Bash Shell Variables and Loops

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Variables

Variables hold reusable values and are foundational to the use of scripts

• export command sets the variable within the current shell session
• set a variable: VARIABLE=value
• call a variable: $VARIABLE
• can set a variable dynamically with read
  • e.g., read VARIABLE
    • gets user input and stores in VARIABLE

Arithmetic

• Shells incorporate a few special conventions to perform arithmetic
  • e.g., expr command is needed to perform arithmetic
• arithmetic operations performed with variables must be contain in back ticks (``)
  • allow the variables to be evaluated
  • e.g., sum=`expr $num1 $num2`

Boolean Operators

Boolean operators are used to perform comparisons.

• e.g., var1 == var2

Operator	Name
==	Is equal to
!=	Is not equal to
-eq	Alternative form of “is equal to”
-ne	Alternative form of “is not equal to”
-gt	Greater than
-lt	Less than
-ge	Greater than or equal to
-le	Less than or equal to

If/Else Statements

• Comparisons are used to create decision branches which help steer the direction of the script’s commands
• If/Else statements help identify a course of action based on a set of comparisons
• boolean comparisons are contained within square brackets
  • there are spaces between the brackets and the comparison statement
  • e.g., [ $num1 -gt $num2 ]

Example

#!/bin/bash
mynum=100
echo "Enter a number"
read num1
if [ $num1 -gt $mynum ]; then
	echo "Your number is bigger"
elif [ $num1 -lt $mynum ]; then
	echo "Your number is smaller"
else
	echo "We picked the same number!"
fi

While Loops

A while loop will continue processing a series of commands until a predetermined condition is met.

while :
do
	something here