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 of the loop, skipping all the remaining commands in that particular loop cycle.
Example 10-19. 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 "$a " 20 done 21 22 # Exercise for the reader: 23 # Why does loop print up to 20? 24 25 echo; echo 26 27 echo Printing Numbers 1 through 20, but something happens after 2. 28 29 ################################################################## 30 31 # Same loop, but substituting 'break' for 'continue'. 32 33 a=0 34 35 while [ "$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 10-20. 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 $outerloop: " 9 10 for innerloop in 1 2 3 4 5 11 do 12 echo -n "$innerloop " 13 14 if [ "$innerloop" -eq 3 ] 15 then 16 break # Try break 2 to see what happens. 17 # ("Breaks" out of both inner and outer loops.) 18 fi 19 done 20 21 echo 22 done 23 24 echo 25 26 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 10-21. 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 $outer: " 7 8 for inner in 1 2 3 4 5 6 7 8 9 10 # inner loop 9 do 10 11 if [ "$inner" -eq 7 ] 12 then 13 continue 2 # Continue at loop on 2nd level, that is "outer loop". 14 # Replace above line with a simple "continue" 15 # to see normal loop behavior. 16 fi 17 18 echo -n "$inner " # 8 9 10 will never echo. 19 done 20 21 done 22 23 echo; echo 24 25 # Exercise for the reader: 26 # Come up with a meaningful use for "continue N" in a script. 27 28 exit 0
The continue N construct is difficult to understand and tricky to use in any meaningful context. It is probably best avoided.