How to Increment & Decrement Variable in Shell Script

Often you may need to increment or decrement variables in shell script. It is mostly done in loops as counter, but can be done elsewhere also. There are several simple ways to increment & decrement variable in shell script. We will learn them in detail, in this article.

How to Increment & Decrement Variable in Shell Script

Here are the different ways to increment & decrement variables in shell script. You can do this using double parenthesis ((…)) or $((…)) or with let command.

1. Using + and – operator

You can simply add/subtract the differential value to increment/decrement variable, using +/- operators. Here is an example to increment shell variable.

i=$((i+1))
((i=i+1))
let "i=i+1"

Here is an example to decrement shell variable.

i=$((i-1))
((i=i-1))
let "i=i-1"

You can use these operators to increment/decrement variables by any value you want.

Here is a simple example to use it in for loop.

for((i=0;i<5;i=i+1))
do
  echo i: $i
done
i:0
i:1
i:2
i:3
i:4

2. Using += and -= operators

In this expression you increment/decrement value of the left operand with the value in the right operand.

Here is an example to increment shell variable by 1.

((i+=1))
let "i+=1"

Here is an example to decrement shell variable by 1.

((i-=1))
let "i-=1"

Here is an example to use it in for loop where we increment shell variable by 2.

for((i=0;i<10;i+=2))
do
  echo i: $i
done
i:0
i:2
i:4
i:6
i:8

3. Using ++ and — operator

You can also use ++ and — operators to increment and decrement values respectively. But these operators will increment/decrement operators only 1, unlike above operators. You can use them as prefix or postfix operators meaning you can add them immediately before or after the operator.

Here is an example to increment shell variable using ++ operator.

#prefix increment
((++i))
let "++i"

#postfix increment
((i++))
let "i++"

Here is an example to decrement shell variable using — operator.

#prefix increment
((--i))
let "--i"

#postfix increment
((i--))
let "i--"

The main difference between prefix and postfix operators is that prefix operators increment/decrement the value first and only then return the updated value, while postfix operators increment/decrement the variable after returning the old value.

Here is an example to show the difference between prefix and postfix operators. First let us look at the prefix increment operator.