here-docs.html

一本完整的描述Unix Shell 编程的工具书的所有范例

><P
><B
>Example 17-8. 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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><HR></DIV
><P
>        It is possible to set a variable from the output of a here document.
	<TABLE
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><PRE
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 here document can supply input to a function in the same
        script.</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="HF"
></A
><P
><B
>Example 17-9. Here documents and functions</B
></P
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><PRE
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 $zipcode"
  30&nbsp;echo
  31&nbsp;
  32&nbsp;exit 0</PRE
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><HR></DIV
><P
><A
NAME="ANONHEREDOC0"
></A
></P
><P
>It is possible to use <SPAN
CLASS="TOKEN"
>:</SPAN
> as a dummy command
        accepting output from a here document. This, in effect, creates an
	<SPAN
CLASS="QUOTE"
>"anonymous"</SPAN
> here document.</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="ANONHEREDOC"
></A
><P
><B
>Example 17-10. <SPAN
CLASS="QUOTE"
>"Anonymous"</SPAN
> Here Document</B
></P
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><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;
   3&nbsp;: &#60;&#60;TESTVARIABLES
   4&nbsp;${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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><HR></DIV
><P
><A
NAME="CBLOCK1"
></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
>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 17-11. Commenting out a block of code</B
></P
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><PRE
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 $?."   # 0
  15&nbsp;# No error shown.
  16&nbsp;
  17&nbsp;
  18&nbsp;#  The above technique also comes in useful for commenting out
  19&nbsp;#+ a block of working code for debugging purposes.
  20&nbsp;#  This saves having to put a "#" at the beginning of each line,
  21&nbsp;#+ then having to go back and delete each "#" later.
  22&nbsp;
  23&nbsp;: &#60;&#60;DEBUGXXX
  24&nbsp;for file in *
  25&nbsp;do
  26&nbsp; cat "$file"
  27&nbsp;done
  28&nbsp;DEBUGXXX
  29&nbsp;
  30&nbsp;exit 0</PRE
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><HR></DIV
><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 17-12. 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="$1"
  28&nbsp;fi  
  29&nbsp;
  30&nbsp;echo "Listing of "$directory":"; echo
  31&nbsp;(printf "PERMISSIONS LINKS OWNER GROUP SIZE MONTH DAY HH:MM PROG-NAME\n" \
  32&nbsp;; ls -l "$directory" | sed 1d) | column -t
  33&nbsp;
  34&nbsp;exit 0</PRE
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><HR></DIV
><P
>Using a <A
HREF="here-docs.html#CATSCRIPTREF"
>cat script</A
> is an
        alternate way of accomplishing this.</P
><P
>      <TABLE
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><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;DOC_REQUEST=70
   2&nbsp;
   3&nbsp;if [ "$1" = "-h"  -o "$1" = "--help" ]     # Request help.
   4&nbsp;then                                       # Use a "cat script" . . .
   5&nbsp;  cat &#60;&#60;DOCUMENTATIONXX
   6&nbsp;List the statistics of a specified directory in tabular format.
   7&nbsp;---------------------------------------------------------------
   8&nbsp;The command line parameter gives the directory to be listed.
   9&nbsp;If no directory specified or directory specified cannot be read,
  10&nbsp;then list the current working directory.
  11&nbsp;
  12&nbsp;DOCUMENTATIONXX
  13&nbsp;exit $DOC_REQUEST
  14&nbsp;fi</PRE
></TD
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>
      </P
><P
>See also <A
HREF="contributed-scripts.html#ISSPAMMER2"
>Example A-27</A
> for one more excellent example
        of a self-documenting script.</P
><DIV
CLASS="NOTE"
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SRC="common/note.png"
HSPACE="5"
ALT="Note"></TD
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ALIGN="LEFT"
VALIGN="TOP"
><P
>Here documents create temporary files, but these
	    files are deleted after opening and are not accessible to
	    any other process.</P
><P
>	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="SCREEN"
> <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>bash -c 'lsof -a -p $$ -d0' &#60;&#60; EOF</B
></TT
>
 <TT
CLASS="PROMPT"
>&#62; </TT
><TT
CLASS="USERINPUT"
><B
>EOF</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>lsof    1213 bozo    0r   REG    3,5    0 30386 /tmp/t1213-0-sh (deleted)</TT
>
 	      </PRE
></TD
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>
	  </P
></TD
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></DIV
><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
>Some utilities will not work inside a
	  <I
CLASS="EMPHASIS"
>here document</I
>.</P
></TD
></TR
></TABLE
></DIV
><P
><A
NAME="INDENTEDLS"
></A
></P
><DIV
CLASS="WARNING"
><TABLE
CLASS="WARNING"
WIDTH="100%"
BORDER="0"
><TR
><TD
WIDTH="25"
ALIGN="CENTER"
VALIGN="TOP"
><IMG
SRC="common/warning.png"
HSPACE="5"
ALT="Warning"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
>The closing <I
CLASS="EMPHASIS"
>limit string</I
>,
	  on the final line of a here document, must start in the
	  <I
CLASS="EMPHASIS"
>first</I
> character position. There can
	  be <I
CLASS="EMPHASIS"
>no leading whitespace</I
>. Trailing
	  whitespace after the limit string likewise causes unexpected
	  behavior. The whitespace prevents the limit string from being
	  recognized.</P
><P
>	 <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;
   3&nbsp;echo "----------------------------------------------------------------------"
   4&nbsp;
   5&nbsp;cat &#60;&#60;LimitString
   6&nbsp;echo "This is line 1 of the message inside the here document."
   7&nbsp;echo "This is line 2 of the message inside the here document."
   8&nbsp;echo "This is the final line of the message inside the here document."
   9&nbsp;     LimitString
  10&nbsp;#^^^^Indented limit string. Error! This script will not behave as expected.
  11&nbsp;
  12&nbsp;echo "----------------------------------------------------------------------"
  13&nbsp;
  14&nbsp;#  These comments are outside the 'here document',
  15&nbsp;#+ and should not echo.
  16&nbsp;
  17&nbsp;echo "Outside the here document."
  18&nbsp;
  19&nbsp;exit 0
  20&nbsp;
  21&nbsp;echo "This line had better not echo."  # Follows an 'exit' command.</PRE
></TD
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>
	 </P
></TD
></TR
></TABLE
></DIV
><P
>For those tasks too complex for a <SPAN
CLASS="QUOTE"
>"here
	  document"</SPAN
>, consider using the <B
CLASS="COMMAND"
>expect</B
>
	  scripting language, which is specifically tailored for feeding
	  input into interactive programs.</P
><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="AEN13393"
>17.1. Here Strings</A
></H1
><P
><A
NAME="HERESTRINGSREF"
></A
></P
><P
>A <I
CLASS="EMPHASIS"
>here string</I
> can be considered as
	   a stripped-down form of <I
CLASS="EMPHASIS"
>here document</I
>. It
	   consists of nothing more than <B
CLASS="COMMAND"
>COMMAND
	   &#60;&#60;&#60;$WORD</B
>, where <TT
CLASS="VARNAME"
>$WORD</TT
>
	   is expanded and fed to the <TT
CLASS="FILENAME"
>stdin</TT
> of
	   <TT
CLASS="VARNAME"
>COMMAND</TT
>.</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="PREPENDEX"
></A
><P
><B
>Example 17-13. Prepending a line to a file</B
></P
><TABLE
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><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# prepend.sh: Add text at beginning of file.
   3&nbsp;#
   4&nbsp;#  Example contributed by Kenny Stauffer,
   5&nbsp;#+ and slightly modified by document author.
   6&nbsp;
   7&nbsp;
   8&nbsp;E_NOSUCHFILE=65
   9&nbsp;
  10&nbsp;read -p "File: " file   # -p arg to 'read' displays prompt.
  11&nbsp;if [ ! -e "$file" ]
  12&nbsp;then   # Bail out if no such file.
  13&nbsp;  echo "File $file not found."
  14&nbsp;  exit $E_NOSUCHFILE
  15&nbsp;fi
  16&nbsp;
  17&nbsp;read -p "Title: " title
  18&nbsp;cat - $file &#60;&#60;&#60;$title &#62; $file.new
  19&nbsp;
  20&nbsp;echo "Modified file is $file.new"
  21&nbsp;
  22&nbsp;exit 0
  23&nbsp;
  24&nbsp;# from 'man bash':
  25&nbsp;# Here Strings
  26&nbsp;# 	A variant of here documents, the format is:
  27&nbsp;# 
  28&nbsp;# 		&#60;&#60;&#60;word
  29&nbsp;# 
  30&nbsp;# 	The word is expanded and supplied to the command on its standard input.</PRE
></TD
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><HR></DIV
><P
>Exercise: Find other uses for <I
CLASS="EMPHASIS"
>here
            strings</I
>.</P
></DIV
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