loops.html

Shall高级编程

>
	      <A
HREF="loops.html#WHILENOBRACKETS"
>are <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>not</I
></SPAN
>
	      mandatory</A
> in a <I
CLASS="FIRSTTERM"
>while</I
> loop.
	      See, for example,  the <A
HREF="internal.html#GETOPTSX"
>getopts
	      construct</A
>.</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="EX25"
></A
><P
><B
>Example 10-14. Simple <I
CLASS="FIRSTTERM"
>while</I
> loop</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;
   3&nbsp;var0=0
   4&nbsp;LIMIT=10
   5&nbsp;
   6&nbsp;while [ "$var0" -lt "$LIMIT" ]
   7&nbsp;#      ^                    ^
   8&nbsp;# Spaces, because these are "test-brackets" . . .
   9&nbsp;do
  10&nbsp;  echo -n "$var0 "        # -n suppresses newline.
  11&nbsp;  #             ^           Space, to separate printed out numbers.
  12&nbsp;
  13&nbsp;  var0=`expr $var0 + 1`   # var0=$(($var0+1))  also works.
  14&nbsp;                          # var0=$((var0 + 1)) also works.
  15&nbsp;                          # let "var0 += 1"    also works.
  16&nbsp;done                      # Various other methods also work.
  17&nbsp;
  18&nbsp;echo
  19&nbsp;
  20&nbsp;exit 0</PRE
></TD
></TR
></TABLE
><HR></DIV
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="EX26"
></A
><P
><B
>Example 10-15. Another <I
CLASS="FIRSTTERM"
>while</I
> loop</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;
   3&nbsp;echo
   4&nbsp;                               # Equivalent to:
   5&nbsp;while [ "$var1" != "end" ]     # while test "$var1" != "end"
   6&nbsp;do
   7&nbsp;  echo "Input variable #1 (end to exit) "
   8&nbsp;  read var1                    # Not 'read $var1' (why?).
   9&nbsp;  echo "variable #1 = $var1"   # Need quotes because of "#" . . .
  10&nbsp;  # If input is 'end', echoes it here.
  11&nbsp;  # Does not test for termination condition until top of loop.
  12&nbsp;  echo
  13&nbsp;done  
  14&nbsp;
  15&nbsp;exit 0</PRE
></TD
></TR
></TABLE
><HR></DIV
><P
><A
NAME="WHMULTCOND"
></A
></P
><P
>A <I
CLASS="FIRSTTERM"
>while loop</I
> may have multiple
	      conditions. Only the final condition determines when the loop
	      terminates. This necessitates a slightly different loop syntax,
	      however.</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="EX26A"
></A
><P
><B
>Example 10-16. <I
CLASS="FIRSTTERM"
>while</I
> loop with multiple conditions</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;
   3&nbsp;var1=unset
   4&nbsp;previous=$var1
   5&nbsp;
   6&nbsp;while echo "previous-variable = $previous"
   7&nbsp;      echo
   8&nbsp;      previous=$var1
   9&nbsp;      [ "$var1" != end ] # Keeps track of what $var1 was previously.
  10&nbsp;      # Four conditions on "while", but only last one controls loop.
  11&nbsp;      # The *last* exit status is the one that counts.
  12&nbsp;do
  13&nbsp;echo "Input variable #1 (end to exit) "
  14&nbsp;  read var1
  15&nbsp;  echo "variable #1 = $var1"
  16&nbsp;done  
  17&nbsp;
  18&nbsp;# Try to figure out how this all works.
  19&nbsp;# It's a wee bit tricky.
  20&nbsp;
  21&nbsp;exit 0</PRE
></TD
></TR
></TABLE
><HR></DIV
><P
><A
NAME="WLOOPCSTYLE"
></A
></P
><P
>As with a <I
CLASS="FIRSTTERM"
>for loop</I
>, a
	      <I
CLASS="FIRSTTERM"
>while loop</I
> may employ C-style syntax
	      by using the double parentheses construct (see also <A
HREF="dblparens.html#CVARS"
>Example 9-33</A
>).</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="WHLOOPC"
></A
><P
><B
>Example 10-17. C-style syntax in a <I
CLASS="FIRSTTERM"
>while</I
> loop</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# wh-loopc.sh: Count to 10 in a "while" loop.
   3&nbsp;
   4&nbsp;LIMIT=10
   5&nbsp;a=1
   6&nbsp;
   7&nbsp;while [ "$a" -le $LIMIT ]
   8&nbsp;do
   9&nbsp;  echo -n "$a "
  10&nbsp;  let "a+=1"
  11&nbsp;done           # No surprises, so far.
  12&nbsp;
  13&nbsp;echo; echo
  14&nbsp;
  15&nbsp;# +=================================================================+
  16&nbsp;
  17&nbsp;# Now, repeat with C-like syntax.
  18&nbsp;
  19&nbsp;((a = 1))      # a=1
  20&nbsp;# Double parentheses permit space when setting a variable, as in C.
  21&nbsp;
  22&nbsp;while (( a &#60;= LIMIT ))   # Double parentheses, and no "$" preceding variables.
  23&nbsp;do
  24&nbsp;  echo -n "$a "
  25&nbsp;  ((a += 1))   # let "a+=1"
  26&nbsp;  # Yes, indeed.
  27&nbsp;  # Double parentheses permit incrementing a variable with C-like syntax.
  28&nbsp;done
  29&nbsp;
  30&nbsp;echo
  31&nbsp;
  32&nbsp;# C programmers can feel right at home in Bash.
  33&nbsp;
  34&nbsp;exit 0</PRE
></TD
></TR
></TABLE
><HR></DIV
><P
><A
NAME="WHILEFUNC"
></A
></P
><P
>	      Inside its test brackets, a <I
CLASS="FIRSTTERM"
>while loop</I
>
	      can call a <A
HREF="functions.html#FUNCTIONREF"
>function</A
>.

	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;t=0
   2&nbsp;
   3&nbsp;condition ()
   4&nbsp;{
   5&nbsp;  ((t++))
   6&nbsp;
   7&nbsp;  if [ $t -lt 5 ]
   8&nbsp;  then
   9&nbsp;    return 0  # true
  10&nbsp;  else
  11&nbsp;    return 1  # false
  12&nbsp;  fi
  13&nbsp;}
  14&nbsp;
  15&nbsp;while condition
  16&nbsp;#     ^^^^^^^^^
  17&nbsp;#     Function call -- four loop iterations.
  18&nbsp;do
  19&nbsp;  echo "Still going: t = $t"
  20&nbsp;done
  21&nbsp;
  22&nbsp;# Still going: t = 1
  23&nbsp;# Still going: t = 2
  24&nbsp;# Still going: t = 3
  25&nbsp;# Still going: t = 4</PRE
></TD
></TR
></TABLE
>
	    
	    
	    
	    </P
><TABLE
CLASS="SIDEBAR"
BORDER="1"
CELLPADDING="5"
><TR
><TD
><DIV
CLASS="SIDEBAR"
><A
NAME="AEN6086"
></A
><P
><A
NAME="WHILENOBRACKETS"
></A
></P
><P
>Similar to the <A
HREF="tests.html#IFGREPREF"
>if-test</A
>
	      construct, a <I
CLASS="FIRSTTERM"
>while</I
> loop can omit the test
	      brackets.
	        <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;while condition
   2&nbsp;do
   3&nbsp;   ...
   4&nbsp;done</PRE
></TD
></TR
></TABLE
></P
></DIV
></TD
></TR
></TABLE
><P
><A
NAME="WHILEREADREF2"
></A
></P
><P
>By coupling the power of the <A
HREF="internal.html#READREF"
>read</A
> command with a
	      <I
CLASS="FIRSTTERM"
>while loop</I
>, we get the handy <A
HREF="internal.html#WHILEREADREF"
>while read</A
> construct, useful
	      for reading and parsing files.
	        <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;cat $filename |   # Supply input from a file.
   2&nbsp;while read line   # As long as there is another line to read ...
   3&nbsp;do
   4&nbsp;  ...
   5&nbsp;done</PRE
></TD
></TR
></TABLE
></P
><P
><A
NAME="WHREDIR"
></A
></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
>A <I
CLASS="FIRSTTERM"
>while loop</I
> may have its
	      <TT
CLASS="FILENAME"
>stdin</TT
> <A
HREF="redircb.html#REDIRREF"
>redirected to a file</A
> by a
	      <SPAN
CLASS="TOKEN"
>&#60;</SPAN
> at its end.</P
><P
>A <I
CLASS="FIRSTTERM"
>while loop</I
> may have its
	      <TT
CLASS="FILENAME"
>stdin</TT
> <A
HREF="internal.html#READPIPEREF"
>	      supplied by a pipe</A
>.</P
></TD
></TR
></TABLE
></DIV
></DD
><DT
><A
NAME="UNTILLOOPREF"
></A
><B
CLASS="COMMAND"
>until</B
></DT
><DD
><P
>This construct tests for a condition at the top of a loop, and keeps
	      looping as long as that condition is false (opposite of
	      <I
CLASS="FIRSTTERM"
>while loop</I
>).</P
><P
><P
><B
CLASS="COMMAND"
>until</B
>  [<TT
CLASS="REPLACEABLE"
><I
> condition-is-true </I
></TT
>]<BR>  do <BR>  <TT
CLASS="REPLACEABLE"
><I
>燾ommand(s)</I
></TT
>... <BR>  done </P
></P
><P
>Note that an <I
CLASS="FIRSTTERM"
>until loop</I
> tests for the
	      terminating condition at the top of the loop, differing from a
	      similar construct in some programming languages.</P
><P
>As is the case with <I
CLASS="FIRSTTERM"
>for loops</I
>,
	      placing the <I
CLASS="FIRSTTERM"
>do</I
> on the same line as
	      the condition test requires a semicolon.</P
><P
><P
><B
CLASS="COMMAND"
>until</B
>  [<TT
CLASS="REPLACEABLE"
><I
> condition-is-true </I
></TT
>]  ;   do </P
></P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="EX27"
></A
><P
><B
>Example 10-18. <I
CLASS="FIRSTTERM"
>until</I
> loop</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;
   3&nbsp;END_CONDITION=end
   4&nbsp;
   5&nbsp;until [ "$var1" = "$END_CONDITION" ]
   6&nbsp;# Tests condition here, at top of loop.
   7&nbsp;do
   8&nbsp;  echo "Input variable #1 "
   9&nbsp;  echo "($END_CONDITION to exit)"
  10&nbsp;  read var1
  11&nbsp;  echo "variable #1 = $var1"
  12&nbsp;  echo
  13&nbsp;done  
  14&nbsp;
  15&nbsp;exit 0</PRE
></TD
></TR
></TABLE
><HR></DIV
></DD
></DL
></DIV
><P
><A
NAME="CHOOSELOOP"
></A
></P
><P
>How to choose between a <I
CLASS="FIRSTTERM"
>for</I
> loop or a
	  <I
CLASS="FIRSTTERM"
>while</I
> loop or
	  <I
CLASS="FIRSTTERM"
>until</I
> loop? In <B
CLASS="COMMAND"
>C</B
>,
	  you would typically use a <I
CLASS="FIRSTTERM"
>for</I
> loop
	  when the number of loop iterations is known beforehand. With
	  <I
CLASS="FIRSTTERM"
>Bash</I
>, however, the situation is
	  fuzzier. The Bash <I
CLASS="FIRSTTERM"
>for</I
> loop is more
	  loosely structured and more flexible than its equivalent in
	  other languages. Therefore, feel free to use whatever type
	  of loop gets the job done in the simplest way.</P
></DIV
></DIV
><H3
CLASS="FOOTNOTES"
>Notes</H3
><TABLE
BORDER="0"
CLASS="FOOTNOTES"
WIDTH="100%"
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="5%"
><A
NAME="FTN.AEN5815"
HREF="loops.html#AEN5815"
>[1]</A
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="95%"
><P
><A
NAME="ITERATIONREF"
></A
><I
CLASS="FIRSTTERM"
>Iteration</I
>:
	  Repeated execution of a command or group of commands --
	  usually, but not always -- while a given condition holds,
	  or until a given condition is met.</P
></TD
></TR
></TABLE
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