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>Advanced Bash-Scripting Guide: An in-depth exploration of the art of shell scripting</TH
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><DIV
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><H1
><A
NAME="LIST-CONS"
></A
>Chapter 26. List Constructs</H1
><P
><A
NAME="LISTCONSREF"
></A
></P
><P
>The <I
CLASS="FIRSTTERM"
>and list</I
> and <I
CLASS="FIRSTTERM"
>or
	list</I
> constructs provide a means of processing a
	number of commands consecutively. These can effectively replace
	complex nested <A
HREF="tests.html#TESTCONSTRUCTS1"
>if/then</A
>
	or even <A
HREF="testbranch.html#CASEESAC1"
>case</A
> statements.</P
><DIV
CLASS="VARIABLELIST"
><P
><B
><A
NAME="LCONS1"
></A
>Chaining together commands</B
></P
><DL
><DT
>and list</DT
><DD
><P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;command-1 &#38;&#38; command-2 &#38;&#38; command-3 &#38;&#38; ... command-n</PRE
></TD
></TR
></TABLE
>
	      Each command executes in turn, provided that
	      the previous command has given a return value of
	      <TT
CLASS="REPLACEABLE"
><I
>true</I
></TT
> (zero). At the first
	      <TT
CLASS="REPLACEABLE"
><I
>false</I
></TT
> (non-zero) return, the
	      command chain terminates (the first command returning
	      <TT
CLASS="REPLACEABLE"
><I
>false</I
></TT
> is the last one to
	      execute).</P
><P
>An interesting use of a two-condition <I
CLASS="FIRSTTERM"
>and
	      list</I
> from an early version of YongYe's <A
HREF="http://bash.deta.in/Tetris_Game.sh"
TARGET="_top"
>Tetris
	      game script</A
>:</P
><P
>           <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;equation()
   2&nbsp;
   3&nbsp;{  # core algorithm used for doubling and halving the coordinates
   4&nbsp;   [[ ${cdx} ]] &#38;&#38; ((y=cy+(ccy-cdy)${2}2))
   5&nbsp;   eval ${1}+=\"${x} ${y} \"
   6&nbsp;}</PRE
></TD
></TR
></TABLE
>
           </P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="EX64"
></A
><P
><B
>Example 26-1. Using an <I
CLASS="FIRSTTERM"
>and list</I
> to test
	      for command-line arguments</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# and list
   3&nbsp;
   4&nbsp;if [ ! -z "$1" ] &#38;&#38; echo "Argument #1 = $1" &#38;&#38; [ ! -z "$2" ] &#38;&#38; \
   5&nbsp;#                ^^                         ^^               ^^
   6&nbsp;echo "Argument #2 = $2"
   7&nbsp;then
   8&nbsp;  echo "At least 2 arguments passed to script."
   9&nbsp;  # All the chained commands return true.
  10&nbsp;else
  11&nbsp;  echo "Fewer than 2 arguments passed to script."
  12&nbsp;  # At least one of the chained commands returns false.
  13&nbsp;fi  
  14&nbsp;# Note that "if [ ! -z $1 ]" works, but its alleged equivalent,
  15&nbsp;#   "if [ -n $1 ]" does not.
  16&nbsp;#     However, quoting fixes this.
  17&nbsp;#  if "[ -n "$1" ]" works.
  18&nbsp;#           ^  ^    Careful!
  19&nbsp;# It is always best to QUOTE the variables being tested.
  20&nbsp;
  21&nbsp;
  22&nbsp;# This accomplishes the same thing, using "pure" if/then statements.
  23&nbsp;if [ ! -z "$1" ]
  24&nbsp;then
  25&nbsp;  echo "Argument #1 = $1"
  26&nbsp;fi
  27&nbsp;if [ ! -z "$2" ]
  28&nbsp;then
  29&nbsp;  echo "Argument #2 = $2"
  30&nbsp;  echo "At least 2 arguments passed to script."
  31&nbsp;else
  32&nbsp;  echo "Fewer than 2 arguments passed to script."
  33&nbsp;fi
  34&nbsp;# It's longer and more ponderous than using an "and list".
  35&nbsp;
  36&nbsp;
  37&nbsp;exit $?</PRE
></TD
></TR
></TABLE
><HR></DIV
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="ANDLIST2"
></A
><P
><B
>Example 26-2. Another command-line arg test using an <I
CLASS="FIRSTTERM"
>and
	      list</I
></B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;
   3&nbsp;ARGS=1        # Number of arguments expected.
   4&nbsp;E_BADARGS=85  # Exit value if incorrect number of args passed.
   5&nbsp;
   6&nbsp;test $# -ne $ARGS &#38;&#38; \
   7&nbsp;#    ^^^^^^^^^^^^ condition #1
   8&nbsp;echo "Usage: `basename $0` $ARGS argument(s)" &#38;&#38; exit $E_BADARGS
   9&nbsp;#                                             ^^
  10&nbsp;#  If condition #1 tests true (wrong number of args passed to script),
  11&nbsp;#+ then the rest of the line executes, and script terminates.
  12&nbsp;
  13&nbsp;# Line below executes only if the above test fails.
  14&nbsp;echo "Correct number of arguments passed to this script."
  15&nbsp;
  16&nbsp;exit 0
  17&nbsp;
  18&nbsp;# To check exit value, do a "echo $?" after script termination.</PRE
></TD
></TR
></TABLE
><HR></DIV
><P
><A
NAME="ANDDEFAULT"
></A
></P
><P
>	      Of course, an <I
CLASS="FIRSTTERM"
>and list</I
> can also
	      <I
CLASS="FIRSTTERM"
>set</I
> variables to a default value.
	        <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;arg1=$@ &#38;&#38; [ -z "$arg1" ] &#38;&#38; arg1=DEFAULT
   2&nbsp;		
   3&nbsp;              # Set $arg1 to command-line arguments, if any.
   4&nbsp;              # But . . . set to DEFAULT if not specified on command-line.</PRE
></TD
></TR
></TABLE
>
            </P
></DD
><DT
><A
NAME="ORLISTREF"
></A
>or list</DT
><DD
><P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;command-1 || command-2 || command-3 || ... command-n</PRE
></TD
></TR
></TABLE
>
	      Each command executes in turn for as long as the previous
	      command returns <SPAN
CLASS="RETURNVALUE"
>false</SPAN
>. At
	      the first <SPAN
CLASS="RETURNVALUE"
>true</SPAN
> return, the
	      command chain terminates (the first command returning
	      <SPAN
CLASS="RETURNVALUE"
>true</SPAN
> is the last one to
	      execute). This is obviously the inverse of the <SPAN
CLASS="QUOTE"
>"and
	      list"</SPAN
>.</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="EX65"
></A
><P
><B
>Example 26-3. Using <I
CLASS="FIRSTTERM"
>or lists</I
> in combination
	      with an <I
CLASS="FIRSTTERM"
>and list</I
></B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;
   3&nbsp;#  delete.sh, a not-so-cunning file deletion utility.
   4&nbsp;#  Usage: delete filename
   5&nbsp;
   6&nbsp;E_BADARGS=85
   7&nbsp;
   8&nbsp;if [ -z "$1" ]
   9&nbsp;then
  10&nbsp;  echo "Usage: `basename $0` filename"
  11&nbsp;  exit $E_BADARGS  # No arg? Bail out.
  12&nbsp;else  
  13&nbsp;  file=$1          # Set filename.
  14&nbsp;fi  
  15&nbsp;
  16&nbsp;
  17&nbsp;[ ! -f "$file" ] &#38;&#38; echo "File \"$file\" not found. \
  18&nbsp;Cowardly refusing to delete a nonexistent file."
  19&nbsp;# AND LIST, to give error message if file not present.
  20&nbsp;# Note echo message continuing on to a second line after an escape.
  21&nbsp;
  22&nbsp;[ ! -f "$file" ] || (rm -f $file; echo "File \"$file\" deleted.")
  23&nbsp;# OR LIST, to delete file if present.
  24&nbsp;
  25&nbsp;# Note logic inversion above.
  26&nbsp;# AND LIST executes on true, OR LIST on false.
  27&nbsp;
  28&nbsp;exit $?</PRE
></TD
></TR
></TABLE
><HR></DIV
><DIV
CLASS="CAUTION"
><TABLE
CLASS="CAUTION"
WIDTH="90%"
BORDER="0"
><TR
><TD
WIDTH="25"
ALIGN="CENTER"
VALIGN="TOP"
><IMG
SRC="common/caution.png"
HSPACE="5"
ALT="Caution"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
>If the first command in an <I
CLASS="FIRSTTERM"
>or
	      list</I
> returns <SPAN
CLASS="RETURNVALUE"
>true</SPAN
>,
	      it <TT
CLASS="REPLACEABLE"
><I
>will</I
></TT
> execute.</P
></TD
></TR
></TABLE
></DIV
></DD
></DL
></DIV
><P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;# ==&#62; The following snippets from the /etc/rc.d/init.d/single
   2&nbsp;#+==&#62; script by Miquel van Smoorenburg
   3&nbsp;#+==&#62; illustrate use of "and" and "or" lists.
   4&nbsp;# ==&#62; "Arrowed" comments added by document author.
   5&nbsp;
   6&nbsp;[ -x /usr/bin/clear ] &#38;&#38; /usr/bin/clear
   7&nbsp;  # ==&#62; If /usr/bin/clear exists, then invoke it.
   8&nbsp;  # ==&#62; Checking for the existence of a command before calling it
   9&nbsp;  #+==&#62; avoids error messages and other awkward consequences.
  10&nbsp;
  11&nbsp;  # ==&#62; . . .
  12&nbsp;
  13&nbsp;# If they want to run something in single user mode, might as well run it...
  14&nbsp;for i in /etc/rc1.d/S[0-9][0-9]* ; do
  15&nbsp;        # Check if the script is there.
  16&nbsp;        [ -x "$i" ] || continue
  17&nbsp;  # ==&#62; If corresponding file in $PWD *not* found,
  18&nbsp;  #+==&#62; then "continue" by jumping to the top of the loop.
  19&nbsp;
  20&nbsp;        # Reject backup files and files generated by rpm.
  21&nbsp;        case "$1" in
  22&nbsp;                *.rpmsave|*.rpmorig|*.rpmnew|*~|*.orig)
  23&nbsp;                        continue;;
  24&nbsp;        esac
  25&nbsp;        [ "$i" = "/etc/rc1.d/S00single" ] &#38;&#38; continue
  26&nbsp;  # ==&#62; Set script name, but don't execute it yet.
  27&nbsp;        $i start
  28&nbsp;done
  29&nbsp;
  30&nbsp;  # ==&#62; . . .</PRE
></TD
></TR
></TABLE
></P
><DIV
CLASS="IMPORTANT"
><TABLE
CLASS="IMPORTANT"
WIDTH="100%"
BORDER="0"
><TR
><TD
WIDTH="25"
ALIGN="CENTER"
VALIGN="TOP"
><IMG
SRC="common/important.png"
HSPACE="5"
ALT="Important"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
>The <A
HREF="exit-status.html#EXITSTATUSREF"
>exit
	  status</A
> of an <TT
CLASS="USERINPUT"
><B
>and list</B
></TT
> or an
	  <TT
CLASS="USERINPUT"
><B
>or list</B
></TT
> is the exit status of the last
	  command executed.</P
></TD
></TR
></TABLE
></DIV
><P
>Clever combinations of <I
CLASS="FIRSTTERM"
>and</I
> and
	<I
CLASS="FIRSTTERM"
>or</I
> lists are possible, but the logic may
	easily become convoluted and require close attention to <A
HREF="opprecedence.html#OPPRECEDENCE1"
>operator precedence rules</A
>, and
	possibly extensive debugging.</P
><P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;false &#38;&#38; true || echo false         # false
   2&nbsp;
   3&nbsp;# Same result as
   4&nbsp;( false &#38;&#38; true ) || echo false     # false
   5&nbsp;# But NOT
   6&nbsp;false &#38;&#38; ( true || echo false )     # (nothing echoed)
   7&nbsp;
   8&nbsp;#  Note left-to-right grouping and evaluation of statements.
   9&nbsp;
  10&nbsp;#  It's usually best to avoid such complexities.
  11&nbsp;
  12&nbsp;#  Thanks, S.C.</PRE
></TD
></TR
></TABLE
>
	</P
><P
>See <A
HREF="contributed-scripts.html#DAYSBETWEEN"
>Example A-7</A
> and <A
HREF="fto.html#BROKENLINK"
>Example 7-4</A
> for illustrations of using <TT
CLASS="USERINPUT"
><B
>and
	/ or list</B
></TT
> constructs to test variables.</P
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