If there is a need to execute set of statements repeatedly, then we need to use looping statements. Below is the list of statements supported by shell script.
6.1. While Loop
6.2. Until Loop
6.3. For Loop
6.4. Select Loop
6.1. While Loop
While loop is a looping statement that is used to execute a set of statement until the condition is true. Once the condition becomes false, it will exit out of the loop. Below is the syntax for while loop.
Syntax:
while [ condition ] do #statements done
Note:
Here we use “while”, “do” and “done” keywords used in while loop.
Example:
#!/bin/bash var=5 while [ $var -ge 1 ] do echo "the number is $var" var=$(($var - 1)) done
Output:
the number is 5 the number is 4 the number is 3 the number is 2 the number is 1
6.2. Until Loop
Until loop is similar to wile loop with a twist. Here the loop will continue to execute till the expression becomes true. As long as the condition fails, the loop continues.
Syntax:
until [ condition ] do #statements done
Note: Here we use “until”, “do” and “done” keywords used in until loop.
Example:
#!/bin/bash var=1 until [ $var -eq 5 ] do echo "the number is $var" var=$(( $var + 1 )) done
Output:
the number is 1 the number is 2 the number is 3 the number is 4
As you see in above output, when the “var” value becomes 5, it will exit the loop. Hence it will not print the number 5.
6.3. For Loop
For loop is similar to while and until loop, but the number of times to loop should be known beforehand. There are 2 kinds of for loop.
1. The bash form of for loop:
Syntax:
for <variable_name> in <list> do #statements done
Note:
Here “for”, “in”, “do”, “done” are the keyword.
list can be a list of numbers, array list, or any other that has a list of items.
<variable_name> holds the list of items, one element at a time.
Example:
#!/bin/bash for var in 2 4 6 8 10 do echo "The variable is $var" done
Output:
The variable is 2 The variable is 4 The variable is 6 The variable is 8 The variable is 10
2. The “c” style of for loop:
for (( <initialization> ; <condition>; <increment_or_decrement> )) do #statements done
Here “for”, “;”, “do”, “done” are the keywords used.
<initialization> is used to initialize the variable
<condition> Here condition to be checked should be written. The statements between “do” and “done” will be executed until the condition become false.
<increment_or_decrement> After every loop, this expression will be executed.
Example:
#!/bin/bash for (( var=0 ; var<=5; var++ )) do echo "The variable is $var" done
Output:
The variable is 0 The variable is 1 The variable is 2 The variable is 3 The variable is 4 The variable is 5
6.4. Select Loop
Usually we use select loop, when we want to display the options to the user. Generally, select loop will be used with case or else-if statements for better control. As you can see from below syntax, there is no way to exit the select loop, break statement to be used to exit out of select loop.
Syntax:
select <variable_name> in <list> do #statements done
Note:
“select” “in” “do” “done” are the keywords used.
The elements in “list” will be displayed to the user as a menu.
Example:
#!/bin/bash echo "Select an operating system from below" select var in linux unix macOS windows do echo "The OS you have selected is $var" done
Output:
Select an operating system from below 1) linux 2) unix 3) macOS 4) windows #? 3 The OS you have selected is macOS
