Looping Statements | Shell Script
Looping Statements in Shell Scripting: There are total 3 looping statements which can be used in bash programming
- while statement
- for statement
- until statement
To alter the flow of loop statements, two commands are used they are,
- break
- continue
Their descriptions and syntax are as follows:
- while statement
Here command is evaluated and based on the result loop will executed, if command raise to false then loop will be terminated
Syntax
- for statement
The for loop operate on lists of items. It repeats a set of commands for every item in a list.
Here var is the name of a variable and word1 to wordN are sequences of characters separated by spaces (words). Each time the for loop executes, the value of the variable var is set to the next word in the list of words, word1 to wordN.
Syntax
- until statement
The until loop is executed as many as times the condition/command evaluates to false. The loop terminates when the condition/command becomes true.
Syntax
Example Programs
Example 1:
Implementing for loop with break statement
php
#Start of for loop for a in 1 2 3 4 5 6 7 8 9 10 do # if a is equal to 5 break the loop if [ $a == 5 ] then break fi # Print the value echo "Iteration no $a" done |
Output
$bash -f main.sh Iteration no 1 Iteration no 2 Iteration no 3 Iteration no 4
Example 2:
Implementing for loop with continue statement
php
for a in 1 2 3 4 5 6 7 8 9 10 do # if a = 5 then continue the loop and # don't move to line 8 if [ $a == 5 ] then continue fi echo "Iteration no $a" done |
Output
$bash -f main.sh Iteration no 1 Iteration no 2 Iteration no 3 Iteration no 4 Iteration no 6 Iteration no 7 Iteration no 8 Iteration no 9 Iteration no 10
Example 3:
Implementing while loop
php
a=0 # -lt is less than operator #Iterate the loop until a less than 10 while [ $a -lt 10 ] do # Print the values echo $a # increment the value a=`expr $a + 1` done |
Output:
$bash -f main.sh 0 1 2 3 4 5 6 7 8 9
Example 4:
Implementing until loop
php
a=0 # -gt is greater than operator #Iterate the loop until a is greater than 10 until [ $a -gt 10 ] do # Print the values echo $a # increment the value a=`expr $a + 1` done |
Output:
$bash -f main.sh 0 1 2 3 4 5 6 7 8 9 10
Note: Shell scripting is a case-sensitive language, which means proper syntax has to be followed while writing the scripts.
Example 5:
PHP
COLORS= "red green blue" # the for loop continues until it reads all the values from the COLORS for COLOR in $COLORS do echo "COLOR: $COLOR" done |
Output:
$bash -f main.sh COLOR: red COLOR: green COLOR: blue
Example 6:
We can even access the positional parameters using loops
We execute a shell script named sample.sh using three parameters
Script Execution: main.sh sample1 sample2 sample3
We can access above three parameters using $@
PHP
echo "Executing script" # the script is executed using the below command # main.sh sample1 sample2 sample # where sample1, sample2 and sample3 are the positional arguments # here $@ contains all the positional arguments. for SAMPLE in $@ do echo "The given sample is: $SAMPLE" done |
Output:
$bash -f main.sh sample1 sample2 sample3 Executing script The given sample is sample1 The given sample is sample2 The given sample is sample3
Example 7: Infinite loop
PHP
while true do # Command to be executed # sleep 1 indicates it sleeps for 1 sec echo "Hi, I am infinity loop" sleep 1 done |
Output:
$bash -f main.sh Hi, I am infinity loop Hi, I am infinity loop Hi, I am infinity loop . . . . It continues
Example 8: Checking for user input
PHP
CORRECT=n while [ "$CORRECT" == "n" ] do # loop discontinues when you enter y i.e.e, when your name is correct # -p stands for prompt asking for the input read -p "Enter your name:" NAME read -p "Is ${NAME} correct? " CORRECT done |
Output:
$bash -f main.sh Enter your name:Ironman Is Ironman correct? n Enter your name:Spiderman Is Spiderman correct? y