loops.html

一本完整的描述Unix Shell 编程的工具书的所有范例

   7 if [ $# -ne $EXPECTED_ARGS ]
   8 # Check for proper no. of command line args.
   9 then
  10    echo "Usage: `basename $0` phone# text-file"
  11    exit $E_BADARGS
  12 fi
  13 
  14 
  15 if [ ! -f "$2" ]
  16 then
  17   echo "File $2 is not a text file"
  18   exit $E_BADARGS
  19 fi
  20   
  21 
  22 fax make $2              # Create fax formatted files from text files.
  23 
  24 for file in $(ls $2.0*)  # Concatenate the converted files.
  25                          # Uses wild card in variable list.
  26 do
  27   fil="$fil $file"
  28 done  
  29 
  30 efax -d /dev/ttyS3 -o1 -t "T$1" $fil   # Do the work.
  31 
  32 
  33 # As S.C. points out, the for-loop can be eliminated with
  34 #    efax -d /dev/ttyS3 -o1 -t "T$1" $2.0*
  35 # but it's not quite as instructive [grin].
  36 
  37&nbsp;exit 0</PRE
></TD
></TR
></TABLE
><HR></DIV
></DD
><DT
><A
NAME="WHILELOOPREF"
></A
><B
CLASS="COMMAND"
>while</B
></DT
><DD
><P
>This construct tests for a condition at the top of a
	      loop, and keeps looping as long as that condition
	      is true (returns a <SPAN
CLASS="RETURNVALUE"
>0</SPAN
> <A
HREF="exit-status.html#EXITSTATUSREF"
>exit status</A
>).  In contrast
	      to a <A
HREF="loops.html#FORLOOPREF1"
>for loop</A
>, a
	      <I
CLASS="FIRSTTERM"
>while loop</I
> finds use in situations
	      where the number of loop repetitions is not known
	      beforehand.</P
><P
><P
><B
CLASS="COMMAND"
>while</B
>  [<TT
CLASS="REPLACEABLE"
><I
>condition</I
></TT
>]<BR>  do <BR>  <TT
CLASS="REPLACEABLE"
><I
>燾ommand</I
></TT
>... <BR>  done </P
></P
><P
>As is the case with <I
CLASS="FIRSTTERM"
>for loops</I
>,
	      placing the <I
CLASS="FIRSTTERM"
>do</I
> on the same line as
	      the condition test requires a semicolon.</P
><P
><P
><B
CLASS="COMMAND"
>while</B
>  [<TT
CLASS="REPLACEABLE"
><I
>condition</I
></TT
>]  ;   do </P
></P
><P
>Note that certain specialized <I
CLASS="FIRSTTERM"
>while
	      loops</I
>, as, for example,  a <A
HREF="internal.html#GETOPTSX"
>getopts construct</A
>, deviate
	      somewhat from the standard template given here.</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="EX25"
></A
><P
><B
>Example 10-14. Simple <B
CLASS="COMMAND"
>while</B
> loop</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;
   3&nbsp;var0=0
   4&nbsp;LIMIT=10
   5&nbsp;
   6&nbsp;while [ "$var0" -lt "$LIMIT" ]
   7&nbsp;do
   8&nbsp;  echo -n "$var0 "        # -n suppresses newline.
   9&nbsp;  #             ^           Space, to separate printed out numbers.
  10&nbsp;
  11&nbsp;  var0=`expr $var0 + 1`   # var0=$(($var0+1))  also works.
  12&nbsp;                          # var0=$((var0 + 1)) also works.
  13&nbsp;                          # let "var0 += 1"    also works.
  14&nbsp;done                      # Various other methods also work.
  15&nbsp;
  16&nbsp;echo
  17&nbsp;
  18&nbsp;exit 0</PRE
></TD
></TR
></TABLE
><HR></DIV
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="EX26"
></A
><P
><B
>Example 10-15. Another <B
CLASS="COMMAND"
>while</B
> 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 = $var1"   # Need quotes because of "#" . . .
  10&nbsp;  # If input is 'end', echoes it here.
  11&nbsp;  # Does not test for termination condition until top of loop.
  12&nbsp;  echo
  13&nbsp;done  
  14&nbsp;
  15&nbsp;exit 0</PRE
></TD
></TR
></TABLE
><HR></DIV
><P
>A <I
CLASS="FIRSTTERM"
>while loop</I
> may have multiple
	      conditions. Only the final condition determines when the loop
	      terminates. This necessitates a slightly different loop syntax,
	      however.</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="EX26A"
></A
><P
><B
>Example 10-16. <B
CLASS="COMMAND"
>while</B
> loop with multiple conditions</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;
   3&nbsp;var1=unset
   4&nbsp;previous=$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 = $var1"
  16&nbsp;done  
  17&nbsp;
  18&nbsp;# Try to figure out how this all works.
  19&nbsp;# It's a wee bit tricky.
  20&nbsp;
  21&nbsp;exit 0</PRE
></TD
></TR
></TABLE
><HR></DIV
><P
>As with a <I
CLASS="FIRSTTERM"
>for loop</I
>, a
	      <I
CLASS="FIRSTTERM"
>while loop</I
> may employ C-like syntax
	      by using the double parentheses construct (see also <A
HREF="dblparens.html#CVARS"
>Example 9-30</A
>).</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="WHLOOPC"
></A
><P
><B
>Example 10-17. C-like syntax in a <B
CLASS="COMMAND"
>while</B
> loop</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# wh-loopc.sh: Count to 10 in a "while" loop.
   3&nbsp;
   4&nbsp;LIMIT=10
   5&nbsp;a=1
   6&nbsp;
   7&nbsp;while [ "$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;# Now, C programmers can feel right at home in Bash.
  33&nbsp;
  34&nbsp;exit 0</PRE
></TD
></TR
></TABLE
><HR></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
>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</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. <B
CLASS="COMMAND"
>until</B
> 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
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