assortedtips.html

Shall高级编程

   9 #     command_test=$(whereis "$CMD" | grep \/)
  10 #  But then the sense of the following test would have to be reversed,
  11 #+ since the $command_test variable holds content only if
  12 #+ the $CMD exists on the system.
  13 #     (Thanks, bojster.)
  14 
  15 
  16 if [[ -z "$command_test" ]]  # Check whether command present.
  17 then
  18   $CMD option1 option2       #  Run command1 with options.
  19 else                         #  Otherwise,
  20   $PlanB                     #+ run command2. 
  21&nbsp;fi</PRE
></TD
></TR
></TABLE
>
          </P
></LI
><LI
><P
><A
NAME="IFGREPFIX"
></A
></P
><P
>An <A
HREF="tests.html#IFGREPREF"
>if-grep test</A
> may not
	    return expected results in an error case, when text is output to
	    <TT
CLASS="FILENAME"
>stderr</TT
>, rather that
	    <TT
CLASS="FILENAME"
>stdout</TT
>.
	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;if ls -l nonexistent_filename | grep -q 'No such file or directory'
   2&nbsp;  then echo "File \"nonexistent_filename\" does not exist."
   3&nbsp;fi</PRE
></TD
></TR
></TABLE
></P
><P
><A
HREF="io-redirection.html#IOREDIRREF"
>Redirecting</A
>
	    <TT
CLASS="FILENAME"
>stderr</TT
> to <TT
CLASS="FILENAME"
>stdout</TT
> fixes
	    this.
	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;if ls -l nonexistent_filename 2&#62;&#38;1 | grep -q 'No such file or directory'
   2&nbsp;#                             ^^^^
   3&nbsp;  then echo "File \"nonexistent_filename\" does not exist."
   4&nbsp;fi
   5&nbsp;
   6&nbsp;# Thanks, Chris Martin, for pointing this out.</PRE
></TD
></TR
></TABLE
></P
></LI
><LI
><P
><A
NAME="SUBSHTMP"
></A
>
        If you absolutely must access a subshell variable outside the
	subshell, here's a way to do it.
	  <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;TMPFILE=tmpfile                  # Create a temp file to store the variable.
   2&nbsp;
   3&nbsp;(   # Inside the subshell ...
   4&nbsp;inner_variable=Inner
   5&nbsp;echo $inner_variable
   6&nbsp;echo $inner_variable &#62;&#62;$TMPFILE  # Append to temp file.
   7&nbsp;)
   8&nbsp;
   9&nbsp;    # Outside the subshell ...
  10&nbsp;
  11&nbsp;echo; echo "-----"; echo
  12&nbsp;echo $inner_variable             # Null, as expected.
  13&nbsp;echo "-----"; echo
  14&nbsp;
  15&nbsp;# Now ...
  16&nbsp;read inner_variable &#60;$TMPFILE    # Read back shell variable.
  17&nbsp;rm -f "$TMPFILE"                 # Get rid of temp file.
  18&nbsp;echo "$inner_variable"           # It's an ugly kludge, but it works.</PRE
></TD
></TR
></TABLE
>
	</P
></LI
><LI
><P
><A
NAME="RUNPARTSREF2"
></A
></P
><P
>The <A
HREF="extmisc.html#RUNPARTSREF"
>run-parts</A
>
	    command is handy for running a set of command
	    scripts in a particular sequence, especially in
	    combination with <A
HREF="system.html#CRONREF"
>cron</A
> or
	    <A
HREF="timedate.html#ATREF"
>at</A
>.</P
></LI
><LI
><P
><A
NAME="RCSREF"
></A
></P
><P
>For doing multiple revisions on a complex script, use the
	    <I
CLASS="FIRSTTERM"
>rcs</I
> Revision Control System package.</P
><P
> Among other benefits of this is automatically updated ID
	    header tags. The <B
CLASS="COMMAND"
>co</B
> command in
	    <I
CLASS="FIRSTTERM"
>rcs</I
> does a parameter replacement of
	    certain reserved key words, for example, replacing
	    <TT
CLASS="PARAMETER"
><I
>#$Id$</I
></TT
> in a script with something like:
	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#$Id: hello-world.sh,v 1.1 2004/10/16 02:43:05 bozo Exp $</PRE
></TD
></TR
></TABLE
></P
></LI
></UL
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN19071"
></A
>33.8.2. Widgets</H2
><P
><A
NAME="WIDGETREF"
></A
></P
><P
>It would be nice to be able to invoke X-Windows widgets
	    from a shell script. There happen to exist several packages
	    that purport to do so, namely <I
CLASS="FIRSTTERM"
>Xscript</I
>,
	    <I
CLASS="FIRSTTERM"
>Xmenu</I
>, and <I
CLASS="FIRSTTERM"
>widtools</I
>.
	    The first two of these no longer seem
	    to be maintained.  Fortunately, it is still
	    possible to obtain <I
CLASS="FIRSTTERM"
>widtools</I
> <A
HREF="http://www.batse.msfc.nasa.gov/~mallozzi/home/software/xforms/src/widtools-2.0.tgz"
TARGET="_top"
>here</A
>.
	    </P
><DIV
CLASS="CAUTION"
><TABLE
CLASS="CAUTION"
WIDTH="100%"
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 <I
CLASS="FIRSTTERM"
>widtools</I
> (widget tools)
	    package requires the <I
CLASS="FIRSTTERM"
>XForms</I
> library to
	    be installed. Additionally, the <A
HREF="filearchiv.html#MAKEFILEREF"
>Makefile</A
> needs some judicious
	    editing before the package will build on a typical Linux
	    system. Finally, three of the six widgets offered do not work
	    (and, in fact, segfault).</P
></TD
></TR
></TABLE
></DIV
><P
><A
NAME="DIALOGREF"
></A
></P
><P
>The <I
CLASS="FIRSTTERM"
>dialog</I
> family of tools offers a method
	    of calling <SPAN
CLASS="QUOTE"
>"dialog"</SPAN
> widgets from a shell script. The
	    original <I
CLASS="FIRSTTERM"
>dialog</I
> utility works in a text
	    console, but its successors, <I
CLASS="FIRSTTERM"
>gdialog</I
>,
	    <I
CLASS="FIRSTTERM"
>Xdialog</I
>, and <I
CLASS="FIRSTTERM"
>kdialog</I
>
	    use X-Windows-based widget sets.</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="DIALOG"
></A
><P
><B
>Example 33-19. Widgets invoked from a shell script</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# dialog.sh: Using 'gdialog' widgets.
   3&nbsp;
   4&nbsp;# Must have 'gdialog' installed on your system to run this script.
   5&nbsp;# Or, you can replace all instance of 'gdialog' below with 'kdialog' ...
   6&nbsp;# Version 1.1 (corrected 04/05/05)
   7&nbsp;
   8&nbsp;# This script was inspired by the following article.
   9&nbsp;#     "Scripting for X Productivity," by Marco Fioretti,
  10&nbsp;#      LINUX JOURNAL, Issue 113, September 2003, pp. 86-9.
  11&nbsp;# Thank you, all you good people at LJ.
  12&nbsp;
  13&nbsp;
  14&nbsp;# Input error in dialog box.
  15&nbsp;E_INPUT=65
  16&nbsp;# Dimensions of display, input widgets.
  17&nbsp;HEIGHT=50
  18&nbsp;WIDTH=60
  19&nbsp;
  20&nbsp;# Output file name (constructed out of script name).
  21&nbsp;OUTFILE=$0.output
  22&nbsp;
  23&nbsp;# Display this script in a text widget.
  24&nbsp;gdialog --title "Displaying: $0" --textbox $0 $HEIGHT $WIDTH
  25&nbsp;
  26&nbsp;
  27&nbsp;
  28&nbsp;# Now, we'll try saving input in a file.
  29&nbsp;echo -n "VARIABLE=" &#62; $OUTFILE
  30&nbsp;gdialog --title "User Input" --inputbox "Enter variable, please:" \
  31&nbsp;$HEIGHT $WIDTH 2&#62;&#62; $OUTFILE
  32&nbsp;
  33&nbsp;
  34&nbsp;if [ "$?" -eq 0 ]
  35&nbsp;# It's good practice to check exit status.
  36&nbsp;then
  37&nbsp;  echo "Executed \"dialog box\" without errors."
  38&nbsp;else
  39&nbsp;  echo "Error(s) in \"dialog box\" execution."
  40&nbsp;        # Or, clicked on "Cancel", instead of "OK" button.
  41&nbsp;  rm $OUTFILE
  42&nbsp;  exit $E_INPUT
  43&nbsp;fi
  44&nbsp;
  45&nbsp;
  46&nbsp;
  47&nbsp;# Now, we'll retrieve and display the saved variable.
  48&nbsp;. $OUTFILE   # 'Source' the saved file.
  49&nbsp;echo "The variable input in the \"input box\" was: "$VARIABLE""
  50&nbsp;
  51&nbsp;
  52&nbsp;rm $OUTFILE  # Clean up by removing the temp file.
  53&nbsp;             # Some applications may need to retain this file.
  54&nbsp;
  55&nbsp;exit $?
  56&nbsp;
  57&nbsp;# Exercise: Rewrite this script using the 'zenity' widget set.</PRE
></TD
></TR
></TABLE
><HR></DIV
><P
><A
NAME="XMESSAGEREF2"
></A
>
	    The <A
HREF="extmisc.html#XMESSAGEREF"
>xmessage</A
> command is
	    a simple method of popping up a message/query window. For
	    example:
	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;xmessage Fatal error in script! -button exit</PRE
></TD
></TR
></TABLE
>
	  </P
><P
><A
NAME="ZENITYREF2"
></A
>
	    The latest entry in the widget sweepstakes is
	    <A
HREF="extmisc.html#ZENITYREF"
>zenity</A
>.
	    This utility pops up
	    <I
CLASS="FIRSTTERM"
>GTK+</I
> dialog widgets-and-windows,
	    and it works very nicely within a script.
	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;get_info ()
   2&nbsp;{
   3&nbsp;  zenity --entry       #  Pops up query window . . .
   4&nbsp;                       #+ and prints user entry to stdout.
   5&nbsp;
   6&nbsp;                       #  Also try the --calendar and --scale options.
   7&nbsp;}
   8&nbsp;
   9&nbsp;answer=$( get_info )   #  Capture stdout in $answer variable.
  10&nbsp;
  11&nbsp;echo "User entered: "$answer""</PRE
></TD
></TR
></TABLE
>
	  </P
><P
>For other methods of scripting with widgets, try
	    <I
CLASS="FIRSTTERM"
>Tk</I
> or <I
CLASS="FIRSTTERM"
>wish</I
>
	    (<I
CLASS="FIRSTTERM"
>Tcl</I
> derivatives),
	    <I
CLASS="FIRSTTERM"
>PerlTk</I
> (<I
CLASS="FIRSTTERM"
>Perl</I
>
	    with <I
CLASS="FIRSTTERM"
>Tk</I
> extensions),
	    <I
CLASS="FIRSTTERM"
>tksh</I
> (<I
CLASS="FIRSTTERM"
>ksh</I
>
	    with <I
CLASS="FIRSTTERM"
>Tk</I
> extensions),
	    <I
CLASS="FIRSTTERM"
>XForms4Perl</I
>
	    (<I
CLASS="FIRSTTERM"
>Perl</I
> with
	    <I
CLASS="FIRSTTERM"
>XForms</I
> extensions),
	    <I
CLASS="FIRSTTERM"
>Gtk-Perl</I
> (<I
CLASS="FIRSTTERM"
>Perl</I
>
	    with <I
CLASS="FIRSTTERM"
>Gtk</I
> extensions), or
	    <I
CLASS="FIRSTTERM"
>PyQt</I
> (<I
CLASS="FIRSTTERM"
>Python</I
>
	    with <I
CLASS="FIRSTTERM"
>Qt</I
> extensions).</P
></DIV
></DIV
><DIV
CLASS="NAVFOOTER"
><HR
ALIGN="LEFT"
WIDTH="100%"><TABLE
SUMMARY="Footer navigation table"
WIDTH="100%"
BORDER="0"
CELLPADDING="0"
CELLSPACING="0"
><TR
><TD
WIDTH="33%"
ALIGN="left"
VALIGN="top"
><A
HREF="optimizations.html"
ACCESSKEY="P"
>Prev</A
></TD
><TD
WIDTH="34%"
ALIGN="center"
VALIGN="top"
><A
HREF="index.html"
ACCESSKEY="H"
>Home</A
></TD
><TD
WIDTH="33%"
ALIGN="right"
VALIGN="top"
><A
HREF="securityissues.html"
ACCESSKEY="N"
>Next</A
></TD
></TR
><TR
><TD
WIDTH="33%"
ALIGN="left"
VALIGN="top"
>Optimizations</TD
><TD
WIDTH="34%"
ALIGN="center"
VALIGN="top"
><A
HREF="miscellany.html"
ACCESSKEY="U"
>Up</A
></TD
><TD
WIDTH="33%"
ALIGN="right"
VALIGN="top"
>Security Issues</TD
></TR
></TABLE
></DIV
></BODY
></HTML
>