This post covers the loop usage of bash shell. NOTE: read inline comments carefully 🙂
1. for loop
|
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 |
#!/bin/bash # loop list, last value remains for test in Alabama Alaska Arizona Arkansas California Colorado do echo The next state is $test done echo "The last state we visited was $test" test=Connecticut echo "Wait, now we're visiting $test" # using escape or quote for test in I don\'t know if "this'll" work do echo "word: $test" done # loop variable & files states="Alabama Alaska Arizona Arkansas Colorado Connecticut Delaware Florida Georgia" statesfile=states.txt for state in $states; do echo $state >> $statesfile done for state in `cat $statesfile`; do echo "Visit beautiful $state" done rm $statesfile # loop directory for file in ~/.b*; do if [ -d "$file" ]; then echo "$file is a directory" elif [ -f "$file" ]; then echo "$file is a file" else echo "$file doesn't exist" fi done # c-style syntax for (( i = 1; i <= 10; i++ )); do echo "The next number is $i" done # IFS (internal field separator) to separator string IFSHOLD=$IFS IFS=$'\n' for entry in `cat /etc/passwd`; do echo "Values in $entry:" IFS=: for value in $entry; do echo " $value" done done IFS=$IFSHOLD |
2. while loop
|
1 2 3 4 5 6 7 |
#!/bin/bash var1=10 while [ $var1 -gt 0 ]; do echo $var1 var1=$[ $var1 - 1 ] done |
3. until loop
|
1 2 3 4 5 6 7 |
#!/bin/bash var1=100 until [ $var1 -eq 0 ]; do echo $var1 var1=$[ $var1 - 25 ] done |
4. break & continue
|
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 |
#!/bin/bash # break for (( a = 1; a < 4; a++ )); do echo "Outer loop: $a" for (( b = 1; b < 100; b++ )); do if [ $b -eq 5 ]; then break fi echo "Inner loop: $b" done done # break outer loop for (( a = 1; a < 4; a++ )); do echo "Outer loop: $a" for (( b = 1; b < 100; b++ )); do if [ $b -eq 5 ]; then break 2 fi echo "Inner loop: $b" done done # continue outer loop for (( a = 1; a <= 5; a++ )); do echo "Iteration $a:" for (( b = 1; b < 3; b++ )); do if [ $a -gt 2 ] && [ $a -lt 4 ]; then continue 2 fi var3=$[ $a * $b ] echo " The result of $a * $b is $var3" done done |
There may be times when you’re in an inner loop but need to stop the outer loop. The break command includes a single command line parameter value: break n where n indicates the level of the loop to break out of. By default, n is 1, indicating to break out of the current loop. If you set n to a value of 2, the break command will stop the next level of the outer loop.
5. redirect & pipe
Finally, you can either pipe or redirect the output of a loop within your shell script.
|
1 2 3 4 5 6 7 8 9 |
#!/bin/bash testfile=testloop.txt for (( a = 1; a < 10; a++ )); do echo "The number is $a" done > $testfile echo "The command is finished." cat $testfile rm $testfile |