高级 Bash 脚本编程指南
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11.3. 循环控制

Tournez cent tours, tournez mille tours,
Tournez souvent et tournez toujours . . .
--Verlaine, "Chevaux de bois"

影响循环行为的命令

break, continue

break 和 continue 循环控制命令 [1] 与其他编程语言中的对应命令完全一致。break 命令终止循环（跳出循环），而 continue 命令则跳转到循环的下一个迭代，跳过该特定循环周期中的所有剩余命令。

示例 11-21. break 和 continue 在循环中的效果

#!/bin/bash

LIMIT=19  # Upper limit

echo
echo "Printing Numbers 1 through 20 (but not 3 and 11)."

a=0

while [ $a -le "$LIMIT" ]
do
 a=$(($a+1))

 if [ "$a" -eq 3 ] || [ "$a" -eq 11 ]  # Excludes 3 and 11.
 then
   continue      # Skip rest of this particular loop iteration.
 fi

 echo -n "$a "   # This will not execute for 3 and 11.
done 

# Exercise:
# Why does the loop print up to 20?

echo; echo

echo Printing Numbers 1 through 20, but something happens after 2.

##################################################################

# Same loop, but substituting 'break' for 'continue'.

a=0

while [ "$a" -le "$LIMIT" ]
do
 a=$(($a+1))

 if [ "$a" -gt 2 ]
 then
   break  # Skip entire rest of loop.
 fi

 echo -n "$a "
done

echo; echo; echo

exit 0

break 命令可以选择性地接受一个参数。一个简单的 break 仅终止它所嵌入的最内层循环，但 break N 会跳出N层循环。

示例 11-22. 跳出多层循环

#!/bin/bash
# break-levels.sh: Breaking out of loops.

# "break N" breaks out of N level loops.

for outerloop in 1 2 3 4 5
do
  echo -n "Group $outerloop:   "

  # --------------------------------------------------------
  for innerloop in 1 2 3 4 5
  do
    echo -n "$innerloop "

    if [ "$innerloop" -eq 3 ]
    then
      break  # Try   break 2   to see what happens.
             # ("Breaks" out of both inner and outer loops.)
    fi
  done
  # --------------------------------------------------------

  echo
done  

echo

exit 0

continue 命令，类似于 break，可以选择性地接受一个参数。一个简单的 continue 会缩短其循环内的当前迭代，并开始下一次迭代。 continue N 终止其循环级别的所有剩余迭代，并继续在循环的下一次迭代中，N层之上。

示例 11-23. 在更高的循环级别继续

#!/bin/bash
# The "continue N" command, continuing at the Nth level loop.

for outer in I II III IV V           # outer loop
do
  echo; echo -n "Group $outer: "

  # --------------------------------------------------------------------
  for inner in 1 2 3 4 5 6 7 8 9 10  # inner loop
  do

    if [[ "$inner" -eq 7 && "$outer" = "III" ]]
    then
      continue 2  # Continue at loop on 2nd level, that is "outer loop".
                  # Replace above line with a simple "continue"
                  # to see normal loop behavior.
    fi  

    echo -n "$inner "  # 7 8 9 10 will not echo on "Group III."
  done  
  # --------------------------------------------------------------------

done

echo; echo

# Exercise:
# Come up with a meaningful use for "continue N" in a script.

exit 0

示例 11-24. 在实际任务中使用 continue N

# Albert Reiner gives an example of how to use "continue N":
# ---------------------------------------------------------

#  Suppose I have a large number of jobs that need to be run, with
#+ any data that is to be treated in files of a given name pattern
#+ in a directory. There are several machines that access
#+ this directory, and I want to distribute the work over these
#+ different boxen.
#  Then I usually nohup something like the following on every box:

while true
do
  for n in .iso.*
  do
    [ "$n" = ".iso.opts" ] && continue
    beta=${n#.iso.}
    [ -r .Iso.$beta ] && continue
    [ -r .lock.$beta ] && sleep 10 && continue
    lockfile -r0 .lock.$beta || continue
    echo -n "$beta: " `date`
    run-isotherm $beta
    date
    ls -alF .Iso.$beta
    [ -r .Iso.$beta ] && rm -f .lock.$beta
    continue 2
  done
  break
done

exit 0

#  The details, in particular the sleep N, are particular to my
#+ application, but the general pattern is:

while true
do
  for job in {pattern}
  do
    {job already done or running} && continue
    {mark job as running, do job, mark job as done}
    continue 2
  done
  break        # Or something like `sleep 600' to avoid termination.
done

#  This way the script will stop only when there are no more jobs to do
#+ (including jobs that were added during runtime). Through the use
#+ of appropriate lockfiles it can be run on several machines
#+ concurrently without duplication of calculations [which run a couple
#+ of hours in my case, so I really want to avoid this]. Also, as search
#+ always starts again from the beginning, one can encode priorities in
#+ the file names. Of course, one could also do this without `continue 2',
#+ but then one would have to actually check whether or not some job
#+ was done (so that we should immediately look for the next job) or not
#+ (in which case we terminate or sleep for a long time before checking
#+ for a new job).

continue N 结构难以理解，并且在任何有意义的上下文中都很难使用。最好避免使用它。

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11.3. 循环控制

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