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>Advanced Bash-Scripting Guide: An in-depth exploration of the art of shell scripting</TH
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><DIV
CLASS="SECT1"
><H1
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><A
NAME="LOOPCONTROL"
></A
>11.3. Loop Control</H1
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>&nbsp;</TD
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><I
><P
><I
>Tournez cent tours, tournez mille tours,</I
></P
><P
><I
>Tournez souvent et tournez toujours . . .</I
></P
><P
><I
>--Verlaine, <SPAN
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>"Chevaux de bois"</SPAN
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></P
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></TD
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><DIV
CLASS="VARIABLELIST"
><P
><B
><A
NAME="BRKCONT1"
></A
>Commands affecting loop behavior</B
></P
><DL
><DT
><B
CLASS="COMMAND"
>break</B
>, <B
CLASS="COMMAND"
>continue</B
></DT
><DD
><P
>The <B
CLASS="COMMAND"
>break</B
> and <B
CLASS="COMMAND"
>continue</B
>
	      loop control commands
		<A
NAME="AEN6981"
HREF="#FTN.AEN6981"
>[1]</A
>
	      correspond exactly to their counterparts in other
	      programming languages. The <B
CLASS="COMMAND"
>break</B
>
	      command terminates the loop (<SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>breaks</I
></SPAN
>
	      out of it), while <B
CLASS="COMMAND"
>continue</B
> causes a jump
	      to the next <A
HREF="loops.html#ITERATIONREF"
>iteration</A
>
	      of the loop, skipping all the remaining commands in that
	      particular loop cycle.</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="EX28"
></A
><P
><B
>Example 11-21. Effects of <I
CLASS="FIRSTTERM"
>break</I
> and
		<B
CLASS="COMMAND"
>continue</B
> in a loop</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;
   3&nbsp;LIMIT=19  # Upper limit
   4&nbsp;
   5&nbsp;echo
   6&nbsp;echo "Printing Numbers 1 through 20 (but not 3 and 11)."
   7&nbsp;
   8&nbsp;a=0
   9&nbsp;
  10&nbsp;while [ $a -le "$LIMIT" ]
  11&nbsp;do
  12&nbsp; a=$(($a+1))
  13&nbsp;
  14&nbsp; if [ "$a" -eq 3 ] || [ "$a" -eq 11 ]  # Excludes 3 and 11.
  15&nbsp; then
  16&nbsp;   continue      # Skip rest of this particular loop iteration.
  17&nbsp; fi
  18&nbsp;
  19&nbsp; echo -n "$a "   # This will not execute for 3 and 11.
  20&nbsp;done 
  21&nbsp;
  22&nbsp;# Exercise:
  23&nbsp;# Why does the loop print up to 20?
  24&nbsp;
  25&nbsp;echo; echo
  26&nbsp;
  27&nbsp;echo Printing Numbers 1 through 20, but something happens after 2.
  28&nbsp;
  29&nbsp;##################################################################
  30&nbsp;
  31&nbsp;# Same loop, but substituting 'break' for 'continue'.
  32&nbsp;
  33&nbsp;a=0
  34&nbsp;
  35&nbsp;while [ "$a" -le "$LIMIT" ]
  36&nbsp;do
  37&nbsp; a=$(($a+1))
  38&nbsp;
  39&nbsp; if [ "$a" -gt 2 ]
  40&nbsp; then
  41&nbsp;   break  # Skip entire rest of loop.
  42&nbsp; fi
  43&nbsp;
  44&nbsp; echo -n "$a "
  45&nbsp;done
  46&nbsp;
  47&nbsp;echo; echo; echo
  48&nbsp;
  49&nbsp;exit 0</PRE
></TD
></TR
></TABLE
><HR></DIV
><P
><A
NAME="BREAKPARAM"
></A
></P
><P
>The <B
CLASS="COMMAND"
>break</B
> command may optionally take a
	      parameter. A plain <B
CLASS="COMMAND"
>break</B
> terminates
	      only the innermost loop in which it is embedded,
	      but a <B
CLASS="COMMAND"
>break N</B
> breaks out of
	      <TT
CLASS="PARAMETER"
><I
>N</I
></TT
> levels of loop.</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="BREAKLEVELS"
></A
><P
><B
>Example 11-22. Breaking out of multiple loop levels</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# break-levels.sh: Breaking out of loops.
   3&nbsp;
   4&nbsp;# "break N" breaks out of N level loops.
   5&nbsp;
   6&nbsp;for outerloop in 1 2 3 4 5
   7&nbsp;do
   8&nbsp;  echo -n "Group $outerloop:   "
   9&nbsp;
  10&nbsp;  # --------------------------------------------------------
  11&nbsp;  for innerloop in 1 2 3 4 5
  12&nbsp;  do
  13&nbsp;    echo -n "$innerloop "
  14&nbsp;
  15&nbsp;    if [ "$innerloop" -eq 3 ]
  16&nbsp;    then
  17&nbsp;      break  # Try   break 2   to see what happens.
  18&nbsp;             # ("Breaks" out of both inner and outer loops.)
  19&nbsp;    fi
  20&nbsp;  done
  21&nbsp;  # --------------------------------------------------------
  22&nbsp;
  23&nbsp;  echo
  24&nbsp;done  
  25&nbsp;
  26&nbsp;echo
  27&nbsp;
  28&nbsp;exit 0</PRE
></TD
></TR
></TABLE
><HR></DIV
><P
>The <B
CLASS="COMMAND"
>continue</B
> command, similar to
	      <B
CLASS="COMMAND"
>break</B
>, optionally takes a parameter. A
	      plain <B
CLASS="COMMAND"
>continue</B
> cuts short the
	      current iteration within its loop and begins the next.
	      A <B
CLASS="COMMAND"
>continue N</B
> terminates all remaining
	      iterations at its loop level and continues with the
	      next iteration at the loop, <TT
CLASS="OPTION"
>N</TT
> levels
	      above.</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="CONTINUELEVELS"
></A
><P
><B
>Example 11-23. Continuing at a higher loop level</B
></P
><TABLE
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><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# The "continue N" command, continuing at the Nth level loop.
   3&nbsp;
   4&nbsp;for outer in I II III IV V           # outer loop
   5&nbsp;do
   6&nbsp;  echo; echo -n "Group $outer: "
   7&nbsp;
   8&nbsp;  # --------------------------------------------------------------------
   9&nbsp;  for inner in 1 2 3 4 5 6 7 8 9 10  # inner loop
  10&nbsp;  do
  11&nbsp;
  12&nbsp;    if [[ "$inner" -eq 7 &#38;&#38; "$outer" = "III" ]]
  13&nbsp;    then
  14&nbsp;      continue 2  # Continue at loop on 2nd level, that is "outer loop".
  15&nbsp;                  # Replace above line with a simple "continue"
  16&nbsp;                  # to see normal loop behavior.
  17&nbsp;    fi  
  18&nbsp;
  19&nbsp;    echo -n "$inner "  # 7 8 9 10 will not echo on "Group III."
  20&nbsp;  done  
  21&nbsp;  # --------------------------------------------------------------------
  22&nbsp;
  23&nbsp;done
  24&nbsp;
  25&nbsp;echo; echo
  26&nbsp;
  27&nbsp;# Exercise:
  28&nbsp;# Come up with a meaningful use for "continue N" in a script.
  29&nbsp;
  30&nbsp;exit 0</PRE
></TD
></TR
></TABLE
><HR></DIV
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="CONTINUENEX"
></A
><P
><B
>Example 11-24. Using <I
CLASS="FIRSTTERM"
>continue N</I
> in an actual task</B
></P
><TABLE
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><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;# Albert Reiner gives an example of how to use "continue N":
   2&nbsp;# ---------------------------------------------------------
   3&nbsp;
   4&nbsp;#  Suppose I have a large number of jobs that need to be run, with
   5&nbsp;#+ any data that is to be treated in files of a given name pattern
   6&nbsp;#+ in a directory. There are several machines that access
   7&nbsp;#+ this directory, and I want to distribute the work over these
   8&nbsp;#+ different boxen.
   9&nbsp;#  Then I usually nohup something like the following on every box:
  10&nbsp;
  11&nbsp;while true
  12&nbsp;do
  13&nbsp;  for n in .iso.*
  14&nbsp;  do
  15&nbsp;    [ "$n" = ".iso.opts" ] &#38;&#38; continue
  16&nbsp;    beta=${n#.iso.}
  17&nbsp;    [ -r .Iso.$beta ] &#38;&#38; continue
  18&nbsp;    [ -r .lock.$beta ] &#38;&#38; sleep 10 &#38;&#38; continue
  19&nbsp;    lockfile -r0 .lock.$beta || continue
  20&nbsp;    echo -n "$beta: " `date`
  21&nbsp;    run-isotherm $beta
  22&nbsp;    date
  23&nbsp;    ls -alF .Iso.$beta
  24&nbsp;    [ -r .Iso.$beta ] &#38;&#38; rm -f .lock.$beta
  25&nbsp;    continue 2
  26&nbsp;  done
  27&nbsp;  break
  28&nbsp;done
  29&nbsp;
  30&nbsp;exit 0
  31&nbsp;
  32&nbsp;#  The details, in particular the sleep N, are particular to my
  33&nbsp;#+ application, but the general pattern is:
  34&nbsp;
  35&nbsp;while true
  36&nbsp;do
  37&nbsp;  for job in {pattern}
  38&nbsp;  do
  39&nbsp;    {job already done or running} &#38;&#38; continue
  40&nbsp;    {mark job as running, do job, mark job as done}
  41&nbsp;    continue 2
  42&nbsp;  done
  43&nbsp;  break        # Or something like `sleep 600' to avoid termination.
  44&nbsp;done
  45&nbsp;
  46&nbsp;#  This way the script will stop only when there are no more jobs to do
  47&nbsp;#+ (including jobs that were added during runtime). Through the use
  48&nbsp;#+ of appropriate lockfiles it can be run on several machines
  49&nbsp;#+ concurrently without duplication of calculations [which run a couple
  50&nbsp;#+ of hours in my case, so I really want to avoid this]. Also, as search
  51&nbsp;#+ always starts again from the beginning, one can encode priorities in
  52&nbsp;#+ the file names. Of course, one could also do this without `continue 2',
  53&nbsp;#+ but then one would have to actually check whether or not some job
  54&nbsp;#+ was done (so that we should immediately look for the next job) or not
  55&nbsp;#+ (in which case we terminate or sleep for a long time before checking
  56&nbsp;#+ for a new job).</PRE
></TD
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><HR></DIV
><DIV
CLASS="CAUTION"
><TABLE
CLASS="CAUTION"
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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
>The <B
CLASS="COMMAND"
>continue N</B
> construct is
	      difficult to understand and tricky to use in any meaningful
	      context. It is probably best avoided.</P
></TD
></TR
></TABLE
></DIV
></DD
></DL
></DIV
></DIV
><H3
CLASS="FOOTNOTES"
>Notes</H3
><TABLE
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><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="5%"
><A
NAME="FTN.AEN6981"
HREF="loopcontrol.html#AEN6981"
>[1]</A
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="95%"
><P
>These are shell <A
HREF="internal.html#BUILTINREF"
>builtins</A
>,
		whereas other loop commands, such as <A
HREF="loops.html#WHILELOOPREF"
>while</A
> and <A
HREF="testbranch.html#CASEESAC1"
>case</A
>, are <A
HREF="internal.html#KEYWORDREF"
>keywords</A
>.</P
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