6&nbsp;
   7&nbsp;echo
   8&nbsp;
   9&nbsp;echo "The name of this script is \"$0\"."
  10&nbsp;# Adds ./ for current directory
  11&nbsp;echo "The name of this script is \"`basename $0`\"."
  12&nbsp;# Strips out path name info (see 'basename')
  13&nbsp;
  14&nbsp;echo
  15&nbsp;
  16&nbsp;if [ -n "$1" ]              # Tested variable is quoted.
  17&nbsp;then
  18&nbsp; echo "Parameter #1 is $1"  # Need quotes to escape #
  19&nbsp;fi 
  20&nbsp;
  21&nbsp;if [ -n "$2" ]
  22&nbsp;then
  23&nbsp; echo "Parameter #2 is $2"
  24&nbsp;fi 
  25&nbsp;
  26&nbsp;if [ -n "$3" ]
  27&nbsp;then
  28&nbsp; echo "Parameter #3 is $3"
  29&nbsp;fi 
  30&nbsp;
  31&nbsp;# ...
  32&nbsp;
  33&nbsp;
  34&nbsp;if [ -n "${10}" ]  # Parameters &#62; $9 must be enclosed in {brackets}.
  35&nbsp;then
  36&nbsp; echo "Parameter #10 is ${10}"
  37&nbsp;fi 
  38&nbsp;
  39&nbsp;echo "-----------------------------------"
  40&nbsp;echo "All the command-line parameters are: "$*""
  41&nbsp;
  42&nbsp;if [ $# -lt "$MINPARAMS" ]
  43&nbsp;then
  44&nbsp;  echo
  45&nbsp;  echo "This script needs at least $MINPARAMS command-line arguments!"
  46&nbsp;fi  
  47&nbsp;
  48&nbsp;echo
  49&nbsp;
  50&nbsp;exit 0</PRE
></TD
></TR
></TABLE
><HR></DIV
><P
><I
CLASS="FIRSTTERM"
>Bracket notation</I
> for positional
	      parameters leads to a fairly simple way of referencing
	      the <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>last</I
></SPAN
> argument passed to a
	      script on the command-line. This also requires <A
HREF="bash2.html#VARREFNEW"
>indirect referencing</A
>.</P
><P
><A
NAME="LASTARGREF"
></A
></P
><P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;args=$#           # Number of args passed.
   2&nbsp;lastarg=${!args}
   3&nbsp;# Note: This is an *indirect reference* to $args ...
   4&nbsp;
   5&nbsp;
   6&nbsp;# Or:       lastarg=${!#}             (Thanks, Chris Monson.)
   7&nbsp;# This is an *indirect reference* to the $# variable.
   8&nbsp;# Note that lastarg=${!$#} doesn't work.</PRE
></TD
></TR
></TABLE
></P
><P
>Some scripts can perform different operations,
	      depending on which name they are invoked with. For this
	      to work, the script needs to check <TT
CLASS="VARNAME"
>$0</TT
>,
	      the name it was invoked by.

	      <A
NAME="AEN2501"
HREF="#FTN.AEN2501"
>[3]</A
>
	      
	      There must also exist symbolic links to all the alternate
	      names of the script. See <A
HREF="external.html#HELLOL"
>Example 16-2</A
>.</P
><P
><A
NAME="NULLVAR"
></A
></P
><DIV
CLASS="TIP"
><TABLE
CLASS="TIP"
WIDTH="90%"
BORDER="0"
><TR
><TD
WIDTH="25"
ALIGN="CENTER"
VALIGN="TOP"
><IMG
SRC="common/tip.png"
HSPACE="5"
ALT="Tip"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
>If a script expects a command-line parameter
	      but is invoked without one, this may cause a <I
CLASS="FIRSTTERM"
>null
	      variable assignment</I
>, generally an undesirable
	      result. One way to prevent this is to append an extra
	      character to both sides of the assignment statement using
	      the expected positional parameter.  </P
></TD
></TR
></TABLE
></DIV
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;variable1_=$1_  # Rather than variable1=$1
   2&nbsp;# This will prevent an error, even if positional parameter is absent.
   3&nbsp;
   4&nbsp;critical_argument01=$variable1_
   5&nbsp;
   6&nbsp;# The extra character can be stripped off later, like so.
   7&nbsp;variable1=${variable1_/_/}
   8&nbsp;# Side effects only if $variable1_ begins with an underscore.
   9&nbsp;# This uses one of the parameter substitution templates discussed later.
  10&nbsp;# (Leaving out the replacement pattern results in a deletion.)
  11&nbsp;
  12&nbsp;#  A more straightforward way of dealing with this is
  13&nbsp;#+ to simply test whether expected positional parameters have been passed.
  14&nbsp;if [ -z $1 ]
  15&nbsp;then
  16&nbsp;  exit $E_MISSING_POS_PARAM
  17&nbsp;fi
  18&nbsp;
  19&nbsp;
  20&nbsp;#  However, as Fabian Kreutz points out,
  21&nbsp;#+ the above method may have unexpected side-effects.
  22&nbsp;#  A better method is parameter substitution:
  23&nbsp;#         ${1:-$DefaultVal}
  24&nbsp;#  See the "Parameter Substition" section
  25&nbsp;#+ in the "Variables Revisited" chapter.</PRE
></TD
></TR
></TABLE
><P
>---</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="EX18"
></A
><P
><B
>Example 4-6. <I
CLASS="FIRSTTERM"
>wh</I
>, <I
CLASS="FIRSTTERM"
>                whois</I
> domain name lookup</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# ex18.sh
   3&nbsp;
   4&nbsp;# Does a 'whois domain-name' lookup on any of 3 alternate servers:
   5&nbsp;#                    ripe.net, cw.net, radb.net
   6&nbsp;
   7&nbsp;# Place this script -- renamed 'wh' -- in /usr/local/bin
   8&nbsp;
   9&nbsp;# Requires symbolic links:
  10&nbsp;# ln -s /usr/local/bin/wh /usr/local/bin/wh-ripe
  11&nbsp;# ln -s /usr/local/bin/wh /usr/local/bin/wh-apnic
  12&nbsp;# ln -s /usr/local/bin/wh /usr/local/bin/wh-tucows
  13&nbsp;
  14&nbsp;E_NOARGS=75
  15&nbsp;
  16&nbsp;
  17&nbsp;if [ -z "$1" ]
  18&nbsp;then
  19&nbsp;  echo "Usage: `basename $0` [domain-name]"
  20&nbsp;  exit $E_NOARGS
  21&nbsp;fi
  22&nbsp;
  23&nbsp;# Check script name and call proper server.
  24&nbsp;case `basename $0` in    # Or:    case ${0##*/} in
  25&nbsp;    "wh"       ) whois $1@whois.tucows.com;;
  26&nbsp;    "wh-ripe"  ) whois $1@whois.ripe.net;;
  27&nbsp;    "wh-apnic" ) whois $1@whois.apnic.net;;
  28&nbsp;    "wh-cw"    ) whois $1@whois.cw.net;;
  29&nbsp;    *          ) echo "Usage: `basename $0` [domain-name]";;
  30&nbsp;esac 
  31&nbsp;
  32&nbsp;exit $?</PRE
></TD
></TR
></TABLE
><HR></DIV
><P
>---</P
><P
><A
NAME="SHIFTREF"
></A
></P
><P
>	      
	      
	      The <B
CLASS="COMMAND"
>shift</B
> command reassigns the positional
	      parameters, in effect shifting them to the left one notch.</P
><P
><TT
CLASS="VARNAME"
>$1</TT
> &#60;--- <TT
CLASS="VARNAME"
>$2</TT
>, <TT
CLASS="VARNAME"
>$2</TT
> &#60;--- <TT
CLASS="VARNAME"
>$3</TT
>, <TT
CLASS="VARNAME"
>$3</TT
> &#60;--- <TT
CLASS="VARNAME"
>$4</TT
>, etc.</P
><P
>The old <TT
CLASS="VARNAME"
>$1</TT
> disappears, but
	      <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
><TT
CLASS="VARNAME"
>$0</TT
> (the script name)
	      does not change</I
></SPAN
>. If you use a large number of
	      positional parameters to a script, <B
CLASS="COMMAND"
>shift</B
>
	      lets you access those past <TT
CLASS="LITERAL"
>10</TT
>, although
	      <A
HREF="othertypesv.html#BRACKETNOTATION"
>{bracket} notation</A
>
	      also permits this.</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="EX19"
></A
><P
><B
>Example 4-7. Using <I
CLASS="FIRSTTERM"
>shift</I
></B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# shft.sh: Using 'shift' to step through all the positional parameters.
   3&nbsp;
   4&nbsp;#  Name this script something like shft.sh,
   5&nbsp;#+ and invoke it with some parameters.
   6&nbsp;#+ For example:
   7&nbsp;#             sh shft.sh a b c def 83 barndoor
   8&nbsp;
   9&nbsp;until [ -z "$1" ]  # Until all parameters used up . . .
  10&nbsp;do
  11&nbsp;  echo -n "$1 "
  12&nbsp;  shift
  13&nbsp;done
  14&nbsp;
  15&nbsp;echo               # Extra linefeed.
  16&nbsp;
  17&nbsp;# But, what happens to the "used-up" parameters?
  18&nbsp;echo "$2"
  19&nbsp;#  Nothing echoes!
  20&nbsp;#  When $2 shifts into $1 (and there is no $3 to shift into $2)
  21&nbsp;#+ then $2 remains empty.
  22&nbsp;#  So, it is not a parameter *copy*, but a *move*.
  23&nbsp;
  24&nbsp;exit
  25&nbsp;
  26&nbsp;#  See also the echo-params.sh script for a "shiftless"
  27&nbsp;#+ alternative method of stepping through the positional params.</PRE
></TD
></TR
></TABLE
><HR></DIV
><P
>The <B
CLASS="COMMAND"
>shift</B
> command can take a numerical
	    parameter indicating how many positions to shift.</P
><P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# shift-past.sh
   3&nbsp;
   4&nbsp;shift 3    # Shift 3 positions.
   5&nbsp;#  n=3; shift $n
   6&nbsp;#  Has the same effect.
   7&nbsp;
   8&nbsp;echo "$1"
   9&nbsp;
  10&nbsp;exit 0
  11&nbsp;
  12&nbsp;# ======================== #
  13&nbsp;
  14&nbsp;
  15&nbsp;$ sh shift-past.sh 1 2 3 4 5
  16&nbsp;4
  17&nbsp;
  18&nbsp;#  However, as Eleni Fragkiadaki, points out,
  19&nbsp;#+ attempting a 'shift' past the number of
  20&nbsp;#+ positional parameters ($#) returns an exit status of 1,
  21&nbsp;#+ and the positional parameters themselves do not change.
  22&nbsp;#  This means possibly getting stuck in an endless loop. . . .
  23&nbsp;#  For example:
  24&nbsp;#      until [ -z "$1" ]
  25&nbsp;#      do
  26&nbsp;#         echo -n "$1 "
  27&nbsp;#         shift 20    #  If less than 20 pos params,
  28&nbsp;#      done           #+ then loop never ends!
  29&nbsp;#
  30&nbsp;# When in doubt, add a sanity check. . . .
  31&nbsp;#           shift 20 || break
  32&nbsp;#                    ^^^^^^^^</PRE
></TD
></TR
></TABLE
></P
><DIV
CLASS="NOTE"
><TABLE
CLASS="NOTE"
WIDTH="90%"
BORDER="0"
><TR
><TD
WIDTH="25"
ALIGN="CENTER"
VALIGN="TOP"
><IMG
SRC="common/note.png"
HSPACE="5"
ALT="Note"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
>The <B
CLASS="COMMAND"
>shift</B
> command works in a similar
	    fashion on parameters passed to a <A
HREF="functions.html#FUNCTIONREF"
>function</A
>.  See <A
HREF="assortedtips.html#MULTIPLICATION"
>Example 36-18</A
>.</P
></TD
></TR
></TABLE
></DIV
></DD
></DL
></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.AEN2450"
HREF="othertypesv.html#AEN2450"
>[1]</A
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="95%"
><P
>Note that <A
HREF="functions.html#PASSEDARGS"
><I
CLASS="FIRSTTERM"
>functions</I
>
		also take positional parameters</A
>.</P
></TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="5%"
><A
NAME="FTN.AEN2464"
HREF="othertypesv.html#AEN2464"
>[2]</A
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="95%"
><P
><A
NAME="ARG0"
></A
>The process calling the
		script sets the <TT
CLASS="VARNAME"
>$0</TT
> parameter. By
		convention, this parameter is the name of the script. See
		the <A
HREF="external.html#MANREF"
>manpage</A
> (manual page)
		for <B
CLASS="COMMAND"
>execv</B
>.</P
><P
>From the <I
CLASS="FIRSTTERM"
>command-line</I
>, however,
	        <TT
CLASS="VARNAME"
>$0</TT
> is the name of the shell.
	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="SCREEN"
> <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>echo $0</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>bash</TT
>
 
 <TT
CLASS="PROMPT"
>tcsh% </TT
><TT
CLASS="USERINPUT"
><B
>echo $0</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>tcsh</TT
></PRE
></TD
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></P
></TD
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><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="5%"
><A
NAME="FTN.AEN2501"
HREF="othertypesv.html#AEN2501"
>[3]</A
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="95%"
><P
>If the the script is <A
HREF="internal.html#SOURCEREF"
>sourced</A
> or <A
HREF="external.html#SYMLINKREF"
>symlinked</A
>, then
                 this will not work. It is safer to check <A
HREF="debugging.html#BASHSOURCEREF"
>$BASH_Source</A
>.</P
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