othertypesv.html

Shall高级编程

><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
><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;# Or:       lastarg=${!#}
   4&nbsp;#           (Thanks, Chris Monson.)
   5&nbsp;# Note that lastarg=${!$#} 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. There must also exist symbolic
	      links to all the alternate names of the script. See <A
HREF="external.html#HELLOL"
>Example 15-2</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-cw
  12&nbsp;# ln -s /usr/local/bin/wh /usr/local/bin/wh-radb
  13&nbsp;
  14&nbsp;E_NOARGS=65
  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 $[e-mail];;
  26&nbsp;    "wh-ripe") whois $[e-mail];;
  27&nbsp;    "wh-radb") whois $[e-mail];;
  28&nbsp;    "wh-cw"  ) whois $[e-mail];;
  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 23 skidoo
   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 line feed.
  16&nbsp;
  17&nbsp;exit 0
  18&nbsp;
  19&nbsp;#  See also the echo-params.sh script for a "shiftless"
  20&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 0,
  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 33-15</A
>.</P
></TD
></TR
></TABLE
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></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.AEN2193"
HREF="othertypesv.html#AEN2193"
>[1]</A
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="95%"
><P
>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
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