special-chars.html

一本完整的描述Unix Shell 编程的工具书的所有范例

CLASS="CAUTION"
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><TD
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SRC="common/caution.png"
HSPACE="5"
ALT="Caution"></TD
><TD
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><P
>No spaces allowed within the braces
	    <I
CLASS="EMPHASIS"
>unless</I
> the spaces are quoted or escaped.</P
><P
><TT
CLASS="USERINPUT"
><B
>echo {file1,file2}\ :{\ A," B",' C'}</B
></TT
></P
><P
><TT
CLASS="COMPUTEROUTPUT"
>file1 : A file1 : B file1 : C file2 : A file2 : B file2 : C</TT
></P
></TD
></TR
></TABLE
></DIV
></DD
><DT
><SPAN
CLASS="TOKEN"
>{}</SPAN
></DT
><DD
><P
><A
NAME="CODEBLOCKREF"
></A
></P
><DIV
CLASS="FORMALPARA"
><P
><B
>Block of code [curly brackets]. </B
>Also referred to as an <SPAN
CLASS="QUOTE"
>"inline group"</SPAN
>,
		this construct, in effect, creates an anonymous
		function. However, unlike a <A
HREF="functions.html#FUNCTIONREF"
>function</A
>, the variables
		in a code block remain visible to the remainder of the
		script.</P
></DIV
><P
> <TABLE
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><TR
><TD
><PRE
CLASS="SCREEN"
> <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>{ local a;
	      a=123; }</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>bash: local: can only be used in a
function</TT
>
 	      </PRE
></TD
></TR
></TABLE
> </P
><P
><TABLE
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><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;a=123
   2&nbsp;{ a=321; }
   3&nbsp;echo "a = $a"   # a = 321   (value inside code block)
   4&nbsp;
   5&nbsp;# Thanks, S.C.</PRE
></TD
></TR
></TABLE
></P
><P
>The code block enclosed in braces may have <A
HREF="io-redirection.html#IOREDIRREF"
>I/O redirected</A
> to and from
	      it.</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="EX8"
></A
><P
><B
>Example 3-1. Code blocks and I/O redirection</B
></P
><TABLE
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><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# Reading lines in /etc/fstab.
   3&nbsp;
   4&nbsp;File=/etc/fstab
   5&nbsp;
   6&nbsp;{
   7&nbsp;read line1
   8&nbsp;read line2
   9&nbsp;} &#60; $File
  10&nbsp;
  11&nbsp;echo "First line in $File is:"
  12&nbsp;echo "$line1"
  13&nbsp;echo
  14&nbsp;echo "Second line in $File is:"
  15&nbsp;echo "$line2"
  16&nbsp;
  17&nbsp;exit 0
  18&nbsp;
  19&nbsp;# Now, how do you parse the separate fields of each line?
  20&nbsp;# Hint: use awk.</PRE
></TD
></TR
></TABLE
><HR></DIV
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="RPMCHECK"
></A
><P
><B
>Example 3-2. Saving the results of a code block to a file</B
></P
><TABLE
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><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# rpm-check.sh
   3&nbsp;
   4&nbsp;# Queries an rpm file for description, listing, and whether it can be installed.
   5&nbsp;# Saves output to a file.
   6&nbsp;# 
   7&nbsp;# This script illustrates using a code block.
   8&nbsp;
   9&nbsp;SUCCESS=0
  10&nbsp;E_NOARGS=65
  11&nbsp;
  12&nbsp;if [ -z "$1" ]
  13&nbsp;then
  14&nbsp;  echo "Usage: `basename $0` rpm-file"
  15&nbsp;  exit $E_NOARGS
  16&nbsp;fi  
  17&nbsp;
  18&nbsp;{ 
  19&nbsp;  echo
  20&nbsp;  echo "Archive Description:"
  21&nbsp;  rpm -qpi $1       # Query description.
  22&nbsp;  echo
  23&nbsp;  echo "Archive Listing:"
  24&nbsp;  rpm -qpl $1       # Query listing.
  25&nbsp;  echo
  26&nbsp;  rpm -i --test $1  # Query whether rpm file can be installed.
  27&nbsp;  if [ "$?" -eq $SUCCESS ]
  28&nbsp;  then
  29&nbsp;    echo "$1 can be installed."
  30&nbsp;  else
  31&nbsp;    echo "$1 cannot be installed."
  32&nbsp;  fi  
  33&nbsp;  echo
  34&nbsp;} &#62; "$1.test"       # Redirects output of everything in block to file.
  35&nbsp;
  36&nbsp;echo "Results of rpm test in file $1.test"
  37&nbsp;
  38&nbsp;# See rpm man page for explanation of options.
  39&nbsp;
  40&nbsp;exit 0</PRE
></TD
></TR
></TABLE
><HR></DIV
><DIV
CLASS="NOTE"
><TABLE
CLASS="NOTE"
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><TR
><TD
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><IMG
SRC="common/note.png"
HSPACE="5"
ALT="Note"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
>Unlike a command group within (parentheses),
	      as above, a code block enclosed by {braces} will
	      <I
CLASS="EMPHASIS"
>not</I
> normally launch a <A
HREF="subshells.html#SUBSHELLSREF"
>subshell</A
>.

		<A
NAME="AEN931"
HREF="#FTN.AEN931"
>[2]</A
>

	      </P
></TD
></TR
></TABLE
></DIV
></DD
><DT
><SPAN
CLASS="TOKEN"
>{} \;</SPAN
></DT
><DD
><DIV
CLASS="FORMALPARA"
><P
><B
>pathname. </B
>Mostly used in <A
HREF="moreadv.html#FINDREF"
>find</A
>
		constructs.  This is <I
CLASS="EMPHASIS"
>not</I
> a shell
		<A
HREF="internal.html#BUILTINREF"
>builtin</A
>.</P
></DIV
><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 <SPAN
CLASS="QUOTE"
>"<SPAN
CLASS="TOKEN"
>;</SPAN
>"</SPAN
> ends
		the <TT
CLASS="OPTION"
>-exec</TT
> option of a
		<B
CLASS="COMMAND"
>find</B
> command sequence.  It needs
		to be escaped to protect it from interpretation by the
		shell.</P
></TD
></TR
></TABLE
></DIV
></DD
><DT
><SPAN
CLASS="TOKEN"
>[ ]</SPAN
></DT
><DD
><DIV
CLASS="FORMALPARA"
><P
><B
>test. </B
></P
></DIV
><P
><A
NAME="LEFTBRACKET"
></A
><A
HREF="tests.html#IFTHEN"
>Test</A
> expression between <B
CLASS="COMMAND"
>[
	       ]</B
>.  Note that <B
CLASS="COMMAND"
>[</B
> is part of
	       the shell builtin <B
CLASS="COMMAND"
>test</B
> (and a synonym
	       for it), <I
CLASS="EMPHASIS"
>not</I
> a link to the external
	       command <TT
CLASS="FILENAME"
>/usr/bin/test</TT
>.</P
></DD
><DT
><SPAN
CLASS="TOKEN"
>[[ ]]</SPAN
></DT
><DD
><DIV
CLASS="FORMALPARA"
><P
><B
>test. </B
></P
></DIV
><P
>Test expression between <SPAN
CLASS="TOKEN"
>[[ ]]</SPAN
> (shell
	       <A
HREF="internal.html#KEYWORDREF"
>keyword</A
>).</P
><P
>See the discussion on the <A
HREF="tests.html#DBLBRACKETS"
>[[ ... ]] construct</A
>.</P
></DD
><DT
><SPAN
CLASS="TOKEN"
>[ ]</SPAN
></DT
><DD
><DIV
CLASS="FORMALPARA"
><P
><B
>array element. </B
></P
></DIV
><P
>In the context of an <A
HREF="arrays.html#ARRAYREF"
>array</A
>,
	       brackets set off the numbering of each element of that array.
	         <TABLE
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><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;Array[1]=slot_1
   2&nbsp;echo ${Array[1]}</PRE
></TD
></TR
></TABLE
></P
></DD
><DT
><SPAN
CLASS="TOKEN"
>[ ]</SPAN
></DT
><DD
><DIV
CLASS="FORMALPARA"
><P
><B
>range of characters. </B
></P
></DIV
><P
>As part of a <A
HREF="regexp.html#REGEXREF"
>regular
	       expression</A
>, brackets delineate a <A
HREF="regexp.html#BRACKETSREF"
>range of characters</A
> to
	       match.</P
></DD
><DT
><SPAN
CLASS="TOKEN"
>(( ))</SPAN
></DT
><DD
><DIV
CLASS="FORMALPARA"
><P
><B
>integer expansion. </B
></P
></DIV
><P
>Expand and evaluate integer expression between
	       <SPAN
CLASS="TOKEN"
>(( ))</SPAN
>.</P
><P
>See the discussion on the <A
HREF="dblparens.html"
>(( ... )) construct</A
>.</P
></DD
><DT
><SPAN
CLASS="TOKEN"
>&#62;</SPAN
> <SPAN
CLASS="TOKEN"
>&#38;&#62;</SPAN
> <SPAN
CLASS="TOKEN"
>&#62;&#38;</SPAN
> <SPAN
CLASS="TOKEN"
>&#62;&#62;</SPAN
> <SPAN
CLASS="TOKEN"
>&#60;</SPAN
></DT
><DD
><DIV
CLASS="FORMALPARA"
><P
><B
><A
HREF="io-redirection.html#IOREDIRREF"
>redirection</A
>. </B
></P
></DIV
><P
><TT
CLASS="USERINPUT"
><B
>scriptname &#62;filename</B
></TT
> redirects the output of
	      <TT
CLASS="FILENAME"
>scriptname</TT
> to file
	      <TT
CLASS="FILENAME"
>filename</TT
>. Overwrite
	      <TT
CLASS="FILENAME"
>filename</TT
> if it already exists.</P
><P
><TT
CLASS="USERINPUT"
><B
>command &#38;&#62;filename</B
></TT
> redirects
	      both the <TT
CLASS="FILENAME"
>stdout</TT
> and the
	      <TT
CLASS="FILENAME"
>stderr</TT
> of <TT
CLASS="FILENAME"
>command</TT
>
	      to <TT
CLASS="FILENAME"
>filename</TT
>.</P
><P
><TT
CLASS="USERINPUT"
><B
>command &#62;&#38;2</B
></TT
> redirects
	      <TT
CLASS="FILENAME"
>stdout</TT
> of <TT
CLASS="FILENAME"
>command</TT
>
	      to <TT
CLASS="FILENAME"
>stderr</TT
>.</P
><P
><TT
CLASS="USERINPUT"
><B
>scriptname &#62;&#62;filename</B
></TT
> appends
	    the output of <TT
CLASS="FILENAME"
>scriptname</TT
>
	    to file <TT
CLASS="FILENAME"
>filename</TT
>. If
	    <TT
CLASS="FILENAME"
>filename</TT
> does not already exist,
	    it will be created.</P
><DIV
CLASS="FORMALPARA"
><P
><B
><A
HREF="process-sub.html#PROCESSSUBREF"
>process substitution</A
>. </B
></P
></DIV
><P
><TT
CLASS="USERINPUT"
><B
>(command)&#62;</B
></TT
></P
><P
><TT
CLASS="USERINPUT"
><B
>&#60;(command)</B
></TT
></P
><P
><A
HREF="comparison-ops.html#LTREF"
>In a different context</A
>,
	      the <SPAN
CLASS="QUOTE"
>"<SPAN
CLASS="TOKEN"
>&#60;</SPAN
>"</SPAN
> and
	      <SPAN
CLASS="QUOTE"
>"<SPAN
CLASS="TOKEN"
>&#62;</SPAN
>"</SPAN
> characters act
	      as <A
HREF="comparison-ops.html#SCOMPARISON1"
>string comparison
	      operators</A
>.</P
><P
><A
HREF="comparison-ops.html#INTLT"
>In yet another context</A
>,
	      the <SPAN
CLASS="QUOTE"
>"<SPAN
CLASS="TOKEN"
>&#60;</SPAN
>"</SPAN
> and
	      <SPAN
CLASS="QUOTE"
>"<SPAN
CLASS="TOKEN"
>&#62;</SPAN
>"</SPAN
> characters act
	      as <A
HREF="comparison-ops.html#ICOMPARISON1"
>integer comparison
	      operators</A
>. See also <A
HREF="moreadv.html#EX45"
>Example 12-9</A
>.</P
></DD
><DT
><SPAN
CLASS="TOKEN"
>&#60;&#60;</SPAN
></DT
><DD
><DIV
CLASS="FORMALPARA"
><P
><B
>redirection used in a <A
HREF="here-docs.html#HEREDOCREF"
>here document</A
>. </B
></P
></DIV
></DD
><DT
><SPAN
CLASS="TOKEN"
>&#60;&#60;&#60;</SPAN
></DT
><DD
><DIV
CLASS="FORMALPARA"
><P
><B
>redirection used in a <A
HREF="here-docs.html#HERESTRINGSREF"
>here string</A
>. </B
></P
></DIV
></DD
><DT
><SPAN
CLASS="TOKEN"
>&#60;</SPAN
>, <SPAN
CLASS="TOKEN"
>&#62;</SPAN
></DT
><DD
><DIV
CLASS="FORMALPARA"
><P
><B
><A
HREF="comparison-ops.html#LTREF"
>ASCII
	    comparison</A
>. </B
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;veg1=carrots
   2&nbsp;veg2=tomatoes
   3&nbsp;
   4&nbsp;if [[ "$veg1" &#60; "$veg2" ]]
   5&nbsp;then
   6&nbsp;  echo "Although $veg1 precede $veg2 in the dictionary,"
   7&nbsp;  echo "this implies nothing about my culinary preferences."
   8&nbsp;else
   9&nbsp;  echo "What kind of dictionary are you using, anyhow?"
  10&nbsp;fi</PRE
></TD
></TR
></TABLE
></P
></DIV
></DD
><DT
><SPAN
CLASS="TOKEN"
>\&#60;</SPAN
>, <SPAN
CLASS="TOKEN"
>\&#62;</SPAN
></DT
><DD
><DIV
CLASS="FORMALPARA"
><P
><B
><A
HREF="regexp.html#ANGLEBRAC"
>word
	    boundary</A
> in a <A
HREF="regexp.html#REGEXREF"
>regular
	    expression</A
>. </B
></P
></DIV
><P
><TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>grep '\&#60;the\&#62;' textfile</B
></TT
></P
></DD
><DT
><SPAN
CLASS="TOKEN"
>|</SPAN
></DT
><DD
><P
><A
NAME="PIPEREF"
></A
></P
><DIV
CLASS="FORMALPARA"
><P
><B
>pipe. </B
>Passes the output of previous command to the input
		of the next one, or to the shell. This is a method of
		chaining commands together.</P
></DIV
><P
>              <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;echo ls -l | sh
   2&nbsp;#  Passes the output of "echo ls -l" to the shell,
   3&nbsp;#+ with the same result as a simple "ls -l".
   4&nbsp;
   5&nbsp;
   6&nbsp;cat *.lst | sort | uniq
   7&nbsp;# Merges and sorts all ".lst" files, then deletes duplicate lines.</PRE
></TD
></TR
></TABLE
>
	    </P
><TABLE
CLASS="SIDEBAR"
BORDER="1"
CELLPADDING="5"
><TR
><TD
><DIV
CLASS="SIDEBAR"
><A
NAME="AEN1202"
></A
><P
>A pipe, as a classic method of interprocess
	      communication, sends the <TT
CLASS="FILENAME"
>stdout</TT
>
	      of one process to the <TT
CLASS="FILENAME"
>stdin</TT
>
	      of another.  In a typical case, a command,
	      such as <A
HREF="external.html#CATREF"
>cat</A
> or <A
HREF="internal.html#ECHOREF"
>echo</A
>, pipes a stream of data to a
	      <SPAN
CLASS="QUOTE"
>"filter"</SPAN
> (a command that transforms its input)
	      for processing.</P
><P
>  
	      <TT
CLASS="USERINPUT"
><B
>cat $filename1 $filename2 | grep $search_word</B
></TT
>
            </P
></DIV
></TD
></TR
></TABLE
><P
><A
NAME="UCREF"
></A
>The output of a command or commands
	      may be piped to a script.

	      <TABLE
BORDER="0"