here-docs.html

Shall高级编程

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CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# Another 'cat' here document, using parameter substitution.
   3&nbsp;
   4&nbsp;# Try it with no command line parameters,   ./scriptname
   5&nbsp;# Try it with one command line parameter,   ./scriptname Mortimer
   6&nbsp;# Try it with one two-word quoted command line parameter,
   7&nbsp;#                           ./scriptname "Mortimer Jones"
   8&nbsp;
   9&nbsp;CMDLINEPARAM=1     #  Expect at least command line parameter.
  10&nbsp;
  11&nbsp;if [ $# -ge $CMDLINEPARAM ]
  12&nbsp;then
  13&nbsp;  NAME=$1          #  If more than one command line param,
  14&nbsp;                   #+ then just take the first.
  15&nbsp;else
  16&nbsp;  NAME="John Doe"  #  Default, if no command line parameter.
  17&nbsp;fi  
  18&nbsp;
  19&nbsp;RESPONDENT="the author of this fine script"  
  20&nbsp;  
  21&nbsp;
  22&nbsp;cat &#60;&#60;Endofmessage
  23&nbsp;
  24&nbsp;Hello, there, $NAME.
  25&nbsp;Greetings to you, $NAME, from $RESPONDENT.
  26&nbsp;
  27&nbsp;# This comment shows up in the output (why?).
  28&nbsp;
  29&nbsp;Endofmessage
  30&nbsp;
  31&nbsp;# Note that the blank lines show up in the output.
  32&nbsp;# So does the "comment".
  33&nbsp;
  34&nbsp;exit 0</PRE
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><P
><A
NAME="HEREPARAMSUB"
></A
></P
><P
>This is a useful script containing a here document with
        parameter substitution.</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="EX72"
></A
><P
><B
>Example 18-6. Upload a file pair to <I
CLASS="FIRSTTERM"
>Sunsite</I
> incoming
	  directory</B
></P
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CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# upload.sh
   3&nbsp;
   4&nbsp;#  Upload file pair (Filename.lsm, Filename.tar.gz)
   5&nbsp;#+ to incoming directory at Sunsite/UNC (ibiblio.org).
   6&nbsp;#  Filename.tar.gz is the tarball itself.
   7&nbsp;#  Filename.lsm is the descriptor file.
   8&nbsp;#  Sunsite requires "lsm" file, otherwise will bounce contributions.
   9&nbsp;
  10&nbsp;
  11&nbsp;E_ARGERROR=65
  12&nbsp;
  13&nbsp;if [ -z "$1" ]
  14&nbsp;then
  15&nbsp;  echo "Usage: `basename $0` Filename-to-upload"
  16&nbsp;  exit $E_ARGERROR
  17&nbsp;fi  
  18&nbsp;
  19&nbsp;
  20&nbsp;Filename=`basename $1`           # Strips pathname out of file name.
  21&nbsp;
  22&nbsp;Server="ibiblio.org"
  23&nbsp;Directory="/incoming/Linux"
  24&nbsp;#  These need not be hard-coded into script,
  25&nbsp;#+ but may instead be changed to command line argument.
  26&nbsp;
  27&nbsp;Password="your.e-mail.address"   # Change above to suit.
  28&nbsp;
  29&nbsp;ftp -n $Server &#60;&#60;End-Of-Session
  30&nbsp;# -n option disables auto-logon
  31&nbsp;
  32&nbsp;user anonymous "$Password"
  33&nbsp;binary
  34&nbsp;bell                             # Ring 'bell' after each file transfer.
  35&nbsp;cd $Directory
  36&nbsp;put "$Filename.lsm"
  37&nbsp;put "$Filename.tar.gz"
  38&nbsp;bye
  39&nbsp;End-Of-Session
  40&nbsp;
  41&nbsp;exit 0</PRE
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><P
><A
NAME="HEREESC"
></A
></P
><P
>Quoting or escaping the <SPAN
CLASS="QUOTE"
>"limit string"</SPAN
> at the
        head of a here document disables parameter substitution within its
	body.</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="EX71C"
></A
><P
><B
>Example 18-7. Parameter substitution turned off</B
></P
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><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;#  A 'cat' here-document, but with parameter substitution disabled.
   3&nbsp;
   4&nbsp;NAME="John Doe"
   5&nbsp;RESPONDENT="the author of this fine script"  
   6&nbsp;
   7&nbsp;cat &#60;&#60;'Endofmessage'
   8&nbsp;
   9&nbsp;Hello, there, $NAME.
  10&nbsp;Greetings to you, $NAME, from $RESPONDENT.
  11&nbsp;
  12&nbsp;Endofmessage
  13&nbsp;
  14&nbsp;#   No parameter substitution when the "limit string" is quoted or escaped.
  15&nbsp;#   Either of the following at the head of the here document would have
  16&nbsp;#+  the same effect.
  17&nbsp;#   cat &#60;&#60;"Endofmessage"
  18&nbsp;#   cat &#60;&#60;\Endofmessage
  19&nbsp;
  20&nbsp;exit 0</PRE
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><P
><A
NAME="HERELIT"
></A
></P
><P
>Disabling parameter substitution permits outputting literal text.
        Generating scripts or even program code is one use for this.</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="GENERATESCRIPT"
></A
><P
><B
>Example 18-8. A script that generates another script</B
></P
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CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# generate-script.sh
   3&nbsp;# Based on an idea by Albert Reiner.
   4&nbsp;
   5&nbsp;OUTFILE=generated.sh         # Name of the file to generate.
   6&nbsp;
   7&nbsp;
   8&nbsp;# -----------------------------------------------------------
   9&nbsp;# 'Here document containing the body of the generated script.
  10&nbsp;(
  11&nbsp;cat &#60;&#60;'EOF'
  12&nbsp;#!/bin/bash
  13&nbsp;
  14&nbsp;echo "This is a generated shell script."
  15&nbsp;#  Note that since we are inside a subshell,
  16&nbsp;#+ we can't access variables in the "outside" script.
  17&nbsp;
  18&nbsp;echo "Generated file will be named: $OUTFILE"
  19&nbsp;#  Above line will not work as normally expected
  20&nbsp;#+ because parameter expansion has been disabled.
  21&nbsp;#  Instead, the result is literal output.
  22&nbsp;
  23&nbsp;a=7
  24&nbsp;b=3
  25&nbsp;
  26&nbsp;let "c = $a * $b"
  27&nbsp;echo "c = $c"
  28&nbsp;
  29&nbsp;exit 0
  30&nbsp;EOF
  31&nbsp;) &#62; $OUTFILE
  32&nbsp;# -----------------------------------------------------------
  33&nbsp;
  34&nbsp;#  Quoting the 'limit string' prevents variable expansion
  35&nbsp;#+ within the body of the above 'here document.'
  36&nbsp;#  This permits outputting literal strings in the output file.
  37&nbsp;
  38&nbsp;if [ -f "$OUTFILE" ]
  39&nbsp;then
  40&nbsp;  chmod 755 $OUTFILE
  41&nbsp;  # Make the generated file executable.
  42&nbsp;else
  43&nbsp;  echo "Problem in creating file: \"$OUTFILE\""
  44&nbsp;fi
  45&nbsp;
  46&nbsp;#  This method can also be used for generating
  47&nbsp;#+ C programs, Perl programs, Python programs, Makefiles,
  48&nbsp;#+ and the like.
  49&nbsp;
  50&nbsp;exit 0</PRE
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><P
><A
NAME="HERECS"
></A
></P
><P
>        It is possible to set a variable from the output of a here document.
	This is actually a devious form of <A
HREF="commandsub.html#COMMANDSUBREF"
>command substitution</A
>.
	<TABLE
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CLASS="PROGRAMLISTING"
>   1&nbsp;variable=$(cat &#60;&#60;SETVAR
   2&nbsp;This variable
   3&nbsp;runs over multiple lines.
   4&nbsp;SETVAR)
   5&nbsp;
   6&nbsp;echo "$variable"</PRE
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>
      </P
><P
><A
NAME="HEREFUNC"
></A
></P
><P
>A here document can supply input to a function in the same
        script.</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="HF"
></A
><P
><B
>Example 18-9. Here documents and functions</B
></P
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CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# here-function.sh
   3&nbsp;
   4&nbsp;GetPersonalData ()
   5&nbsp;{
   6&nbsp;  read firstname
   7&nbsp;  read lastname
   8&nbsp;  read address
   9&nbsp;  read city 
  10&nbsp;  read state 
  11&nbsp;  read zipcode
  12&nbsp;} # This certainly looks like an interactive function, but...
  13&nbsp;
  14&nbsp;
  15&nbsp;# Supply input to the above function.
  16&nbsp;GetPersonalData &#60;&#60;RECORD001
  17&nbsp;Bozo
  18&nbsp;Bozeman
  19&nbsp;2726 Nondescript Dr.
  20&nbsp;Baltimore
  21&nbsp;MD
  22&nbsp;21226
  23&nbsp;RECORD001
  24&nbsp;
  25&nbsp;
  26&nbsp;echo
  27&nbsp;echo "$firstname $lastname"
  28&nbsp;echo "$address"
  29&nbsp;echo "$city, $state $zipcode"
  30&nbsp;echo
  31&nbsp;
  32&nbsp;exit 0</PRE
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><P
><A
NAME="ANONHEREDOC0"
></A
></P
><P
>It is possible to use <SPAN
CLASS="TOKEN"
>:</SPAN
> as a dummy command
        accepting output from a here document. This, in effect, creates an
	<SPAN
CLASS="QUOTE"
>"anonymous"</SPAN
> here document.</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="ANONHEREDOC"
></A
><P
><B
>Example 18-10. <SPAN
CLASS="QUOTE"
>"Anonymous"</SPAN
> Here Document</B
></P
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><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;
   3&nbsp;: &#60;&#60;TESTVARIABLES
   4&nbsp;${HOSTNAME?}${USER?}${MAIL?}  # Print error message if one of the variables not set.
   5&nbsp;TESTVARIABLES
   6&nbsp;
   7&nbsp;exit 0</PRE
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><P
><A
NAME="CBLOCK1"
></A
></P
><DIV
CLASS="TIP"
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VALIGN="TOP"
><IMG
SRC="common/tip.png"
HSPACE="5"
ALT="Tip"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
>A variation of the above technique permits <SPAN
CLASS="QUOTE"
>"commenting
        out"</SPAN
> blocks of code.</P
></TD
></TR
></TABLE
></DIV
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="COMMENTBLOCK"
></A
><P
><B
>Example 18-11. Commenting out a block of code</B
></P
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CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# commentblock.sh
   3&nbsp;
   4&nbsp;: &#60;&#60;COMMENTBLOCK
   5&nbsp;echo "This line will not echo."
   6&nbsp;This is a comment line missing the "#" prefix.
   7&nbsp;This is another comment line missing the "#" prefix.
   8&nbsp;
   9&nbsp;&#38;*@!!++=
  10&nbsp;The above line will cause no error message,
  11&nbsp;because the Bash interpreter will ignore it.
  12&nbsp;COMMENTBLOCK
  13&nbsp;
  14&nbsp;echo "Exit value of above \"COMMENTBLOCK\" is $?."   # 0
  15&nbsp;# No error shown.
  16&nbsp;echo
  17&nbsp;
  18&nbsp;
  19&nbsp;#  The above technique also comes in useful for commenting out
  20&nbsp;#+ a block of working code for debugging purposes.
  21&nbsp;#  This saves having to put a "#" at the beginning of each line,
  22&nbsp;#+ then having to go back and delete each "#" later.
  23&nbsp;
  24&nbsp;echo "Just before commented-out code block."
  25&nbsp;#  The lines of code between the double-dashed lines will not execute.
  26&nbsp;#  ===================================================================
  27&nbsp;: &#60;&#60;DEBUGXXX
  28&nbsp;for file in *
  29&nbsp;do
  30&nbsp; cat "$file"
  31&nbsp;done
  32&nbsp;DEBUGXXX
  33&nbsp;#  ===================================================================
  34&nbsp;echo "Just after commented-out code block."
  35&nbsp;
  36&nbsp;exit 0
  37&nbsp;
  38&nbsp;
  39&nbsp;
  40&nbsp;######################################################################
  41&nbsp;#  Note, however, that if a bracketed variable is contained within
  42&nbsp;#+ the commented-out code block,
  43&nbsp;#+ then this could cause problems.
  44&nbsp;#  for example:
  45&nbsp;
  46&nbsp;
  47&nbsp;#/!/bin/bash
  48&nbsp;
  49&nbsp;  : &#60;&#60;COMMENTBLOCK
  50&nbsp;  echo "This line will not echo."
  51&nbsp;  &#38;*@!!++=
  52&nbsp;  ${foo_bar_bazz?}
  53&nbsp;  $(rm -rf /tmp/foobar/)
  54&nbsp;  $(touch my_build_directory/cups/Makefile)
  55&nbsp;COMMENTBLOCK
  56&nbsp;
  57&nbsp;
  58&nbsp;$ sh commented-bad.sh
  59&nbsp;commented-bad.sh: line 3: foo_bar_bazz: parameter null or not set
  60&nbsp;
  61&nbsp;# The remedy for this is to strong-quote the 'COMMENTBLOCK' in line 49, above.
  62&nbsp;
  63&nbsp;  : &#60;&#60;'COMMENTBLOCK'
  64&nbsp;
  65&nbsp;# Thank you, Kurt Pfeifle, for pointing this out.</PRE
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><P
><A
NAME="HSELFDOC"
></A
></P
><DIV
CLASS="TIP"
><TABLE
CLASS="TIP"
WIDTH="100%"
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
>Yet another twist of this nifty trick makes
        <SPAN
CLASS="QUOTE"
>"self-documenting"</SPAN
> scripts possible.</P
></TD
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></DIV
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="SELFDOCUMENT"
></A
><P
><B
>Example 18-12. A self-documenting script</B
></P
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><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# self-document.sh: self-documenting script
   3&nbsp;# Modification of "colm.sh".
   4&nbsp;
   5&nbsp;DOC_REQUEST=70
   6&nbsp;
   7&nbsp;if [ "$1" = "-h"  -o "$1" = "--help" ]     # Request help.
   8&nbsp;then
   9&nbsp;  echo; echo "Usage: $0 [directory-name]"; echo
  10&nbsp;  sed --silent -e '/DOCUMENTATIONXX$/,/^DOCUMENTATIONXX$/p' "$0" |
  11&nbsp;  sed -e '/DOCUMENTATIONXX$/d'; exit $DOC_REQUEST; fi
  12&nbsp;
  13&nbsp;
  14&nbsp;: &#60;&#60;DOCUMENTATIONXX
  15&nbsp;List the statistics of a specified directory in tabular format.
  16&nbsp;---------------------------------------------------------------
  17&nbsp;The command line parameter gives the directory to be listed.
  18&nbsp;If no directory specified or directory specified cannot be read,
  19&nbsp;then list the current working directory.
  20&nbsp;
  21&nbsp;DOCUMENTATIONXX
  22&nbsp;
  23&nbsp;if [ -z "$1" -o ! -r "$1" ]
  24&nbsp;then
  25&nbsp;  directory=.
  26&nbsp;else
  27&nbsp;  directory="$1"
  28&nbsp;fi