assortedtips.html

Shall高级编程

><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;
   3&nbsp;SUCCESS=0
   4&nbsp;E_BADINPUT=65
   5&nbsp;
   6&nbsp;test "$1" -ne 0 -o "$1" -eq 0 2&#62;/dev/null
   7&nbsp;# An integer is either equal to 0 or not equal to 0.
   8&nbsp;# 2&#62;/dev/null suppresses error message.
   9&nbsp;
  10&nbsp;if [ $? -ne "$SUCCESS" ]
  11&nbsp;then
  12&nbsp;  echo "Usage: `basename $0` integer-input"
  13&nbsp;  exit $E_BADINPUT
  14&nbsp;fi
  15&nbsp;
  16&nbsp;let "sum = $1 + 25"             # Would give error if $1 not integer.
  17&nbsp;echo "Sum = $sum"
  18&nbsp;
  19&nbsp;# Any variable, not just a command line parameter, can be tested this way.
  20&nbsp;
  21&nbsp;exit 0</PRE
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>
          </P
></LI
><LI
><P
><A
NAME="RVT"
></A
>The 0 - 255 range for function return
	    values is a severe limitation. Global variables and
	    other workarounds are often problematic. An alternative
	    method for a function to communicate a value back to
	    the main body of the script is to have the function
	    write to <TT
CLASS="FILENAME"
>stdout</TT
> (usually with
	    <A
HREF="internal.html#ECHOREF"
>echo</A
>) the <SPAN
CLASS="QUOTE"
>"return
	    value,"</SPAN
> and assign this to a variable. This is
	    actually a variant of <A
HREF="commandsub.html#COMMANDSUBREF"
>command
	    substitution.</A
></P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="MULTIPLICATION"
></A
><P
><B
>Example 33-15. Return value trickery</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# multiplication.sh
   3&nbsp;
   4&nbsp;multiply ()                     # Multiplies params passed.
   5&nbsp;{                               # Will accept a variable number of args.
   6&nbsp;
   7&nbsp;  local product=1
   8&nbsp;
   9&nbsp;  until [ -z "$1" ]             # Until uses up arguments passed...
  10&nbsp;  do
  11&nbsp;    let "product *= $1"
  12&nbsp;    shift
  13&nbsp;  done
  14&nbsp;
  15&nbsp;  echo $product                 #  Will not echo to stdout,
  16&nbsp;}                               #+ since this will be assigned to a variable.
  17&nbsp;
  18&nbsp;mult1=15383; mult2=25211
  19&nbsp;val1=`multiply $mult1 $mult2`
  20&nbsp;echo "$mult1 X $mult2 = $val1"
  21&nbsp;                                # 387820813
  22&nbsp;
  23&nbsp;mult1=25; mult2=5; mult3=20
  24&nbsp;val2=`multiply $mult1 $mult2 $mult3`
  25&nbsp;echo "$mult1 X $mult2 X $mult3 = $val2"
  26&nbsp;                                # 2500
  27&nbsp;
  28&nbsp;mult1=188; mult2=37; mult3=25; mult4=47
  29&nbsp;val3=`multiply $mult1 $mult2 $mult3 $mult4`
  30&nbsp;echo "$mult1 X $mult2 X $mult3 X $mult4 = $val3"
  31&nbsp;                                # 8173300
  32&nbsp;
  33&nbsp;exit 0</PRE
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><HR></DIV
><P
>The same technique also works for alphanumeric
	    strings. This means that a function can <SPAN
CLASS="QUOTE"
>"return"</SPAN
>
	    a non-numeric value.</P
><P
>	  <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;capitalize_ichar ()          #  Capitalizes initial character
   2&nbsp;{                            #+ of argument string(s) passed.
   3&nbsp;
   4&nbsp;  string0="$@"               # Accepts multiple arguments.
   5&nbsp;
   6&nbsp;  firstchar=${string0:0:1}   # First character.
   7&nbsp;  string1=${string0:1}       # Rest of string(s).
   8&nbsp;
   9&nbsp;  FirstChar=`echo "$firstchar" | tr a-z A-Z`
  10&nbsp;                             # Capitalize first character.
  11&nbsp;
  12&nbsp;  echo "$FirstChar$string1"  # Output to stdout.
  13&nbsp;
  14&nbsp;}  
  15&nbsp;
  16&nbsp;newstring=`capitalize_ichar "every sentence should start with a capital letter."`
  17&nbsp;echo "$newstring"          # Every sentence should start with a capital letter.</PRE
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> 
	  </P
><P
>It is even possible for a function to <SPAN
CLASS="QUOTE"
>"return"</SPAN
>
	    multiple values with this method.</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="SUMPRODUCT"
></A
><P
><B
>Example 33-16. Even more return value trickery</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# sum-product.sh
   3&nbsp;# A function may "return" more than one value.
   4&nbsp;
   5&nbsp;sum_and_product ()   # Calculates both sum and product of passed args.
   6&nbsp;{
   7&nbsp;  echo $(( $1 + $2 )) $(( $1 * $2 ))
   8&nbsp;# Echoes to stdout each calculated value, separated by space.
   9&nbsp;}
  10&nbsp;
  11&nbsp;echo
  12&nbsp;echo "Enter first number "
  13&nbsp;read first
  14&nbsp;
  15&nbsp;echo
  16&nbsp;echo "Enter second number "
  17&nbsp;read second
  18&nbsp;echo
  19&nbsp;
  20&nbsp;retval=`sum_and_product $first $second`      # Assigns output of function.
  21&nbsp;sum=`echo "$retval" | awk '{print $1}'`      # Assigns first field.
  22&nbsp;product=`echo "$retval" | awk '{print $2}'`  # Assigns second field.
  23&nbsp;
  24&nbsp;echo "$first + $second = $sum"
  25&nbsp;echo "$first * $second = $product"
  26&nbsp;echo
  27&nbsp;
  28&nbsp;exit 0</PRE
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><HR></DIV
></LI
><LI
><P
><A
NAME="PASSARRAY"
></A
></P
><P
>Next in our bag of tricks are techniques for passing
	    an <A
HREF="arrays.html#ARRAYREF"
>array</A
> to a
	    <A
HREF="functions.html#FUNCTIONREF"
>function</A
>, then
	    <SPAN
CLASS="QUOTE"
>"returning"</SPAN
> an array back to the main body of
	    the script.</P
><P
>Passing an array involves loading the space-separated
	    elements of the array into a variable with <A
HREF="commandsub.html#COMMANDSUBREF"
>command substitution</A
>. <A
NAME="RETARRAY"
></A
>Getting an array back as the <SPAN
CLASS="QUOTE"
>"return
	    value"</SPAN
> from a function uses the previously mentioned
	    strategem of <A
HREF="internal.html#ECHOREF"
>echoing</A
> the
	    array in the function, then invoking command substitution
	    and the <B
CLASS="COMMAND"
>( ... )</B
> operator to assign it to
	    an array.</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="ARRFUNC"
></A
><P
><B
>Example 33-17. Passing and returning arrays</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# array-function.sh: Passing an array to a function and...
   3&nbsp;#                   "returning" an array from a function
   4&nbsp;
   5&nbsp;
   6&nbsp;Pass_Array ()
   7&nbsp;{
   8&nbsp;  local passed_array   # Local variable.
   9&nbsp;  passed_array=( `echo "$1"` )
  10&nbsp;  echo "${passed_array[@]}"
  11&nbsp;  #  List all the elements of the new array
  12&nbsp;  #+ declared and set within the function.
  13&nbsp;}
  14&nbsp;
  15&nbsp;
  16&nbsp;original_array=( element1 element2 element3 element4 element5 )
  17&nbsp;
  18&nbsp;echo
  19&nbsp;echo "original_array = ${original_array[@]}"
  20&nbsp;#                      List all elements of original array.
  21&nbsp;
  22&nbsp;
  23&nbsp;# This is the trick that permits passing an array to a function.
  24&nbsp;# **********************************
  25&nbsp;argument=`echo ${original_array[@]}`
  26&nbsp;# **********************************
  27&nbsp;#  Pack a variable
  28&nbsp;#+ with all the space-separated elements of the original array.
  29&nbsp;#
  30&nbsp;# Note that attempting to just pass the array itself will not work.
  31&nbsp;
  32&nbsp;
  33&nbsp;# This is the trick that allows grabbing an array as a "return value".
  34&nbsp;# *****************************************
  35&nbsp;returned_array=( `Pass_Array "$argument"` )
  36&nbsp;# *****************************************
  37&nbsp;# Assign 'echoed' output of function to array variable.
  38&nbsp;
  39&nbsp;echo "returned_array = ${returned_array[@]}"
  40&nbsp;
  41&nbsp;echo "============================================================="
  42&nbsp;
  43&nbsp;#  Now, try it again,
  44&nbsp;#+ attempting to access (list) the array from outside the function.
  45&nbsp;Pass_Array "$argument"
  46&nbsp;
  47&nbsp;# The function itself lists the array, but...
  48&nbsp;#+ accessing the array from outside the function is forbidden.
  49&nbsp;echo "Passed array (within function) = ${passed_array[@]}"
  50&nbsp;# NULL VALUE since this is a variable local to the function.
  51&nbsp;
  52&nbsp;echo
  53&nbsp;
  54&nbsp;exit 0</PRE
></TD
></TR
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><HR></DIV
><P
>For a more elaborate example of passing arrays to
	    functions, see <A
HREF="contributed-scripts.html#LIFESLOW"
>Example A-10</A
>.</P
></LI
><LI
><P
><A
NAME="CSTYLE"
></A
></P
><P
>Using the <A
HREF="dblparens.html"
>double parentheses
	    construct</A
>, it is possible to use C-style syntax
	    for setting and incrementing/decrementing variables
	    and in <A
HREF="loops.html#FORLOOPREF1"
>for</A
> and <A
HREF="loops.html#WHILELOOPREF"
>while</A
> loops.	See <A
HREF="loops.html#FORLOOPC"
>Example 10-12</A
> and <A
HREF="loops.html#WHLOOPC"
>Example 10-17</A
>.</P
></LI
><LI
><P
><A
NAME="SETPUM"
></A
></P
><P
>Setting the <A
HREF="variables2.html#PATHREF"
>path</A
> and <A
HREF="system.html#UMASKREF"
>umask</A
> at the beginning of a script makes
	    it more <SPAN
CLASS="QUOTE"
>"portable"</SPAN
> -- more likely to run on a
	    <SPAN
CLASS="QUOTE"
>"foreign"</SPAN
> machine whose user may have bollixed up the
	    <TT
CLASS="VARNAME"
>$PATH</TT
> and <B
CLASS="COMMAND"
>umask</B
>.
	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;PATH=/bin:/usr/bin:/usr/local/bin ; export PATH
   3&nbsp;umask 022   # Files that the script creates will have 755 permission.
   4&nbsp;
   5&nbsp;# Thanks to Ian D. Allen, for this tip.</PRE
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></P
></LI
><LI
><P
><A
NAME="FILTEROUTP"
></A
></P
><P
>A useful scripting technique is to
	    <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>repeatedly</I
></SPAN
> feed the output of a filter
	    (by piping) back to the <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>same filter</I
></SPAN
>, but
	    with a different set of arguments and/or options. Especially
	    suitable for this are <A
HREF="textproc.html#TRREF"
>tr</A
> and
	    <A
HREF="textproc.html#GREPREF"
>grep</A
>.</P
><P
>	  <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;# From "wstrings.sh" example.
   2&nbsp;
   3&nbsp;wlist=`strings "$1" | tr A-Z a-z | tr '[:space:]' Z | \
   4&nbsp;tr -cs '[:alpha:]' Z | tr -s '\173-\377' Z | tr Z ' '`</PRE
></TD
></TR
></TABLE
>
	  </P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="AGRAM"
></A
><P
><B
>Example 33-18. Fun with anagrams</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# agram.sh: Playing games with anagrams.
   3&nbsp;
   4&nbsp;# Find anagrams of...
   5&nbsp;LETTERSET=etaoinshrdlu
   6&nbsp;FILTER='.......'       # How many letters minimum?
   7&nbsp;#       1234567
   8&nbsp;
   9&nbsp;anagram "$LETTERSET" | # Find all anagrams of the letterset...
  10&nbsp;grep "$FILTER" |       # With at least 7 letters,
  11&nbsp;grep '^is' |           # starting with 'is'
  12&nbsp;grep -v 's$' |         # no plurals
  13&nbsp;grep -v 'ed$'          # no past tense verbs
  14&nbsp;# Possible to add many combinations of conditions and filters.
  15&nbsp;
  16&nbsp;#  Uses "anagram" utility
  17&nbsp;#+ that is part of the author's "yawl" word list package.
  18&nbsp;#  http://ibiblio.org/pub/Linux/libs/yawl-0.3.2.tar.gz
  19&nbsp;#  http://personal.riverusers.com/~thegrendel/yawl-0.3.2.tar.gz
  20&nbsp;
  21&nbsp;exit 0                 # End of code.
  22&nbsp;
  23&nbsp;
  24&nbsp;bash$ sh agram.sh
  25&nbsp;islander
  26&nbsp;isolate
  27&nbsp;isolead
  28&nbsp;isotheral
  29&nbsp;
  30&nbsp;
  31&nbsp;
  32&nbsp;#  Exercises:
  33&nbsp;#  ---------
  34&nbsp;#  Modify this script to take the LETTERSET as a command-line parameter.
  35&nbsp;#  Parameterize the filters in lines 11 - 13 (as with $FILTER),
  36&nbsp;#+ so that they can be specified by passing arguments to a function.
  37&nbsp;
  38&nbsp;#  For a slightly different approach to anagramming,
  39&nbsp;#+ see the agram2.sh script.</PRE
></TD
></TR
></TABLE
><HR></DIV
><P
>See also <A
HREF="procref1.html#CONSTAT"
>Example 27-3</A
>, <A
HREF="textproc.html#CRYPTOQUOTE"
>Example 15-24</A
>, and <A
HREF="contributed-scripts.html#SOUNDEX"
>Example A-9</A
>.</P
></LI
><LI
><P
><A
NAME="COMMBLAHD"
></A
></P
><P
>Use <SPAN
CLASS="QUOTE"
>"<A
HREF="here-docs.html#ANONHEREDOC0"
>anonymous here
	    documents</A
>"</SPAN
> to comment out blocks of code,
	    to save having to individually comment out each line with
	    a <SPAN
CLASS="TOKEN"
>#</SPAN
>.  See <A
HREF="here-docs.html#COMMENTBLOCK"
>Example 18-11</A
>.</P
></LI
><LI
><P
><A
NAME="WHATISREF3"
></A
></P
><P
>Running a script on a machine that relies on a command
	    that might not be installed is dangerous. Use <A
HREF="filearchiv.html#WHATISREF"
>whatis</A
> to avoid potential problems
	    with this.</P
><P
>	    <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;CMD=command1                 # First choice.
   2&nbsp;PlanB=command2               # Fallback option.
   3&nbsp;
   4&nbsp;command_test=$(whatis "$CMD" | grep 'nothing appropriate')
   5&nbsp;#  If 'command1' not found on system , 'whatis' will return
   6&nbsp;#+ "command1: nothing appropriate."
   7&nbsp;#
   8&nbsp;#  A safer alternative is: