extmisc.html

Shall高级编程

><DIV
CLASS="EXAMPLE"
><HR><A
NAME="UNITCONVERSION"
></A
><P
><B
>Example 15-60. Converting meters to miles</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# unit-conversion.sh
   3&nbsp;
   4&nbsp;
   5&nbsp;convert_units ()  # Takes as arguments the units to convert.
   6&nbsp;{
   7&nbsp;  cf=$(units "$1" "$2" | sed --silent -e '1p' | awk '{print $2}')
   8&nbsp;  # Strip off everything except the actual conversion factor.
   9&nbsp;  echo "$cf"
  10&nbsp;}  
  11&nbsp;
  12&nbsp;Unit1=miles
  13&nbsp;Unit2=meters
  14&nbsp;cfactor=`convert_units $Unit1 $Unit2`
  15&nbsp;quantity=3.73
  16&nbsp;
  17&nbsp;result=$(echo $quantity*$cfactor | bc)
  18&nbsp;
  19&nbsp;echo "There are $result $Unit2 in $quantity $Unit1."
  20&nbsp;
  21&nbsp;#  What happens if you pass incompatible units,
  22&nbsp;#+ such as "acres" and "miles" to the function?
  23&nbsp;
  24&nbsp;exit 0</PRE
></TD
></TR
></TABLE
><HR></DIV
></DD
><DT
><A
NAME="M4REF"
></A
><B
CLASS="COMMAND"
>m4</B
></DT
><DD
><P
>A hidden treasure, <B
CLASS="COMMAND"
>m4</B
> is a
	      powerful macro processing filter,
		 <A
NAME="AEN13268"
HREF="#FTN.AEN13268"
>[5]</A
>
	      virtually a complete language. Although
	      originally written as a pre-processor for
	      <I
CLASS="FIRSTTERM"
>RatFor</I
>, <B
CLASS="COMMAND"
>m4</B
>
	      turned out to be useful as a stand-alone utility. In
	      fact, <B
CLASS="COMMAND"
>m4</B
> combines some of the
	      functionality of <A
HREF="internal.html#EVALREF"
>eval</A
>,
	      <A
HREF="textproc.html#TRREF"
>tr</A
>, and <A
HREF="awk.html#AWKREF"
>awk</A
>, in addition to its extensive
	      macro expansion facilities.</P
><P
>The April, 2002 issue of <A
HREF="http://www.linuxjournal.com"
TARGET="_top"
><I
CLASS="CITETITLE"
>Linux Journal</I
></A
>
	      has a very nice article on <B
CLASS="COMMAND"
>m4</B
> and
	      its uses.</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="M4"
></A
><P
><B
>Example 15-61. Using <I
CLASS="FIRSTTERM"
>m4</I
></B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# m4.sh: Using the m4 macro processor
   3&nbsp;
   4&nbsp;# Strings
   5&nbsp;string=abcdA01
   6&nbsp;echo "len($string)" | m4                            #   7
   7&nbsp;echo "substr($string,4)" | m4                       # A01
   8&nbsp;echo "regexp($string,[0-1][0-1],\&#38;Z)" | m4          # 01Z
   9&nbsp;
  10&nbsp;# Arithmetic
  11&nbsp;echo "incr(22)" | m4                                #  23
  12&nbsp;echo "eval(99 / 3)" | m4                            #  33
  13&nbsp;
  14&nbsp;exit 0</PRE
></TD
></TR
></TABLE
><HR></DIV
></DD
><DT
><A
NAME="XMESSAGEREF"
></A
><B
CLASS="COMMAND"
>xmessage</B
></DT
><DD
><P
>This X-based variant of
              <A
HREF="internal.html#ECHOREF"
>echo</A
> pops up a message/query
	      window on the desktop.</P
><P
>	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;xmessage Left click to continue -button okay</PRE
></TD
></TR
></TABLE
>
	      </P
></DD
><DT
><A
NAME="ZENITYREF"
></A
><B
CLASS="COMMAND"
>zenity</B
></DT
><DD
><P
>The
	      <A
HREF="http://freshmeat.net/projects/zenity"
TARGET="_top"
>zenity</A
>
	      utility is adept at displaying
	      <I
CLASS="FIRSTTERM"
>GTK+</I
> dialog <A
HREF="assortedtips.html#WIDGETREF"
>widgets</A
> and <A
HREF="assortedtips.html#ZENITYREF2"
>very suitable for scripting
	      purposes</A
>.</P
></DD
><DT
><A
NAME="DOEXECREF"
></A
><B
CLASS="COMMAND"
>doexec</B
></DT
><DD
><P
>The <B
CLASS="COMMAND"
>doexec</B
> command enables passing
	      an arbitrary list of arguments to a <I
CLASS="FIRSTTERM"
>binary
	      executable</I
>. In particular, passing
	      <TT
CLASS="PARAMETER"
><I
>argv[0]</I
></TT
> (which corresponds to <A
HREF="othertypesv.html#POSPARAMREF1"
>$0</A
> in a script) lets the
	      executable be invoked by various names, and it can then
	      carry out different sets of actions, according to the name
	      by which it was called. What this amounts to is roundabout
	      way of passing options to an executable.</P
><P
>For example, the <TT
CLASS="FILENAME"
>/usr/local/bin</TT
> directory might
	      contain a binary called <SPAN
CLASS="QUOTE"
>"aaa"</SPAN
>.  Invoking
	      <B
CLASS="COMMAND"
>doexec /usr/local/bin/aaa list</B
>
	      would <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>list</I
></SPAN
> all those files
	      in the current working directory beginning with an
	      <SPAN
CLASS="QUOTE"
>"a"</SPAN
>, while invoking (the same executable
	      with) <B
CLASS="COMMAND"
>doexec /usr/local/bin/aaa delete </B
>
	      would <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>delete</I
></SPAN
> those files.</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 various behaviors of the executable
              must be defined within the code of the executable itself,
              analogous to something like the following in a shell script:
                <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;case `basename $0` in
   2&nbsp;"name1" ) do_something;;
   3&nbsp;"name2" ) do_something_else;;
   4&nbsp;"name3" ) do_yet_another_thing;;
   5&nbsp;*       ) bail_out;;
   6&nbsp;esac</PRE
></TD
></TR
></TABLE
></P
></TD
></TR
></TABLE
></DIV
></DD
><DT
><B
CLASS="COMMAND"
>dialog</B
></DT
><DD
><P
>The <A
HREF="assortedtips.html#DIALOGREF"
>dialog</A
> family of tools
	      provide a method of calling interactive
	      <SPAN
CLASS="QUOTE"
>"dialog"</SPAN
> boxes from a script. The more
	      elaborate variations of <B
CLASS="COMMAND"
>dialog</B
> --
	      <B
CLASS="COMMAND"
>gdialog</B
>, <B
CLASS="COMMAND"
>Xdialog</B
>,
	      and <B
CLASS="COMMAND"
>kdialog</B
> -- actually invoke X-Windows
	      <A
HREF="assortedtips.html#WIDGETREF"
>widgets</A
>.</P
></DD
><DT
><A
NAME="SOXREF"
></A
><B
CLASS="COMMAND"
>sox</B
></DT
><DD
><P
>The <B
CLASS="COMMAND"
>sox</B
>, or
	      <SPAN
CLASS="QUOTE"
>"<B
CLASS="COMMAND"
>so</B
>und
	      e<B
CLASS="COMMAND"
>x</B
>change"</SPAN
> command plays and
	      performs transformations on sound files. In fact,
	      the <TT
CLASS="FILENAME"
>/usr/bin/play</TT
> executable
	      (now deprecated) is nothing but a shell wrapper for
	      <I
CLASS="FIRSTTERM"
>sox</I
>.</P
><P
>For example, <B
CLASS="COMMAND"
>sox soundfile.wav
	      soundfile.au</B
> changes a WAV sound file into a
	      (Sun audio format) AU sound file.</P
><P
>Shell scripts are ideally suited for batch-processing
	      <B
CLASS="COMMAND"
>sox</B
> operations on
	      sound files. For examples, see the <A
HREF="http://osl.iu.edu/~tveldhui/radio/"
TARGET="_top"
> Linux Radio
	      Timeshift HOWTO</A
> and the <A
HREF="http://savannah.nongnu.org/projects/audiodo"
TARGET="_top"
>MP3do
	      Project</A
>.</P
></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.AEN12881"
HREF="extmisc.html#AEN12881"
>[1]</A
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="95%"
><P
>This is actually a script adapted from
		the Debian Linux distribution.</P
></TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="5%"
><A
NAME="FTN.AEN12974"
HREF="extmisc.html#AEN12974"
>[2]</A
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="95%"
><P
>The <I
CLASS="FIRSTTERM"
>print queue</I
> is
	      the group of jobs <SPAN
CLASS="QUOTE"
>"waiting in line"</SPAN
> to be
	      printed.</P
></TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="5%"
><A
NAME="FTN.AEN13038"
HREF="extmisc.html#AEN13038"
>[3]</A
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="95%"
><P
>For an excellent overview of this
		  topic, see Andy Vaught's article, <A
HREF="http://www2.linuxjournal.com/lj-issues/issue41/2156.html"
TARGET="_top"
>Introduction
		  to Named Pipes</A
>, in the September, 1997 issue of
		  <A
HREF="http://www.linuxjournal.com"
TARGET="_top"
><I
CLASS="CITETITLE"
>Linux
		  Journal</I
></A
>.</P
></TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="5%"
><A
NAME="FTN.AEN13075"
HREF="extmisc.html#AEN13075"
>[4]</A
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="95%"
><P
><SPAN
CLASS="ACRONYM"
>EBCDIC</SPAN
> (pronounced
		  <SPAN
CLASS="QUOTE"
>"ebb-sid-ick"</SPAN
>) is an acronym for Extended
		  Binary Coded Decimal Interchange Code. This is an IBM
		  data format no longer in much use. A bizarre
		  application of the <TT
CLASS="OPTION"
>conv=ebcdic</TT
> option
		  of <B
CLASS="COMMAND"
>dd</B
> is as a quick 'n easy, but
		  not very secure text file encoder.
		    <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;cat $file | dd conv=swab,ebcdic &#62; $file_encrypted
   2&nbsp;# Encode (looks like gibberish).		    
   3&nbsp;# Might as well switch bytes (swab), too, for a little extra obscurity.
   4&nbsp;
   5&nbsp;cat $file_encrypted | dd conv=swab,ascii &#62; $file_plaintext
   6&nbsp;# Decode.</PRE
></TD
></TR
></TABLE
>
                </P
></TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="5%"
><A
NAME="FTN.AEN13268"
HREF="extmisc.html#AEN13268"
>[5]</A
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="95%"
><P
>A <I
CLASS="FIRSTTERM"
>macro</I
> is a
		 symbolic constant that expands into a command string
		 or a set of operations on parameters.</P
></TD
></TR
></TABLE
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