moreadv.html

Shall高级编程

  34 /g' | \
  35 #  Filter out periods and commas, and
  36 #+ change space between words to linefeed,
  37 sort | uniq -c | sort -nr
  38 #  Finally prefix occurrence count and sort numerically.
  39 ########################################################
  40 
  41 #  This does the same job as the "wf.sh" example,
  42 #+ but a bit more ponderously, and it runs more slowly (why?).
  43 
  44&nbsp;exit 0</PRE
></TD
></TR
></TABLE
><HR></DIV
></DD
><DT
><A
NAME="EXPRREF"
></A
><TT
CLASS="USERINPUT"
><B
>expr</B
></TT
></DT
><DD
><P
>All-purpose expression evaluator:
	      Concatenates and evaluates the arguments according
	      to the operation given (arguments must be separated
	      by spaces). Operations may be arithmetic, comparison,
	      string, or logical.</P
><DIV
CLASS="VARIABLELIST"
><DL
><DT
><TT
CLASS="USERINPUT"
><B
>expr 3 + 5</B
></TT
></DT
><DD
><P
>returns <TT
CLASS="LITERAL"
>8</TT
></P
></DD
><DT
><TT
CLASS="USERINPUT"
><B
>expr 5 % 3</B
></TT
></DT
><DD
><P
>returns 2</P
></DD
><DT
><TT
CLASS="USERINPUT"
><B
>expr 1 / 0</B
></TT
></DT
><DD
><P
>returns the error message, <SPAN
CLASS="ERRORCODE"
>expr: division by
		    zero</SPAN
></P
><P
>Illegal arithmetic operations not allowed.</P
></DD
><DT
><TT
CLASS="USERINPUT"
><B
>expr 5 \* 3</B
></TT
></DT
><DD
><P
>returns 15</P
><P
>The multiplication operator
		  must be escaped when used in an arithmetic expression
		  with <B
CLASS="COMMAND"
>expr</B
>.</P
></DD
><DT
><TT
CLASS="USERINPUT"
><B
>y=`expr $y + 1`</B
></TT
></DT
><DD
><P
>Increment a variable, with the same effect
		    as <TT
CLASS="USERINPUT"
><B
>let y=y+1</B
></TT
> and
		    <TT
CLASS="USERINPUT"
><B
>y=$(($y+1))</B
></TT
>. This is an
		    example of <A
HREF="arithexp.html#ARITHEXPREF"
>arithmetic
		    expansion</A
>.</P
></DD
><DT
><A
NAME="EXPEXTRSUB"
></A
><TT
CLASS="USERINPUT"
><B
>z=`expr substr
		$string $position $length`</B
></TT
></DT
><DD
><P
>Extract substring of $length characters, starting
		    at $position.</P
></DD
></DL
></DIV
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="EX45"
></A
><P
><B
>Example 15-9. Using <I
CLASS="FIRSTTERM"
>expr</I
></B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;
   3&nbsp;# Demonstrating some of the uses of 'expr'
   4&nbsp;# =======================================
   5&nbsp;
   6&nbsp;echo
   7&nbsp;
   8&nbsp;# Arithmetic Operators
   9&nbsp;# ---------- ---------
  10&nbsp;
  11&nbsp;echo "Arithmetic Operators"
  12&nbsp;echo
  13&nbsp;a=`expr 5 + 3`
  14&nbsp;echo "5 + 3 = $a"
  15&nbsp;
  16&nbsp;a=`expr $a + 1`
  17&nbsp;echo
  18&nbsp;echo "a + 1 = $a"
  19&nbsp;echo "(incrementing a variable)"
  20&nbsp;
  21&nbsp;a=`expr 5 % 3`
  22&nbsp;# modulo
  23&nbsp;echo
  24&nbsp;echo "5 mod 3 = $a"
  25&nbsp;
  26&nbsp;echo
  27&nbsp;echo
  28&nbsp;
  29&nbsp;# Logical Operators
  30&nbsp;# ------- ---------
  31&nbsp;
  32&nbsp;#  Returns 1 if true, 0 if false,
  33&nbsp;#+ opposite of normal Bash convention.
  34&nbsp;
  35&nbsp;echo "Logical Operators"
  36&nbsp;echo
  37&nbsp;
  38&nbsp;x=24
  39&nbsp;y=25
  40&nbsp;b=`expr $x = $y`         # Test equality.
  41&nbsp;echo "b = $b"            # 0  ( $x -ne $y )
  42&nbsp;echo
  43&nbsp;
  44&nbsp;a=3
  45&nbsp;b=`expr $a \&#62; 10`
  46&nbsp;echo 'b=`expr $a \&#62; 10`, therefore...'
  47&nbsp;echo "If a &#62; 10, b = 0 (false)"
  48&nbsp;echo "b = $b"            # 0  ( 3 ! -gt 10 )
  49&nbsp;echo
  50&nbsp;
  51&nbsp;b=`expr $a \&#60; 10`
  52&nbsp;echo "If a &#60; 10, b = 1 (true)"
  53&nbsp;echo "b = $b"            # 1  ( 3 -lt 10 )
  54&nbsp;echo
  55&nbsp;# Note escaping of operators.
  56&nbsp;
  57&nbsp;b=`expr $a \&#60;= 3`
  58&nbsp;echo "If a &#60;= 3, b = 1 (true)"
  59&nbsp;echo "b = $b"            # 1  ( 3 -le 3 )
  60&nbsp;# There is also a "\&#62;=" operator (greater than or equal to).
  61&nbsp;
  62&nbsp;
  63&nbsp;echo
  64&nbsp;echo
  65&nbsp;
  66&nbsp;
  67&nbsp;
  68&nbsp;# String Operators
  69&nbsp;# ------ ---------
  70&nbsp;
  71&nbsp;echo "String Operators"
  72&nbsp;echo
  73&nbsp;
  74&nbsp;a=1234zipper43231
  75&nbsp;echo "The string being operated upon is \"$a\"."
  76&nbsp;
  77&nbsp;# length: length of string
  78&nbsp;b=`expr length $a`
  79&nbsp;echo "Length of \"$a\" is $b."
  80&nbsp;
  81&nbsp;# index: position of first character in substring
  82&nbsp;#        that matches a character in string
  83&nbsp;b=`expr index $a 23`
  84&nbsp;echo "Numerical position of first \"2\" in \"$a\" is \"$b\"."
  85&nbsp;
  86&nbsp;# substr: extract substring, starting position &#38; length specified
  87&nbsp;b=`expr substr $a 2 6`
  88&nbsp;echo "Substring of \"$a\", starting at position 2,\
  89&nbsp;and 6 chars long is \"$b\"."
  90&nbsp;
  91&nbsp;
  92&nbsp;#  The default behavior of the 'match' operations is to
  93&nbsp;#+ search for the specified match at the ***beginning*** of the string.
  94&nbsp;#
  95&nbsp;#        uses Regular Expressions
  96&nbsp;b=`expr match "$a" '[0-9]*'`               #  Numerical count.
  97&nbsp;echo Number of digits at the beginning of \"$a\" is $b.
  98&nbsp;b=`expr match "$a" '\([0-9]*\)'`           #  Note that escaped parentheses
  99&nbsp;#                   ==      ==              + trigger substring match.
 100&nbsp;echo "The digits at the beginning of \"$a\" are \"$b\"."
 101&nbsp;
 102&nbsp;echo
 103&nbsp;
 104&nbsp;exit 0</PRE
></TD
></TR
></TABLE
><HR></DIV
><DIV
CLASS="IMPORTANT"
><TABLE
CLASS="IMPORTANT"
WIDTH="90%"
BORDER="0"
><TR
><TD
WIDTH="25"
ALIGN="CENTER"
VALIGN="TOP"
><IMG
SRC="common/important.png"
HSPACE="5"
ALT="Important"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
>The <A
HREF="special-chars.html#NULLREF"
>:</A
> operator
	      can substitute for <B
CLASS="COMMAND"
>match</B
>. For example,
	      <TT
CLASS="USERINPUT"
><B
>b=`expr $a : [0-9]*`</B
></TT
> is the
	      exact equivalent of <TT
CLASS="USERINPUT"
><B
>b=`expr match $a
	      [0-9]*`</B
></TT
> in the above listing.</P
><P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;
   3&nbsp;echo
   4&nbsp;echo "String operations using \"expr \$string : \" construct"
   5&nbsp;echo "==================================================="
   6&nbsp;echo
   7&nbsp;
   8&nbsp;a=1234zipper5FLIPPER43231
   9&nbsp;
  10&nbsp;echo "The string being operated upon is \"`expr "$a" : '\(.*\)'`\"."
  11&nbsp;#     Escaped parentheses grouping operator.            ==  ==
  12&nbsp;
  13&nbsp;#       ***************************
  14&nbsp;#+          Escaped parentheses
  15&nbsp;#+           match a substring
  16&nbsp;#       ***************************
  17&nbsp;
  18&nbsp;
  19&nbsp;#  If no escaped parentheses...
  20&nbsp;#+ then 'expr' converts the string operand to an integer.
  21&nbsp;
  22&nbsp;echo "Length of \"$a\" is `expr "$a" : '.*'`."   # Length of string
  23&nbsp;
  24&nbsp;echo "Number of digits at the beginning of \"$a\" is `expr "$a" : '[0-9]*'`."
  25&nbsp;
  26&nbsp;# ------------------------------------------------------------------------- #
  27&nbsp;
  28&nbsp;echo
  29&nbsp;
  30&nbsp;echo "The digits at the beginning of \"$a\" are `expr "$a" : '\([0-9]*\)'`."
  31&nbsp;#                                                             ==      ==
  32&nbsp;echo "The first 7 characters of \"$a\" are `expr "$a" : '\(.......\)'`."
  33&nbsp;#         =====                                          ==       ==
  34&nbsp;# Again, escaped parentheses force a substring match.
  35&nbsp;#
  36&nbsp;echo "The last 7 characters of \"$a\" are `expr "$a" : '.*\(.......\)'`."
  37&nbsp;#         ====                  end of string operator  ^^
  38&nbsp;#  (actually means skip over one or more of any characters until specified
  39&nbsp;#+  substring)
  40&nbsp;
  41&nbsp;echo
  42&nbsp;
  43&nbsp;exit 0</PRE
></TD
></TR
></TABLE
></P
></TD
></TR
></TABLE
></DIV
></DD
></DL
></DIV
><P
>The above script illustrates how
	      <B
CLASS="COMMAND"
>expr</B
> uses the <I
CLASS="FIRSTTERM"
>escaped
	      parentheses -- \( ... \) --</I
> grouping operator
	      in tandem with <A
HREF="regexp.html#REGEXREF"
>regular
	      expression</A
> parsing to match a substring.
	      Here is a another example, this time from <SPAN
CLASS="QUOTE"
>"real
	      life."</SPAN
>

	        <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;# Strip the whitespace from the beginning and end.
   2&nbsp;LRFDATE=`expr "$LRFDATE" : '[[:space:]]*\(.*\)[[:space:]]*$'`
   3&nbsp;
   4&nbsp;#  From Peter Knowles' "booklistgen.sh" script
   5&nbsp;#+ for converting files to Sony Librie/PRS-50X format.
   6&nbsp;#  (http://booklistgensh.peterknowles.com)</PRE
></TD
></TR
></TABLE
>

	      </P
><P
><A
HREF="wrapper.html#PERLREF"
>Perl</A
>,
	      <A
HREF="sedawk.html#SEDREF"
>sed</A
>, and <A
HREF="awk.html#AWKREF"
>awk</A
> have far superior string
	      parsing facilities.  A short <B
CLASS="COMMAND"
>sed</B
> or
	      <B
CLASS="COMMAND"
>awk</B
> <SPAN
CLASS="QUOTE"
>"subroutine"</SPAN
> within
	      a script (see <A
HREF="wrapper.html"
>Section 33.3</A
>) is an attractive
	      alternative to <B
CLASS="COMMAND"
>expr</B
>.</P
><P
>See <A
HREF="string-manipulation.html"
>Section 9.2</A
> for more on
              using <B
CLASS="COMMAND"
>expr</B
> in string operations.</P
></DIV
><H3
CLASS="FOOTNOTES"
>Notes</H3
><TABLE
BORDER="0"
CLASS="FOOTNOTES"
WIDTH="100%"
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="5%"
><A
NAME="FTN.AEN9532"
HREF="moreadv.html#AEN9532"
>[1]</A
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="95%"
><P
>And even when <I
CLASS="FIRSTTERM"
>xargs</I
> is
		not strictly necessary, it can speed up execution of a command
		involving <A
HREF="timedate.html#BATCHPROCREF"
>batch-processing</A
> of multiple
		files.</P
></TD
></TR
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