Looping Statements

• We want to install 10 applications using apt. The flow chart will look some thing as shown below
  
• To perform looping in shell
  • while
  • for
  • until
• To alter loops we have two interesting commands
  • break
  • continue
• for loop to print sequence of values from 1 to 10

#!/bin/bash

# This program will print numbers from 1 to 10
for index in {1..10}
do
    echo $index
done

• Same loop in while

#!/bin/bash
index=1

while [[ $index -le 10 ]]
do
    echo $index
    # increment the value
    index=`expr $index + 1`
    #index=$((index + 1))
done

• loops with strings

SOFTWARES="git tree nano vim"

for SOFTWARE in $SOFTWARES
do
    echo "SOFTWARE to be insttalled is $SOFTWARE"
done

• To read the input from the user

#!/bin/bash

TO_BE_INSTALLED=y
while [ "$TO_BE_INSTALLED" == "y" ]
do
    # ask the software to be installed
    read -p "Enter the sofware which you want to install: " NAME
    echo "Software to be installed is $NAME"
    read -p "If you want to install more softwares enter y and any other key to exit: " TO_BE_INSTALLED
done

• Change the above program, when user enter y or Y or Yes or yes continue any other key exit.

#!/bin/bash

TO_BE_INSTALLED=y
while [[ "$TO_BE_INSTALLED" == "y" || "$TO_BE_INSTALLED" == "Y" || "$TO_BE_INSTALLED" == "Yes" || "$TO_BE_INSTALLED" == "yes" ]]
do
    # ask the software to be installed
    read -p "Enter the sofware which you want to install: " NAME
    echo "Software to be installed is $NAME"
    read -p "If you want to install more softwares enter y and any other key to exit: " TO_BE_INSTALLED
done

• to write multiple or conditions

# Single POSIX test command with -o operator (not recommended anymore).
# Quotes strongly recommended to guard against empty or undefined variables.
while [ "$stats" -gt 300 -o "$stats" -eq 0 ]

# Two POSIX test commands joined in a list with ||.
# Quotes strongly recommended to guard against empty or undefined variables.
while [ "$stats" -gt 300 ] || [ "$stats" -eq 0 ]

# Two bash conditional expressions joined in a list with ||.
while [[ $stats -gt 300 ]] || [[ $stats -eq 0 ]]

# A single bash conditional expression with the || operator.
while [[ $stats -gt 300 || $stats -eq 0 ]]

# Two bash arithmetic expressions joined in a list with ||.
# $ optional, as a string can only be interpreted as a variable
while (( stats > 300 )) || (( stats == 0 ))

# And finally, a single bash arithmetic expression with the || operator.
# $ optional, as a string can only be interpreted as a variable
while (( stats > 300 || stats == 0 ))

Arthimetic

• Refer Here for bash arthimetic

Figuring out if the input is even or odd

• Solution

#!/bin/bash
read -p "Enter the number: " NUMBER
REMAINDER=$(($NUMBER % 2))
if [[ $REMAINDER -eq 0 ]]
then
    echo "$NUMBER is even"
else
    echo "$NUMBER is odd"
fi

Exercises

• Take 3 inputs p,t,r and calculate simple intrest
• Take 1 input r (radius of circle) and calculate area A=Ï€r2 Refer Here
• Take marks from 6 subjects of your choice and calculate average marks. based of that decide grades