10.3. Loop Control
Commands Affecting Loop Behavior
- break, continue
The break and continue loop control commands [1] correspond exactly to their counterparts in other programming languages. The break command terminates the loop (breaks out of it), while continue causes a jump to the next iteration (repetition) of the loop, skipping all the remaining commands in that particular loop cycle.
Example 10-20. Effects of break and continue in a loop
1 #!/bin/bash 2 3 LIMIT=19 # Upper limit 4 5 echo 6 echo "Printing Numbers 1 through 20 (but not 3 and 11)." 7 8 a=0 9 10 while [ $a -le "$LIMIT" ] 11 do 12 a=$(($a+1)) 13 14 if [ "$a" -eq 3 ] || [ "$a" -eq 11 ] # Excludes 3 and 11. 15 then 16 continue # Skip rest of this particular loop iteration. 17 fi 18 19 echo -n "$a " # This will not execute for 3 and 11. 20 done 21 22 # Exercise: 23 # Why does loop print up to 20? 24 25 echo; echo 26 27 echo Printing Numbers 1 through 20, but something happens after 2. 28 29 ################################################################## 30 31 # Same loop, but substituting 'break' for 'continue'. 32 33 a=0 34 35 while [ "$a" -le "$LIMIT" ] 36 do 37 a=$(($a+1)) 38 39 if [ "$a" -gt 2 ] 40 then 41 break # Skip entire rest of loop. 42 fi 43 44 echo -n "$a " 45 done 46 47 echo; echo; echo 48 49 exit 0
The break command may optionally take a parameter. A plain break terminates only the innermost loop in which it is embedded, but a break N breaks out of N levels of loop.
Example 10-21. Breaking out of multiple loop levels
1 #!/bin/bash 2 # break-levels.sh: Breaking out of loops. 3 4 # "break N" breaks out of N level loops. 5 6 for outerloop in 1 2 3 4 5 7 do 8 echo -n "Group $outerloop: " 9 10 # -------------------------------------------------------- 11 for innerloop in 1 2 3 4 5 12 do 13 echo -n "$innerloop " 14 15 if [ "$innerloop" -eq 3 ] 16 then 17 break # Try break 2 to see what happens. 18 # ("Breaks" out of both inner and outer loops.) 19 fi 20 done 21 # -------------------------------------------------------- 22 23 echo 24 done 25 26 echo 27 28 exit 0The continue command, similar to break, optionally takes a parameter. A plain continue cuts short the current iteration within its loop and begins the next. A continue N terminates all remaining iterations at its loop level and continues with the next iteration at the loop, N levels above.
Example 10-22. Continuing at a higher loop level
1 #!/bin/bash 2 # The "continue N" command, continuing at the Nth level loop. 3 4 for outer in I II III IV V # outer loop 5 do 6 echo; echo -n "Group $outer: " 7 8 # -------------------------------------------------------------------- 9 for inner in 1 2 3 4 5 6 7 8 9 10 # inner loop 10 do 11 12 if [ "$inner" -eq 7 ] 13 then 14 continue 2 # Continue at loop on 2nd level, that is "outer loop". 15 # Replace above line with a simple "continue" 16 # to see normal loop behavior. 17 fi 18 19 echo -n "$inner " # 7 8 9 10 will never echo. 20 done 21 # -------------------------------------------------------------------- 22 23 done 24 25 echo; echo 26 27 # Exercise: 28 # Come up with a meaningful use for "continue N" in a script. 29 30 exit 0
Example 10-23. Using continue N in an actual task
1 # Albert Reiner gives an example of how to use "continue N": 2 # --------------------------------------------------------- 3 4 # Suppose I have a large number of jobs that need to be run, with 5 #+ any data that is to be treated in files of a given name pattern in a 6 #+ directory. There are several machines that access this directory, and 7 #+ I want to distribute the work over these different boxen. Then I 8 #+ usually nohup something like the following on every box: 9 10 while true 11 do 12 for n in .iso.* 13 do 14 [ "$n" = ".iso.opts" ] && continue 15 beta=${n#.iso.} 16 [ -r .Iso.$beta ] && continue 17 [ -r .lock.$beta ] && sleep 10 && continue 18 lockfile -r0 .lock.$beta || continue 19 echo -n "$beta: " `date` 20 run-isotherm $beta 21 date 22 ls -alF .Iso.$beta 23 [ -r .Iso.$beta ] && rm -f .lock.$beta 24 continue 2 25 done 26 break 27 done 28 29 # The details, in particular the sleep N, are particular to my 30 #+ application, but the general pattern is: 31 32 while true 33 do 34 for job in {pattern} 35 do 36 {job already done or running} && continue 37 {mark job as running, do job, mark job as done} 38 continue 2 39 done 40 break # Or something like `sleep 600' to avoid termination. 41 done 42 43 # This way the script will stop only when there are no more jobs to do 44 #+ (including jobs that were added during runtime). Through the use 45 #+ of appropriate lockfiles it can be run on several machines 46 #+ concurrently without duplication of calculations [which run a couple 47 #+ of hours in my case, so I really want to avoid this]. Also, as search 48 #+ always starts again from the beginning, one can encode priorities in 49 #+ the file names. Of course, one could also do this without `continue 2', 50 #+ but then one would have to actually check whether or not some job 51 #+ was done (so that we should immediately look for the next job) or not 52 #+ (in which case we terminate or sleep for a long time before checking 53 #+ for a new job).
The continue N construct is difficult to understand and tricky to use in any meaningful context. It is probably best avoided.