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Looping Statements in Shell Scripting:
Three keywords to loop in Unix/Linux shell:
The statements in the until loop are executed until the command in the condition becomes true.
The following is the syntax of the until loop:
until command do Statement(s) to be executed done
The code below demonstrates the use of the until loop.
a=5 until [ $a -lt 1 ] # a < 1 do echo $a # print a a=`expr $a - 1` # a = a - 1 done
Output 5 4 3 2 1
The statements in the while loop are executed until the command in the condition becomes false.
The following is the syntax of the while loop:
while command do Statement(s) to be executed done
The code below demonstrates the use of the while loop.
a=5 while [ $a -gt 0 ] # a > 0 do echo $a # print a a=`expr $a - 1` # a = a - 1 done
Output 5 4 3 2 1
The for loop operates on a list of items. The statements in the for loop are executed for all items in the list.
The following is the syntax of the for loop:
for var in word1 word2 ... wordN do Statement(s) to be executed done
The code below demonstrates the use of the for loop.
for a in 5 4 3 2 1 do echo $a # print a done
Output 5 4 3 2 1
We use the following keywords to alter the normal flow of a loop in Unix/Linux:
The code below demonstrates the use of the break statement.
for a in 2 5 3 1 9 8 0 7 6 4 do if [ $a -eq 0 ] # a == 0 then break fi echo $a done
Output 2 5 3 1 9 8
We use the continue statement to terminate iteration(s) of the loop. It executes the line above continue and ignores all the lines below continue. It then starts execution from the start of the next iteration in the loop.
The code below demonstrates the use of the continue statement.
for a in 1 2 3 4 5 6 7 8 9 10 do if [ `expr $a % 2` -eq 0 ] # a % 2 == 0 then continue fi echo $a done
Output 1 3 5 7 9