special-chars.html

Shall高级编程

	      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 15-9</A
>.</P
></DD
><DT
><A
NAME="HEREDOCRRREF"
></A
><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
><A
NAME="HERESTRINGREF"
></A
><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 $veg2 in the dictionary,"
   7&nbsp;  echo -n "this does not necessarily imply anything "
   8&nbsp;  echo "about my culinary preferences."
   9&nbsp;else
  10&nbsp;  echo "What kind of dictionary are you using, anyhow?"
  11&nbsp;fi</PRE
></TD
></TR
></TABLE
></P
></DIV
></DD
><DT
><SPAN
CLASS="TOKEN"
>\&#60;</SPAN
>, <SPAN
CLASS="TOKEN"
>\&#62;</SPAN
></DT
><DD
><DIV
CLASS="FORMALPARA"
><P
><B
><A
HREF="regexp.html#ANGLEBRAC"
>word
	    boundary</A
> in a <A
HREF="regexp.html#REGEXREF"
>regular
	    expression</A
>. </B
></P
></DIV
><P
><TT
CLASS="PROMPT"
>bash$ </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 (<TT
CLASS="FILENAME"
>stdout</TT
>
		of a previous command to the input
		(<TT
CLASS="FILENAME"
>stdin</TT
>) 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="AEN1380"
></A
><P
>	      A pipe, as a classic method of interprocess
	      communication, sends the <TT
CLASS="FILENAME"
>stdout</TT
>
	      of one <A
HREF="special-chars.html#PROCESSREF"
>process</A
> 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
	      <A
NAME="FILTERDEF"
></A
>
	      <I
CLASS="FIRSTTERM"
>filter</I
>, a command that
	      transforms its input for processing.
		<A
NAME="AEN1389"
HREF="#FTN.AEN1389"
>[5]</A
>
	      </P
><P
>  
	      <TT
CLASS="USERINPUT"
><B
>cat $filename1 $filename2 | grep $search_word</B
></TT
>
            </P
><P
>For an interesting note on the complexity of using UNIX
	      pipes, see <A
HREF="http://www.faqs.org/faqs/unix-faq/faq/part3/"
TARGET="_top"
>the UNIX FAQ,
	      Part 3</A
>.</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"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# uppercase.sh : Changes input to uppercase.
   3&nbsp;
   4&nbsp;tr 'a-z' 'A-Z'
   5&nbsp;#  Letter ranges must be quoted
   6&nbsp;#+ to prevent filename generation from single-letter filenames.
   7&nbsp;
   8&nbsp;exit 0</PRE
></TD
></TR
></TABLE
>
              Now, let us pipe the output of <B
CLASS="COMMAND"
>ls -l</B
> to this
	      script.

	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="SCREEN"
> <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>ls -l | ./uppercase.sh</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>-RW-RW-R--    1 BOZO  BOZO       109 APR  7 19:49 1.TXT
 -RW-RW-R--    1 BOZO  BOZO       109 APR 14 16:48 2.TXT
 -RW-R--R--    1 BOZO  BOZO       725 APR 20 20:56 DATA-FILE</TT
>
 	      </PRE
></TD
></TR
></TABLE
>
	    </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 <TT
CLASS="FILENAME"
>stdout</TT
> of each process in
	       a pipe must be read as the <TT
CLASS="FILENAME"
>stdin</TT
>
	       of the next. If this is not the case, the data stream
	       will <I
CLASS="FIRSTTERM"
>block</I
>, and the pipe will not
	       behave as expected.
	         <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;cat file1 file2 | ls -l | sort
   2&nbsp;# The output from "cat file1 file2" disappears.</PRE
></TD
></TR
></TABLE
>
             </P
><P
>A pipe runs as a <A
HREF="othertypesv.html#CHILDREF"
>child
	       process</A
>, and therefore cannot alter script
	       variables.
	         <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;variable="initial_value"
   2&nbsp;echo "new_value" | read variable
   3&nbsp;echo "variable = $variable"     # variable = initial_value</PRE
></TD
></TR
></TABLE
>
             </P
><P
>If one of the commands in the pipe
	       aborts, this prematurely terminates execution of the
	       pipe. Called a <I
CLASS="FIRSTTERM"
>broken pipe</I
>, this
	       condition sends a <I
CLASS="FIRSTTERM"
>SIGPIPE</I
> <A
HREF="debugging.html#SIGNALD"
>signal</A
>.</P
></TD
></TR
></TABLE
></DIV
></DD
><DT
><SPAN
CLASS="TOKEN"
>&#62;|</SPAN
></DT
><DD
><DIV
CLASS="FORMALPARA"
><P
><B
>force redirection (even if
		the <A
HREF="options.html#NOCLOBBERREF"
>noclobber option</A
>
		is set). </B
>This will forcibly overwrite an existing file.</P
></DIV
></DD
><DT
><SPAN
CLASS="TOKEN"
>||</SPAN
></DT
><DD
><DIV
CLASS="FORMALPARA"
><P
><B
><A
HREF="operations.html#ORREF"
>OR logical operator</A
>. </B
>In a <A
HREF="tests.html#TESTCONSTRUCTS1"
>test
		construct</A
>, the <SPAN
CLASS="TOKEN"
>||</SPAN
> operator causes
		a return of <SPAN
CLASS="RETURNVALUE"
>0</SPAN
> (success) if
		<SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>either</I
></SPAN
> of the linked test conditions
		is true.</P
></DIV
></DD
><DT
><A
NAME="BGJOB"
></A
><SPAN
CLASS="TOKEN"
>&#38;</SPAN
></DT
><DD
><DIV
CLASS="FORMALPARA"
><P
><B
>Run job in background. </B
>A command followed by an <SPAN
CLASS="TOKEN"
>&#38;</SPAN
>
	        will run in the background.</P
></DIV
><P
>	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="SCREEN"
> <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>sleep 10 &#38;</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>[1] 850</TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>[1]+  Done                    sleep 10</TT
>
 	      </PRE
></TD
></TR
></TABLE
>
	    </P
><P
>Within a script, commands and even <A
HREF="loops.html#FORLOOPREF1"
>loops</A
> may run in the
	      background.</P
><P
><A
NAME="BGLOOP0"
></A
></P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="BGLOOP"
></A
><P
><B
>Example 3-3. Running a loop in the background</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# background-loop.sh
   3&nbsp;
   4&nbsp;for i in 1 2 3 4 5 6 7 8 9 10            # First loop.
   5&nbsp;do
   6&nbsp;  echo -n "$i "
   7&nbsp;done &#38; # Run this loop in background.
   8&nbsp;       # Will sometimes execute after second loop.
   9&nbsp;
  10&nbsp;echo   # This 'echo' sometimes will not display.
  11&nbsp;
  12&nbsp;for i in 11 12 13 14 15 16 17 18 19 20   # Second loop.
  13&nbsp;do
  14&nbsp;  echo -n "$i "
  15&nbsp;done  
  16&nbsp;
  17&nbsp;echo   # This 'echo' sometimes will not display.
  18&nbsp;
  19&nbsp;# ======================================================
  20&nbsp;
  21&nbsp;# The expected output from the script:
  22&nbsp;# 1 2 3 4 5 6 7 8 9 10 
  23&nbsp;# 11 12 13 14 15 16 17 18 19 20 
  24&nbsp;
  25&nbsp;# Sometimes, though, you get:
  26&nbsp;# 11 12 13 14 15 16 17 18 19 20 
  27&nbsp;# 1 2 3 4 5 6 7 8 9 10 bozo $
  28&nbsp;# (The second 'echo' doesn't execute. Why?)
  29&nbsp;
  30&nbsp;# Occasionally also:
  31&nbsp;# 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
  32&nbsp;# (The first 'echo' doesn't execute. Why?)
  33&nbsp;
  34&nbsp;# Very rarely something like:
  35&nbsp;# 11 12 13 1 2 3 4 5 6 7 8 9 10 14 15 16 17 18 19 20 
  36&nbsp;# The foreground loop preempts the background one.
  37&nbsp;
  38&nbsp;exit 0
  39&nbsp;
  40&nbsp;#  Nasimuddin Ansari suggests adding    sleep 1
  41&nbsp;#+ after the   echo -n "$i"   in lines 6 and 14,
  42&nbsp;#+ for some real fun.</PRE
></TD
></TR
></TABLE
><HR></DIV
><DIV
CLASS="CAUTION"
><TABLE
CLASS="CAUTION"
WIDTH="90%"
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
>A command run in the background within a
	      script may cause the script to hang, waiting
	      for a keystroke. Fortunately, there is a <A
HREF="internal.html#WAITHANG"
>remedy</A
> for this.</P
></TD
></TR
></TABLE
></DIV
></DD
><DT
><A
NAME="LOGICALAND"
></A
><SPAN
CLASS="TOKEN"
>&#38;&#38;</SPAN
></DT
><DD
><DIV
CLASS="FORMALPARA"
><P
><B
><A
HREF="operations.html#LOGOPS1"
>AND logical
	    operator</A
>. </B
>In a <A
HREF="tests.html#TESTCONSTRUCTS1"
>test
		construct</A
>, the <SPAN
CLASS="TOKEN"
>&#38;&#38;</SPAN
> operator causes
		a return of <SPAN
CLASS="RETURNVALUE"
>0</SPAN
> (success) only if
		<SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>both</I
></SPAN
> the linked test conditions
		are true.</P
></DIV
></DD
><DT
><A
NAME="DASHREF"
></A
><SPAN
CLASS="TOKEN"
>-</SPAN
></DT
><DD
><DIV
CLASS="FORMALPARA"
><P
><B
>option, prefix. </B
>Option flag for a command or filter. Prefix for
		an operator. Prefix for a <A
HREF="parameter-substitution.html#DEFPARAM1"
>default parameter</A
>
		in <A
HREF="parameter-substitution.html#PARAMSUBREF"
>parameter
		substitution</A
>.</P
></DIV
><P
><TT
CLASS="USERINPUT"
><B
>COMMAND -[Option1][Option2][...]</B
></TT
></P
><P
><TT
CLASS="USERINPUT"
><B
>ls -al</B
></TT
></P
><P
><TT
CLASS="USERINPUT"
><B
>sort -dfu $filename</B
></TT
></P
><P
>	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;if [ $file1 -ot $file2 ]
   2&nbsp;then #      ^
   3&nbsp;  echo "File $file1 is older than $file2."
   4&nbsp;fi
   5&nbsp;
   6&nbsp;if [ "$a" -eq "$b" ]
   7&nbsp;then      ^
   8&nbsp;  echo "$a is equal to $b."
   9&nbsp;fi
  10&nbsp;
  11&nbsp;if [ "$c" -eq 24 -a "$d" -eq 47 ]
  12&nbsp;then      ^              ^
  13&nbsp;  echo "$c equals 24 and $d equals 47."
  14&nbsp;fi
  15&nbsp;
  16&nbsp;
  17&nbsp;param2=${param1:-$DEFAULTVAL}
  18&nbsp;#               ^</PRE
></TD
></TR
></TABLE
>  
	      </P
><P
><A
NAME="DOUBLEDASHREF"
></A
></P
><P
><B
CLASS="COMMAND"
>--</B
></P
><P
>The <I
CLASS="FIRSTTERM"
>double-dash</I
>
		<TT
CLASS="OPTION"
>--</TT
> prefixes <I
CLASS="FIRSTTERM"
>long</I
>
		(verbatim) options to commands.</P
><P
><TT
CLASS="USERINPUT"
><B
>sort --ignore-leading-blanks</B
></TT
></P
><P
>Used with a <A
HREF="internal.html#BUILTINREF"
>Bash
	        builtin</A
>, it means the <I
CLASS="FIRSTTERM"
>end of
		options</I
> to that particular command.</P
><DIV
CLASS="TIP"
><TABLE
CLASS="TIP"
WIDTH="90%"
BORDER="0"
><TR
><TD
WIDTH="25"
ALIGN="CENTER"
VALIGN="TOP"
><IMG
SRC="common/tip.png"
HSPACE="5"
ALT="Tip"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
>This provides a handy means of removing
		  files whose <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>names begin with a dash</I
></SPAN
>.
	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="SCREEN"
> <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>ls -l</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>-rw-r--r-- 1 bozo bozo 0 Nov 25 12:29 -badname</TT
>
 
 
 <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>rm -- -badname</B
></TT
>
 
 <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>ls -l</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>total 0</TT
></PRE
></TD
></TR
></TABLE
>
	    </P
></TD
></TR
></TABLE
></DIV
><P
>The <I
CLASS="FIRSTTERM"
>double-dash</I
> is also used in
	        conjunction with <A
HREF="internal.html#SETREF"
>set</A
>.</P
><P
><TT
CLASS="USERINPUT"
><B
>set -- $variable</B
></TT
> (as in <A
HREF="internal.html#SETPOS"
>Example 14-18</A
>)</P
></DD
><DT
><A
NAME="DASHREF2"
></A
><SPAN
CLASS="TOKEN"
>-</SPAN
></DT
><DD
><DIV
CLASS="FORMALPARA"
><P
><B
>redirection from/to <TT
CLASS="FILENAME"
>stdin</TT
> or <TT
CLASS="FILENAME"